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Journal of Dairy Science Vol. 85 No. 11 2948-2963
© 2002 by American Dairy Science Association ®
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Performance of High Producing Dairy Cows with Three Different Feeding Systems Combining Pasture and Total Mixed Rations

F. Bargo1, L. D. Muller, J. E. Delahoy and T. W. Cassidy

Department of Dairy and Animal Science The Pennsylvania State University, University Park 16802

Corresponding author:
Lawrence D. Muller; e-mail:
lmuller{at}psu.edu.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Forty-five Holsteins cows in early to mid lactation were used to compare three feeding systems combining pasture and total mixed rations (TMR) on animal performance in a 21-wk repeated-measures experiment. The three treatments were: 1) pasture plus concentrate (PC), 2) pasture plus partial TMR (pTMR), and 3) TMR (non-pasture). Total dry matter intake, using chromic oxide as a marker, was 21.6, 25.2, and 26.7 kg/d for PC, pTMR, and TMR, respectively. Milk production was highest for TMR (38.1 kg/d), lowest on PC (28.5 kg/d), and intermediate for pTMR (32.0 kg/d). Cows on pTMR and TMR had higher milk fat and true protein percentages than cows on PC. Cows on PC gained less body weight and lost more body condition compared with cows on pTMR and TMR. Initial concentrations of plasma nonesterified fatty acids were higher on PC (302 µeq/L) than on pTMR (130 µeq/L) and TMR (225 µeq/L). Plasma and milk urea nitrogen were lower on both pTMR and TMR than on PC. Combining pasture and TMR resulted in higher milk production, milk fat and protein percentage, and maintenance in body condition score compared to pasture plus concentrate. The TMR feeding system resulted in the highest total dry matter intake and milk production.

Key Words: pasture • concentrate • partial total mixed rations • milk performance

Abbreviation key: IVDMD = in vitro dry matter digestibility, MUN = milk urea nitrogen, PC = pasture plus concentrate, PUN = plasma urea nitrogen, pTMR = pasture plus partial TMR


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Energy is the primary limiting nutrient for high producing dairy cows on pasture. Kolver and Muller (1998) reported that the lower milk production (29.6 vs. 44.1 kg/d) from high producing dairy cows consuming only high quality pasture compared with cows consuming a nutritionally balanced TMR was due to a lower DMI (19.0 vs. 23.4 kg/d) and energy intake. That study indicated that high producing dairy cows on pasture need supplemental energy to reach the genetic potential for milk production. Recently, some studies conducted in the US with high producing dairy cows on pasture (Bargo et al., 2002, Reis and Combs, 2000a, 2000b; Soriano et al., 2000) reported that supplementation with 8 to 9 kg/d of corn-based concentrates increased total DMI to approximately 21 kg/d and sustained milk production around 30 kg/d. These studies also reported that energy supplementation reduced milk fat percentage, which was associated with the high starch intake and the low effective fiber of pasture. Although concentrate supplementation increased total DMI and milk production compared to only pasture, both were still lower than when feeding a TMR. These studies were also relatively short (between 9 and 12 wk) or conducted as non-continuous designed experiments (e.g. Latin square designs) where BW and BCS changes could not be evaluated.

Many dairy farmers in the United States avoid using pasture because milk production per cow is lower than with a confinement feeding system (Kolver and Muller, 1998; White et al., 2002). Some management challenges when pasture is the only forage include low milk production per cow, low milk fat and protein content, variations in production because of climate conditions, difficulty in budgeting pasture availability, and inaccurate estimation of total and pasture DMI (Muller and Fales, 1998). Supplementation of pasture with a TMR, a feeding system that is called partial TMR because the pasture grazed by the cows is not physically part of the TMR, may reduce these challenges. Many dairy producers have the equipment and experience with TMR feeding systems because TMR are often used during the non-grazing season. Some of the potential advantages of feeding a partial TMR include the provision of a more uniform ration throughout the year and the grazing season, an easier and more accurate monitor of pasture and total DMI, less chance of rumen digestive problems due to slug feeding of grain because some forage is fed with the concentrate rather than fed separately, and higher milk production per cow with a higher milk fat and protein content.

There are no published studies that have compared pasture as the only forage plus concentrate, a TMR, and the combination of both pasture and TMR. Previous studies have compared milk performance of high producing dairy cows on TMR versus pasture as the only feed (Kolver and Muller, 1998; Kolver et al., 2000), TMR versus pasture plus concentrate (White et al., 2002), or TMR versus TMR plus pasture (Soriano et al., 2001). Those studies were either short-term studies of 4 to 6 wk (Kolver and Muller, 1998; Soriano et al., 2001) or did not include a partial TMR treatment (White et al., 2002). Our objective was to compare three feeding systems combining pasture and a TMR during a 21-wk trial on animal performance.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Cows and Treatments
Forty-five Holstein cows (15 primiparous and 30 multiparous) [BW, 624 ± 56 kg; milk yield, 44.9 ± 7.5 kg/d; parity, 2.19 ± 1.24; DIM, 109 ± 39 (mean ± SD)] were used in a 21-wk trial starting on May 8 to compare three feeding systems on animal performance. Cows were selected from the dairy herd of The Pennsylvania State University Dairy Cattle Research and Education Center (University Park, PA), which averaged 11,436 kg of milk and 363 kg of protein per cow per lactation in 1999. Cows were stratified in groups of three by lactation number and DIM, and within group randomly assigned to one of three dietary treatments: 1) pasture plus concentrate (PC), 2) pasture plus TMR or partial TMR (pTMR), and 3) TMR (non-pasture).

Cows on the PC treatment grazed a pasture with an average botanical composition of 50% smooth bromegrass (Bromus inermis L.), 33% orchardgrass (Dactylis glomerata L.), 7% Kentucky bluegrass (Poa pratensis L.), and 10% weeds and dead material. Pasture was fertilized five times with urea at a rate of 50 kg N/ha, once before the start of the trial on April 20, and during the trial on wk 5 (June 7), 7 (June 19), 13 (July 31), and 18 (September 5). Pre-grazing pasture mass (kg DM/ha) was measured weekly by cutting 15 quadrats (0.124 m2/quadrat) of pasture to the ground level and drying at 55°C in a forced air oven. Pre-grazing pasture mass was were used to adjust the size of the paddock to maintain the targeted amount of pasture offered per cow or pasture allowance. The targeted pasture allowance was 30 kg DM/d for the PC treatment, to minimize pasture DMI reduction with supplementation (substitution rate) without affecting milk production (Bargo et al., 2002), and 15 kg DM/d for the pTMR treatment. Using a temporary polywire, a new paddock was constructed daily, and divided by another polywire to offer a new portion after each milking. A second polywire fence was used to prevent back-grazing. New paddocks were provided each morning at approximately 0700 h. After the a.m. milking, 2/3 of the daily paddock was offered to thirty cows (fifteen on the PC treatment and fifteen on the pTMR treatment) to provide a pasture allowance of 15 kg DM per cow per 12 h. After the p.m. milking, the other 1/3 portion of the paddock was offered to the fifteen cows on the PC treatment to provide a pasture allowance of 15 kg DM per cow per 12 h to complete the daily allowance of 30 kg DM/cow. Cows on the PC treatment were fed twice daily after each milking with a corn-based concentrate (Table 1Go) offered at 1 kg/4 kg of milk, based on the pre-trial milk production. An upper limit of 10 kg DM/d per cow was established to reduce the risk of metabolic problems in the rumen. The amount of concentrate offered was readjusted in the middle of the trial (wk 10) using the milk production from wk 8 and 9.


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  		Table 1. Ingredient and chemical composition (mean ± SD) of the concentrate used to supplement the cows on the PC and pTMR treatments and the TMR fed to the cows on the pTMR and TMR treatments.
 
Cows on the pTMR treatment grazed between the a.m. and p.m. milking with the cows on the PC treatment. After the p.m. milking, cows on the pTMR treatment were housed overnight in a free-stall barn where the group was offered 17 kg DM/d per cow of the same TMR fed to the TMR treatment (Table 1Go). Cows on the pTMR treatment were fed 2 to 3 kg/d of the same concentrate fed to the PC treatment (Table 1Go), with the objective of having a similar forage to concentrate ratio in the total ration between the PC and pTMR treatments.

Cows on the TMR treatment remained in a free-stall barn and were group fed (NRC, 1989; Table 1Go) with approximately 28 kg DM/d per cow of TMR once daily at 0730 h. Feed was pushed up three times per d.

All cows were milked twice daily at 0530 and 1730 h and received bST injections every 2 wk. Walking distance from the pasture to the milking parlor averaged 0.9 km (range: 0.75 to 1.20 km), therefore cows on the PC treatment walked 3.6 km/d, and cows on the pTMR walked 1.8 km/d on average.

Experimental Measures and Sample Analyses
Total dry matter intake was measured during wk 2 (May 15), 6 (June 12), 14 (August 7), and 19 (September 11). Cows on the pTMR and TMR treatments were placed in a tie-stall barn during these periods and fed individually to measure DMI of TMR. Total DMI was also estimated on the three treatments using Cr2O3 as an indigestible fecal marker. The Cr2O3 was administered twice daily (10 g/d) after each milking (0700 and 1900 h, approximately) for 11 d. Fecal grab samples were collected at 0700 and 1900 h from d 7 to 11 and immediately frozen (–20°C).

During the same days that fecal samples were collected, samples of concentrate and TMR were collected, and pasture samples were plucked by hand to the approximate height to which cows grazed. Hand-plucked pasture samples were also taken every week. Samples were dried at 55°C in a forced air oven and ground through a 1-mm screen (Wiley Mill, Thomas Scientific, Philadelphia, PA). Concentrate and TMR samples were composited by period, while hand-plucked pasture samples taken daily during the intake periods were kept as daily samples, and hand-plucked pastures taken weekly were kept as weekly samples. Concentrate, TMR, and pasture samples were analyzed for DM, CP, ash (AOAC, 1990), soluble CP (Krishnamoorthy et al., 1982), ADF and NDF (Ankom Daisy II, ANKOM Technology Corp., Fairport, NY), NSC (Smith, 1981; modified to use potassium ferricyanide as the colorimetric indicator), and in vitro DM digestibility (IVDMD) by a two-stage procedure (Tilley and Terry, 1963). Samples of corn silage, alfalfa silage, and alfalfa hay used in the TMR were also taken during the intake periods and analyzed for DM, CP, ADF, NDF, non-fiber carbohydrates, ether extract, and minerals by wet chemistry (Dairy One, Forage Analysis Laboratory, Ithaca, NY; Table 2Go).


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  		Table 2. Chemical composition of forages used in the TMR fed to the cows on the pTMR and TMR treatments.
 
Fecal samples were thawed, dried at 55°C in a forced air oven, and ground through a 1-mm screen (Wiley Mill, Thomas Scientific, Philadelphia, PA). A composited sample per cow was made for each period of intake measures. Fecal samples were analyzed for CP (AOAC, 1990), NDF (Ankom Daisy II, ANKOM Technology Corp., Fairport, NY), and Cr (Parker et al., 1989). The concentrate DMI on the PC and pTMR treatments, and the TMR DMI on the pTMR and TMR treatments were measured daily in every cow as the difference between offered and refused the same five days that fecal grab samples were taken. Total DMI on the TMR treatment was also determined by the equation: total DMI (TMR) = [(g Cr/d)/(g Cr/g fecal DM)] / (1 – IVDMD of TMR). Pasture DMI was estimated on the PC treatment using the equation: pasture DMI = [(g Cr/d)/(g Cr/g fecal DM) – concentrate DMI x (1 – IVDMD of concentrate)] / (1 – IVDMD of pasture). Pasture DMI was estimated on the pTMR treatment using the equation: pasture DMI = [(g Cr/d)/(g Cr/g fecal DM) – concentrate DMI x (1 – IVDMD of concentrate) - TMR DMI x (1 - IVDMD of TMR)] / (1 – IVDMD of pasture). Total DMI on the PC treatment was determined as concentrate DMI plus pasture DMI, on the pTMR treatment as pasture DMI plus concentrate DMI plus TMR DMI, and on the TMR treatment as TMR DMI. Apparent digestibilities of DM, NDF, and CP were determined with the fecal output estimated using Cr2O3 as marker and DMI, and nutrient concentration in feces and feed.

Milk production was recorded daily from d 1 (May 8) to 147 (October 1). Milk samples were collected weekly during the 21 wk of the experiment and preserved with 2-bromo-2-nitropropane-1,3 diol. Milk fat and true protein were analyzed by infrared spectrophotometry (Foss 605B Milk-Scan; Foss Electric, Hillerod, Denmark; AOAC, 1990) by the Pennsylvania DHIA milk testing laboratory. Cows were weighed after the p.m. milking on two consecutive days every 2 wk starting on wk 1. The same weeks, the body condition of the cows was scored by two experienced independent observers using the five-point BCS scale (1 = thin, 5 = fat; Wildman et al., 1982).

Every 2 wk starting on wk 1, at 0600 h and before the cows received the concentrate or the TMR, blood samples were collected from the coccygeal vessels into one 20-ml evacuated tube containing sodium heparin, and one 10-ml evacuated tube containing potassium oxylate-sodium fluoride (glycolytic inhibitor). Blood was immediately placed on ice and transported to the laboratory. Samples were centrifuged at 3000 x g for 15 minutes at 4°C. Plasma was analyzed for glucose (Glucose Kit no. 510, Sigma Chemical Co., St. Louis, MO), urea N (Stanbio Urea Nitrogen Kit 580, Stanbio Laboratory, Inc., San Antonio, TX), and NEFA (Wako NEFA C-Kit no. 990-75401, Wako Chemicals USA, Inc., Richmond, VA).

Urine samples were taken by vulval stimulation twice daily after each milking on two consecutive days on wk 2 (May 18 and 19), 6 (June 14 and 15), 14 (August 9 and 10), and 19 (September 13 and 14). Samples were acidified with HCl to maintain a pH below 2 and stored at –20°C. Urine samples were thawed, composited in one sample per cow per period, and analyzed for allantoin (Chen, 1989) and creatinine (Sigma Kit no. 555-A; Sigma Chemical Co., St. Louis, MO).

Grazing behavior was measured using automatic behavior recorders (Rutter et al., 1997) in the cows on the PC and pTMR treatments from May 29 to June 13, from June 26 to July 11, and from August 21 to September 6. Each day, recorders were put on four cows, two on each pasture treatment, after the a.m. milking and before cows were moved to the pasture. During the morning, recorders were installed from 0700 to 1730 h (10 h and 30 min). Before the p.m. milking, recorders were removed from the cows to avoid damage during the milking and feeding, and also to download the information recorded during that period of time. During the night, recorders were installed on the same two cows on the PC treatment from 1830 to 0600 h (11 h and 30 min). Before the a.m. milking, recorders were again removed and the information downloaded. The total time recorders were on the cows averaged 22 h. A total of 25 records per treatment were analyzed using the software program IGER GRAZE (Rutter et al., 1997).

Statistical Analyses
Data were analyzed as repeated measures using the PROC MIXED procedure of SAS (1999). The model included the fixed effects of treatments, parity (primiparous or multiparous), week, treatment by week interaction, the random effect of cows nested within treatment, and the residual error. For each variable analyzed, cow nested within treatment was subjected to three covariance structures: compound symmetry, autoregressive order 1, and unstructured covariance. The covariance that resulted in the smallest Akaike’s information criterion and Schwarz Bayesian criterion was used. Least squares means and SEM are reported for all data. Interactions between treatment and parity were not significant, therefore least squares means are only presented for treatments. When significant (P < 0.05) effects due to dietary treatments were detected, mean separation was conducted by the PDIFF option in SAS (1999).

A total of forty-five cows were originally assigned to the trial. The final number of cows used in the data analysis was forty-three. One cow on the PC treatment died during wk 8 for reasons unrelated to the treatment. One cow on the TMR treatment was removed for health problems unrelated to the treatment and did not complete 8 wk in the study. Therefore data on these cows were not used in the analysis.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Weather Data
Total precipitations were 82, 105, 87, 87, and 51 mm for May, June, July, August, and September, respectively. Average precipitation in the previous 10-yr period (1989–1999) for May, June, July, August, and September were 82, 89, 90, 100, and 105 mm, respectively. There was a shortage of precipitation during September compared with the previous years average. Mean monthly temperatures during May, June, July, August, and September were 16.8, 20.6, 20.2, 20.3, and 16.2°C, respectively. Average low temperatures during May, June, July, August, and September were 10.9, 15.7, 15.0, 15.5, and 10.7°C, respectively. For the previous 10-yr period (1989–1999), average low temperatures for those months were 8.9, 14.2, 16.5, 15.2, and 11.0°C, respectively. Average high temperatures during May, June, July, August, and September were 22.6, 25.5, 25.3, 25.0, and 21.7°C, respectively. For the previous 10-yr period (1989–1999), average high temperatures for those months were 20.6, 25.4, 27.4, 26.3, and 21.9°C, respectively. The first day of the experiment (May 8), sunrise occurred at 0502 h and sunset at 1915 h, while the last day of the experiment (October 1) sunrise occurred at 0608 h and sunset at 1754 h. On average, day lengths during May, June, July, August, and September were 14 h 25 min, 14 h 59 min, 14 h 42 min, 13 h 44 min, and 12 h 27 min, respectively.

Pasture Management and Quality
Pasture quality is shown in Table 3Go. The measurements are from hand-plucked pasture samples and represent the pasture selected and consumed by the cows. Pasture quality averaged 26% CP, 14% NSC, 26% ADF, 50% NDF, and 58% IVDMD. Pasture quality was within the range summarized by Muller and Fales (1998) for cool-season grasses in Pennsylvania. The IVDMD of the pasture was similar to that reported by Kolver et al. (1998) for an orchardgrass pasture in Pennsylvania (60%).


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  		Table 3. Chemical composition of the hand-plucked pasture samples during the four intake measurement periods.
 
The pasture had important variations in CP content during the 21-wk experiment (Figure 1Go), which were related to climate and pasture factors and to nitrogen fertilization during the experiment. Minimum levels of CP in pasture were around 20% during wk 3, 4, 12, and 18. Four peaks of CP content were observed during wk 6, 8, 14, and 19 (Figure 1Go), which coincided with the week after each application of 50 kg N/ha.


Figure 1
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  		Figure 1. Variations in CP content in the pasture and average weekly milk urea nitrogen (MUN) of dairy cows with three different feeding systems: pasture plus concentrate (PC), partial total mixed ration (pTMR), and total mixed ration (TMR). Arrows indicate fertilization with 50 kg N/ha (wk 5, 7, 13, and 18).

 
Intake and Apparent Digestibility of Nutrients
Intake and apparent digestibility of nutrients are shown in Table 4Go. The total diet of the PC treatment was 60% pasture and 40% concentrate; the pTMR treatment was 30% pasture, 61% TMR and 9% concentrate; and the TMR treatment was 100% TMR (DM basis). The forage:concentrate ratio (DM basis) was approximately 60:40 for both the PC and pTMR treatments, and 50:50 for the TMR treatment. Compared with the PC treatment, total DMI was 5.1 and 3.6 kg/d higher for the TMR and pTMR treatments, respectively (P < 0.05). The data reported in Table 4Go corresponds to the total DMI estimated using Cr2O3 as fecal marker, recognizing that this technique may overestimate DMI. The DMI of the TMR, measured by individual feeding, averaged 24.3 kg/d (23.9, 24.6, 23.9, and 24.9 kg/d for May, June, August, and September, respectively) or 2.4 kg/d less than determined by the Cr2O3 method. The indirect technique using of Cr2O3 overestimated DMI on the TMR treatment by about 10%, which agrees with the value reported by Holden et al. (1994) for dry cows. If a 10% overestimation is assumed for all treatments, the total DMI expressed as kg/d and percentage of BW is 19.5 kg/d and 3.24%, 22.7 kg/d and 3.59%, and 24.3 kg/d and 3.73% for the PC, pTMR, and TMR treatments, respectively. The discussion here focuses on the intake estimated by the Cr2O3 method because measuring DMI by individual feeding was only possible for the TMR treatment. Total DMI on the PC treatment was similar to that reported by Buckley et al. (2000), who found that high genetic merit dairy cows had higher total DMI than medium genetic merit dairy cows when fed pasture plus concentrate (20.9 vs. 19.3 kg/d).


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  		Table 4. Intake and apparent digestibility of DM, CP, and NDF, and intake of NEL of dairy cows with three different feeding systems.
 
Total tract apparent DM digestibility did not differ among treatments and averaged 63.3% (P > 0.05). Similar values for DM digestibility (62.7%) were reported for high producing dairy cows grazing grass-legume pasture and supplemented with 10 kg/d of a corn-based concentrate (Reis and Combs, 2000b). Dann et al. (1999) reported a total tract apparent DM digestibility of 61.3% for lactating dairy cows consuming a TMR similar to the one fed in this study, which is comparable to the 62.3% on the TMR treatment.

A significant treatment x period interaction was found for total DMI, indicating that the difference in total DMI among treatments differed between periods (Figure 2Go). While the total DMI on the TMR treatment remained relatively constant, both the PC and pTMR treatments had larger variations. The PC and the pTMR treatments had a reduction in total DMI during June and August compared to May and September, which was associated with a reduction in pasture DMI during these two periods (Figure 2Go). The reduction in total and pasture DMI on both the PC and pTMR treatments may be attributed to higher temperatures during June (25.5°C) and August (25.0°C). The average high temperatures during the 11 d of intake measurements averaged 19.2, 25.3, 24.8, and 22.2°C for the intake periods in May, June, August, and September, respectively.


Figure 2
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  		Figure 2. Total and pasture DMI estimated by Cr2O3 in each of the four periods by dairy cows with three different feeding systems: pasture plus concentrate (PC), partial total mixed ration (pTMR), and total mixed ration (TMR). Least square means with different superscripts differ (P < 0.05). Overall SEM = 0.5 kg/d.

 
Total CP intake did not differ among treatments and averaged 4.7 kg/d, even with a lower total DMI on both the PC and pTMR treatments (P > 0.05; Table 4Go). The CP content in the total diets averaged 21.8, 19.1, and 16.9% in the PC, pTMR, and TMR treatments, respectively. The high CP content in the total diet of the PC treatment is a result of the high CP content of the pasture (26.3%). Total tract apparent CP digestibility was lower on the pTMR and TMR treatments compared to the PC treatment (P < 0.05). Kolver et al. (1998) reported CP digestibility of the total diet higher than 70% for high producing cows fed pasture and 9.2 kg/d of a corn-based concentrate. Dann et al. (1999) reported similar CP digestibility values (58.7%) to the 57.9% on the TMR treatment.

The highest total NDF intake was found on the pTMR treatment, and the lowest on the PC treatment (P < 0.05; Table 4Go). Total diet NDF concentration averaged 36.5, 34.5, and 30.7% for the PC, pTMR, and TMR treatments, respectively. Total NDF intake as percentage of BW was lower for both the PC and TMR treatments than for the pTMR treatment (P < 0.05). The TMR treatment had lower NDF digestibility (41.4%) than the PC (45.2%) and pTMR (43.7%) treatments (P < 0.05). The inclusion of pasture on those two treatments explains this difference because of the highly digestible fiber in good quality pastures (NRC, 2001). Reis and Combs (2000b) reported a 45.9 and 47.4% digestibility of NDF for grazing cows supplemented with 5 or 10 kg/d of corn-based concentrates, respectively. For TMR diets, Dann et al. (1999) reported a NDF digestibility of 38.7%, which is similar to the 41.4% on the TMR treatment of this study.

The TMR treatment had higher NEL intake than the PC and pTMR treatments, respectively (P < 0.05; Table 4Go). The energy concentrations of the PC, pTMR and TMR treatments were 1.63, 1.60, and 1.64 Mcal/kg, respectively. The higher energy intake for the TMR treatment was a result of a higher total DMI because the NEL was similar among treatments.

Milk Production
Milk production was higher (P < 0.05) for cows fed the TMR treatment than on the other two treatments (Table 5Go). Cows on the TMR treatment produced 19% more milk (6.1 kg/d) than cows on the pTMR treatment and 33% more milk (9.6 kg/d) than cows on the PC treatment. The combination of pasture with TMR (pTMR) resulted in a 13% higher milk production (3.5 kg/d) than the PC treatment (P < 0.05). Wu et al. (2001) reported a 7.7 kg/d difference in milk production between fall calving cows grazing during the following summer and supplemented with 7.9 kg/d of concentrate compared with the projected milk production if cows had remained on a TMR diet. In a 4-yr study, White et al. (2002) compared the milk production for the entire lactation from Holstein and Jersey cows fed a TMR or pasture-based diet supplemented with concentrate and hay or silage. Cows fed the pasture-based diet produced 11% less milk per lactation than cows fed the TMR (White et al., 2002). Kolver et al. (2000) reported that Dutch Holstein Friesian cows produced significantly more milk on a 100% TMR diet than on a 100% pasture diet (22.6 vs. 14.9 kg/d).


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  		Table 5. Milk production and composition of dairy cows with three different feeding systems.
 
The differences in milk production among treatments are likely related to differences in energy maintenance requirements related to walking and grazing activity, and to differences in energy intake (Table 4Go). The energy cost of grazing is attributed to the physical effort of eating grass and the extra time selecting the grass (Agnew et al., 2000). Grazing cows also need to walk between the pasture and the milking center twice daily (Agnew et al., 2000). Using the equations developed by the NRC (2001) to account for the energy expended for these activities, the PC treatment had a 5.4 Mcal/d increase in maintenance requirements. Based on 0.64 Mcal NEL/kg of milk, the activity could account for 8.4 kg of milk/d or 88% of the difference in milk production between the PC and TMR treatments. The pTMR had 2.5 Mcal/d increase in the maintenance requirements. Based on 0.67 Mcal NEL/kg of milk, the activity could account for 3.7 kg of milk/d or 61% of the difference in milk production between the pTMR and TMR treatments. It is thus likely that some combination of increased activity and decreased energy intake accounted for much of the differences in milk production among treatments.

The average milk production obtained on the TMR treatment for cows between 109 to 256 DIM (average DIM for this trial) represents the milk production of high genetic merit dairy cows on a well managed confinement feeding system based on a nutritionally balanced TMR and administered bST. This level of production through the entire lactation would result in 11,621 kg per lactation (305 days x 38.1 kg/d), which is similar to the average herd milk production (11,436 kg per lactation) of The Pennsylvania State University Dairy Cattle Research and Education Center. A similar milk production per lactation (11,391 kg) is estimated it we consider that the cows averaged 40 kg/d during the first 108 days (0 to 108 DIM) before this experiment, 38.1 kg/d for the 147 days of the experiment (109 to 256 DIM), and 29.4 kg/d for the last 50 d of lactation (257 to 305 DIM) after this experiment.

The average milk production obtained on the PC treatment for cows between 109 to 256 DIM represents the production level of high genetic merit dairy cows supplemented at a rate of 1 kg of concentrate per 4 kg of milk on intensively managed pasture-based feeding systems and administered bST. Bargo et al. (2002) reported that high producing dairy cows grazing similar pastures and supplemented with 8.6 kg/d of a corn-based concentrate produced 29.8 kg/d of milk. The slightly higher milk production obtained in that study could be related to the shorter duration (12 wk) compared to the current study (21 wk). Two previous long-term studies (24 to 25 wk) at The Pennsylvania State University reported lower milk production for high producing dairy cows on pasture-based diets (Hoffman et al., 1993; Fales et al., 1995) than this trial. Hoffman et al. (1993) reported a milk production of 24.1 kg/d for cows supplemented with a corn-based concentrate at a rate of 1 kg of concentrate/4 kg of milk with no bST. Fales et al. (1995) reported a milk production of 25.5 kg/d for grazing cows supplemented with 6.1 kg/d of grain. Milk production from 17.5 to 24.5 kg/d was reported by Dhiman et al. (1999) for dairy cows on diets containing 33 or 67% of pasture during a 20-wk study. The lower milk production between those studies and our study may be explained by differences in stage of lactation of cows, the use of bST, and the type of pasture. Overall, the cows on the PC treatment are estimated to produce about 9665 kg of milk per lactation assuming a milk production of 40 kg/d during the first 108 d (0 to 108 DIM) before this experiment, 28.5 kg/d for the 147 days of the experiment (109 to 256 DIM), and 21.1 kg/d for the last 50 d of lactation (257 to 305 DIM) after this experiment. This would represent 85% of the milk production per lactation of the TMR treatment.

There is limited published information on feeding systems combining pasture and TMR, therefore comparisons of the milk production obtained in the pTMR treatment for cows from 109 to 256 DIM are limited. Milk production on the pTMR treatment was 3.5 kg/d higher than the PC treatment, and 6.1 kg/d lower than the TMR treatment (Table 5Go), indicating that milk production in the system combining both feedstuffs is closer to the pasture-based feeding system than to the TMR-based feeding system. In a 6-wk study, Soriano et al. (2001) found that milk production was significantly higher (29.1 vs. 27.6 kg/d) for cows fed a TMR than for cows grazing a pasture in the morning and fed a TMR in the afternoon. Dhiman et al. (1999) compared the milk production of dairy cows on pasture diets where pasture was 33, 67, or 100% of the total diet. The one-third pasture treatment received 11.6 kg DM/d of a supplement containing 25% alfalfa hay, 48% high moisture corn, 18% roasted cracked soybeans, 6% soybean meal, and 2.7% mineral and vitamin mix and could be considered equivalent to the pTMR treatment. The 33% pasture treatment produced 40% more milk (24.5 vs. 17.5 kg/d) than the 67% pasture treatment (Dhiman et al., 1999).

Milk Production Persistency
Lactation curves of the three treatments are shown in Figure 3Go. Milk production averaged 45 kg/d for all the cows during the 6 wk prior to the experiment. Cows on both the PC and pTMR treatments experienced a rapid decrease in milk production during the 2 wk of adjustment (adj1 and adj2 in Figure 3Go), when pasture was gradually incorporated to the diets of these two treatments. The management of the PC and pTMR cows during the adjustment period consisted on a gradual increase in the time that cows were allowed to graze during a 2-wk period. Cows on both treatments were moved to the pasture at 0800 h for 2 h during the first 2 d, then for 4 h during the subsequent 3 d, and then for 8 h during the subsequent 4 d. During these first 9 d of the adjustment period, cows were fed a TMR ad libitum and housed in a free-stall barn when not on pasture. During the last 5 d of the adjustment period, the cows on the PC treatment remained on pasture day and night and started to be supplemented with concentrate, whereas cows on the pTMR treatment grazed during the day, were fed a TMR, and were housed in a free-stall barn at night. During these 2 wk, the milk production of cows on the PC treatment decreased 18% (from 45.7 to 37.7 kg) or 0.57 kg/d, and the milk production of cows on the pTMR treatment decreased 15% (from 44.9 to 38.4 kg) or 0.46 kg/d.


Figure 3
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  		Figure 3. Average weekly milk production of dairy cows with three different feeding systems: pasture plus concentrate (PC), partial total mixed ration (pTMR), and total mixed ration (TMR). Figure shows 6 wk prior to the study, 2 wk of adjustment to pasture, and 21 wk of the trial. Significant differences (P < 0.05) between TMR and PC from wk 1 to 21, between TMR and pTMR from wk 1 to 19, and between pTMR and PC from wk 4 to 6, 12, 14, and 16 to 21. Overall SEM = 1.2 kg/d.

 
A rapid decrease in milk production during the initial weeks of grazing has been previously reported for high producing dairy cows (Fales et al., 1995; Hoffman et al., 1993; Jones-Endsley et al., 1997; Wu et al., 2001). Milk production declined about 25% during the first 8 wk when cows grazed good quality grass pasture and were supplemented at a rate of 1 kg concentrate/4 kg of milk (Hoffman et al., 1993). A decrease in milk production from 43.0 to 30.4 kg/d occurred during the first 16 d (0.79 kg/d) when cows supplemented with 6.4 to 9.6 kg/d of concentrate were moved to pasture (Jones-Endsley et al., 1997). Fall-calving dairy cows grazing during the summer and supplemented with concentrate had a reduction between 2.2 and 4.2 kg (0.31 and 0.60 kg/d) during the first week on pasture (Wu et al., 2001). The amount of milk production reduction depends on the stage of lactation of cows, time of the year, and pasture quality. Possible reasons for this rapid decrease in milk production could be a short adjustment period (2 wk or less), changes in the rumen environment with a negative impact on rumen microbial population, a nutritionally unbalanced diet during the transition period, and increased energy maintenance requirements due to walking and grazing activity.

When comparing wk 1 to 21, after the 2 wk adjustment, milk production decreased 16.7 kg (from 37.8 to 21.1 kg/d) on the PC treatment, 9.4 kg (from 37.2 to 27.8 kg/d) on the pTMR treatment, and 16.0 kg (from 45.4 to 29.4 kg/d) on the TMR treatment. The reduction in milk production was –0.11, –0.06, and –0.11 kg/d for the PC, the pTMR, and the TMR treatments, respectively. This indicates that both the PC and TMR treatments had a similar milk production persistency, while the pTMR treatment had a higher milk production persistency during the 21-wk trial. Pulido and Leaver (2001) reported a milk production decreases in the range of –0.10 to –0.18 kg/d for grazing dairy cows supplemented with concentrate and producing more than 30 kg/d of milk.

Milk Composition
The use of TMR on both the pTMR and the TMR treatments increased milk fat percentage (P < 0.05) compared to the PC treatment (3.33 vs. 3.13%; Table 5Go). The lower milk fat percentage on the PC treatment may be related to the highly digestible fiber in good quality pasture and the feeding of concentrate separate from the forage twice daily (NRC, 2001). White et al. (2001) also reported lower milk fat percentage (3.23 vs. 3.33%) for Holstein cows fed pasture plus concentrate compared to Holstein cows fed a TMR.

True protein percentage in milk was higher (P < 0.05) on the TMR treatment than on the PC treatment (2.99 vs. 2.82%; Table 5Go), while the true protein percentage in the pTMR treatment was numerically between those two treatments. The increase in true milk protein percentage and yield on the TMR treatment compared to the PC treatment is likely related to the higher energy intake (Table 4Go).

Fat and true protein yield followed the results found for milk production and 3.5% FCM production (Table 5Go). Both fat and protein yield were higher (P < 0.05) for the TMR treatment, with the lowest values for the PC treatment (P < 0.05). The pTMR treatment was between these treatments. Cows on the TMR treatment produced 18% more fat and 21% more true protein than cows on the pTMR treatment, and 38% more fat and 41% more true protein than cows on the PC treatment. Somatic cell score did not differ among treatments (P > 0.05; Table 5Go).

Milk urea nitrogen (MUN) decreased (P < 0.05) from 14.9 mg/dl on the PC treatment to 10.6 mg/dl on the TMR treatment (Table 5Go). Weekly variations of MUN in the three treatments during the 21 wk of the experiment are shown in Figure 1Go. The PC treatment had greater weekly variations in MUN with peaks of more than 16 mg/dl occurring during wk 6, 8, 14, and 19 (16.5, 17.4, 23.2, and 18.6 mg/dl, respectively). These four MUN peaks on the PC treatment coincided with peaks in the percentage of CP in the pasture during wk 6, 8, 14 and 19 (Figure 1Go). Weekly MUN for the pTMR treatment were intermediate between the PC and TMR treatments during most weeks. Data from 82 dairy herds on pasture-based feeding systems in Chile (Wittwer et al., 1999) reported high seasonal variation in bulk MUN (mean: 13.5 mg/dl, range: 4.1 to 32.1 mg/dl). The highest values were found during early spring (15.8 to 16.3 mg/dl) and the lowest during late summer (9.9 to 10.2 mg/dl). Peaks were correlated with CP content of pasture that averaged 16.5% in spring and 10.7% in the summer in non-fertilized pastures (Wittwer et al., 1999).

Body Weight and Body Condition Score
The initial BW did not differ among treatments (P > 0.05) and averaged 624 kg (Table 6Go). Cows on the three treatments gained BW as shown by the higher final BW (P > 0.05). The cows on the TMR treatment had a greater increase (P < 0.05) in BW in comparison with both the PC and the pTMR treatments (76 vs. 34 kg).


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  		Table 6. Body weight and body condition score of dairy cows with three different feeding systems.
 
The initial BCS did not differ among treatments (P > 0.05) and averaged 2.85 (Table 6Go). Body condition score changes through the 21-wk. The final BCS was higher for the TMR treatment compared to the PC treatment (P < 0.05), while final BCS on the pTMR did not differ compared to the PC and TMR treatments (P > 0.05). Comparing the initial and the final BCS within each treatment, cows on the PC treatment lost BCS (–0.20), cows on the pTMR treatment maintained BCS (0.01) and cows on the TMR treatment gained BCS (0.19).

Although cows on the PC treatment lost BCS, the magnitude of that loss (–0.20 over 21 wk) was small. It is well known that BCS loss and the rate of BCS loss are related to reproductive efficiency (Staples et al., 1992). Kolver and Muller (1998) in a short term study of 4 wk reported losses of BCS (–0.50) for high producing dairy cows consuming only pasture while cows consuming a TMR maintained BCS. Washburn et al. (2002) found that over the entire lactation, Holstein cows on a pasture-based feeding system had between 0.3 and 0.6 lower BCS than Holstein cows on a TMR-based feeding system. However, reproductive performance did not differ between feeding systems with 45.2% of first-service conception and 57.9% overall percentages of pregnancy in 75 d (Washburn et al., 2002).

Plasma and Urine Metabolites
Plasma and urine metabolites data are presented in Table 7Go. Glucose concentration was not affected by treatments and averaged 64.8 mg/dl (P > 0.05). Cows on the PC treatment had higher (P < 0.05) plasma urea nitrogen (PUN) concentrations than cows on the pTMR and TMR treatments (17.2 vs. 13.8 mg/dl; P < 0.05). A significant treatment x wk interaction was observed for PUN concentration (P < 0.05). The PC treatment had larger variations in PUN concentration (maximum of 20.2 mg/dl in wk 19 and minimum of 13.9 mg/dl in wk 9). These variations reflect changes in CP content in the pasture (Figure 1Go) and the subsequent difference in CP intake. The pTMR treatment had smaller variations in PUN concentration (maximum of 17.4 mg/dl in wk 13 and minimum of 12.1 mg/dl in wk 5), as pasture was only 30% of the diet (Table 4Go). The TMR treatment had the smallest variation in PUN concentration (maximum of 16.9 mg/dl in wk 13 and minimum of 12.4 mg/dl in wk 9), because 100% of the diet was a TMR with a more constant CP content.


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  		Table 7. Blood and urine metabolites of dairy cows with three different feeding systems.
 
The initial NEFA concentrations on both the PC and TMR treatments were significantly higher than final NEFA concentrations, suggesting body fat mobilization at the beginning of the experiment in these two treatments (P < 0.05; Table 7Go). The NEFA concentrations on the pTMR treatment remained relatively constant (i.e. initial concentration did not differ from final concentration; P > 0.05), indicating lower mobilization of body fat reserves during the initial weeks of the experiment. Final NEFA concentrations did not differ among treatments and averaged 115.3 µeq/L (P > 0.05). A significant treatment x week interaction was found for NEFA concentrations (P < 0.05). Cows on the PC treatment had a drastic reduction in the NEFA concentration from the start until the end of the trial. The initial high NEFA concentration on the PC treatment is consistent with the loss of BCS and the lower gain in BW (Table 6Go). Although cows on the TMR treatment gained BCS and more BW than cows on the PC treatment (Table 6Go), high levels of NEFA concentration were observed at the beginning of the trial with a lower decrease towards the end. This would indicate mobilization of body reserves in order to sustain the high levels of milk production enhanced by the bST injections (> 38 kg/d). The pTMR treatment had a relative stable concentration of NEFA during the trial, indicating less mobilization of body reserves, which is supported by the maintenance in BCS and the lower BW gain (Table 6Go) and milk production level (Table 5Go) than the TMR treatment.

Allantoin and creatinine concentrations, and allantoin/creatinine ratio in spot urine samples are shown in Table 7Go. Both allantoin and creatinine concentration were higher (P < 0.05) on the TMR treatment than on the PC and pTMR treatments. Carruthers and Neil (1997) reported similar values for allantoin concentrations (1992.1 mg/L) for dairy cows grazing ryegrass pasture and supplemented with NSC. The allantoin/creatinine ratio, an index of total allantoin excretion in urine and an indicator of rumen microbial protein synthesis (Gonda, 1995), was not affected by dietary treatments and averaged 2.92 (P > 0.05). Gonda (1995) reported an average allantoin/creatinine ratio of 3.02 for dairy cows between 98 to 112 DIM on a confinement feeding system. The lack of significance in the allantoin/creatinine ratio indicates that the rumen microbial protein synthesis was not affected by the feeding system or that this technique was not sensitive enough to detect differences in microbial protein synthesis among treatments.

Grazing Behavior
The measures of grazing behavior (grazing time, biting rate and bite mass) are presented in Table 8Go. Total grazing time averaged 9.5 h/d (572 min/d) for the PC treatment, and 4.2 h/half day (252 min/half day) for the pTMR treatment (P < 0.05). Research from Northern Ireland (Sayers, 1999) reported a grazing time of 7.9 h/d (473 min/d) for dairy cows grazing ryegrass pasture and supplemented with 6 kg/d of concentrate. In a second study, grazing time was 7 h/d (480 min/d) or 6.6 h/d (398 min/d) when supplemented with 5 or 10 kg/d of a starch-based concentrate, respectively (Sayers, 1999). The high grazing time found on the PC treatment (> 9 h/d) is within the range cited by Rook (2000), who reported a maximum grazing time of 12 h/d for dairy cows.


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  		Table 8. Grazing behavior of dairy cows with two different feeding systems.
 
When the cows on the PC and pTMR treatments grazed together, grazing time did not differ between treatments and averaged 4.3 h (259 min; P > 0.05; Table 8Go) for the 10.5 h grazing period during the day. This suggests that cows probably reached the maximum potential grazing time for that 10.5 h period, and that the pasture DMI of 7.5 kg per cow obtained on the pTMR treatment may be near the maximum expected pasture DMI for that amount of pasture offered. If feeding systems that combine pasture and TMR have the goal of increasing the proportion of pasture DMI in the diet (higher than the 30% of total DMI obtained in the pTMR treatment), cows may need to graze in the evening to provide the opportunity to increase grazing time per d. For example, if cows on the pTMR treatment were allowed to graze after the p.m. milking for 90 min, assuming the same biting rate (59 bites/min) and bite mass (0.55 g DM/bite) as obtained on the PC treatment, would represent an extra 2.9 kg/d pasture DMI and result in a pasture DMI of 10.4 kg/d or 41% of the total diet. After the p.m. milking, cows on the PC treatment grazed 4.9 h (295 min), or 29 min longer than during the day. Rook (2000) concluded from several studies that grazing ruminants consume a larger meal before sunset. The biting rate averaged 56 bites/min (P > 0.05; Table 8Go) and was not expected to be affected by treatments. Biting rate is primarily affected by animal factors such as stage of lactation and body size (McGilloway and Mayne, 1996; Rook, 2000), which were similar in both treatments. The biting rate did not differ between treatments during the morning (55 bites/min; P > 0.05), while the biting rates was higher on the PC treatment during the evening (59 bites/min). In his review, Rook (2000) concluded that biting rate of dairy cows is higher in the evening than in the morning. Gibb et al. (1998) reported that biting rate showed a quadratic effect of time during the day and reached a maximum value during the evening (52.6, 47.5, 51.6, and 59.4 bites/min at 0700, 1130, 1600, and 1900 h, respectively). Total bites were 31,508 bites/d for the PC treatment, and 14,316 bites/half day for the pTMR treatment (P < 0.05). Total daily bites were on the PC treatment were below the maximum (40,000 bites/d) estimated by Phillips and Leaver (1986). The number of bites between the a.m and p.m. milking were not affected by treatments and averaged 14,294 bites (P > 0.05). During the evening, cows of the PC treatment had a higher number of bites than during daytime grazing (17,236 vs. 14,272).

For both the total number of bites and the biting rate, a significant period effect was found with a decrease in both variables between periods (P < 0.05; Table 8Go). Both variables are determined by animal factors including stage of lactation, milk production, and body size (McGilloway and Mayne, 1996; Rook, 2000), which explains the total bites and biting rate decreased with the reduction in milk production associated with the advance in the stage of lactation. In our study, a positive relationship was found between milk production and the number of bites per d for the PC treatment (R2 = 0.74; Figure 4Go), which resulted in an increase of 5 kg/d of milk for every 10,000 bites. It should be noted that the highest producing cows exceeded 40,000 bites/d compared to the average of 31,508 bites/d. An increase in grazing time (min/d) and rate of pasture intake (g DM/min) were previously reported for dairy cows on a ryegrass pasture producing between 16.9 to 35.5 kg/d of milk and supplemented with different levels of concentrate from 0 to 6 kg/d (Pulido and Leaver, 2001). Bite mass was not affected by treatments and averaged 0.52 g DM/bite (P > 0.05; Table 8Go). This variable is mainly determined by pasture structural characteristics such as height and bulk density (Rook, 2000).


Figure 4
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  		Figure 4. Relationship between milk production and number of bites per d on the pasture plus concentrate (PC) treatment.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
ACKNOWLEDGEMENTS
 REFERENCES
 
The TMR feeding system maximized total DMI and milk production. The TMR treatment had the highest total DMI, the PC treatment the lowest, and the pTMR treatment intermediate. The TMR treatment produced 19% more milk than the pTMR treatment, and 33% more than the PC treatment. Fat and true protein percentages in milk were higher on both the TMR and pTMR treatments than on the PC treatment. Cows on the three treatments gained BW, however, the cows on the TMR treatment had a greater increase in BW and BCS than cows on the PC and pTMR treatments.

Animal performance was improved combining pasture and TMR compared to PC. Milk production on the feeding system combining pasture and TMR was intermediate between the TMR and PC feeding systems, but closer to the pasture-based feeding system. The combination of pasture and TMR also resulted in higher milk fat and protein percentage, a higher body condition score, and lower MUN, PUN, and NEFA concentrations than the PC treatment.


   ACKNOWLEDGEMENTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
REFERENCES
 
This research was partially supported by Agway, Inc. The authors thank Julia Amick, Marisa Bazzini, Paul Kononoff, Maria Long, and Nadine Salomon for assistance in sampling and laboratory analyses; and Jim Homan for assistance in pasture management (fencing and watering).


   FOOTNOTES
 
1 Current address: Dairy Nutrition Services, Inc., Chandler, AZ 85244; email: fbargo{at}dns-ans.com Back

Received for publication January 25, 2002. Accepted for publication March 24, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 


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