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* Department of Dairy Science, University of Wisconsin, Madison 53706
Research Centre, Agriculture and Agri-Food Canada, Lethbridge, AB, Canada T1J 4B1
Corresponding author:
D. K. Combs; e-mail:
dkcombs{at}facstaff.wisc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Abbreviation key: CS = coarse silage, DC = dry corn, DOMI = digestible organic matter intake, eNDF = effective NDF, ERD = effective rumen degradability, FS = fine silage, HMC = high moisture corn, RFC = ruminally fermentable carbohydrate
Key Words: particle size • ruminally fermentable carbohydrate • production • digestibility
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Milk fat percentage response (Armentano and Pereira, 1997), time spent chewing (Sudweeks et al., 1981), and ruminal pH (Mertens, 1997) have been used to determine the effectiveness of fiber in diets for dairy cows. The effectiveness of NDF is related to the total ability of a feed to replace forage in a ration so that the percentage of fat in milk produced by cows eating the ration is effectively maintained (Mertens, 1997). Diets low in effective fiber and high in fermentable carbohydrates can affect ruminal fermentation negatively. In vitro studies have shown that fiber digestion is greatly depressed when pH declines below 6.0, and that the optimal ruminal pH for fiber digestion is around 6.5 (Shriver et al., 1986). However, ruminal pH of growing cattle and high producing dairy cows is often below 6.0 despite diets balanced to include a minimum amount of forage to ensure rumen function. A decrease in fiber digestion because of low ruminal pH can decrease ruminal digestion of the diet (Shriver et al., 1986) and, therefore, negatively affect production.
The Cornell Net Carbohydrate and Protein System indicates that diets containing less than 20% NDF from forage reduce microbial yield (Russell et al., 1992). Strobel and Russell (1986) reported that mixed ruminal bacteria incubated in vitro with a mixed carbohydrate substrate produced 13.6 mg DM/mmol ATP produced at pH 5.7 versus 21.2 mg DM/mmol ATP produced at pH 6.7. Depression in carbohydrate utilization at low pH is a primary factor contributing to the lower protein yield, but this does not completely explain the decrease in protein yield. Because certain amino acids are often limiting postruminally (NRC, 2001), depressions in microbial protein synthesis could have a negative effect on animal productivity. However, information is lacking concerning the in vivo effects of diets low in effective fiber and high in fermentable carbohydrates on the protein yield of rumen microbes.
The objectives of this study were to investigate the effects of, and interactions between, level of ruminally fermentable carbohydrate in the diet and forage particle size on milk production, digestibility, and microbial yield at constant level of dietary NDF.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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. All diets were formulated to meet or exceed the requirements of a 600-kg multiparous cow producing 45 kg of milk/d using CPM-Dairy (1997). Diet formulations are given in Table 2.
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Cows were cared for according to guidelines of The Research Animal and Resource Committee of the Univ. of Wisconsin-Madison, and all experimental procedures performed on the animals were approved. Cows were housed in stalls bedded with rubber mattresses and wood shavings and were milked twice daily at 0300 and 1500 h in a milking parlor. Cows were turned outside for 1 to 2 h daily after being milked, except on days when total urine output was recorded. Milk was sampled on consecutive p.m. and a.m. milkings on 2 d during each period, and milk components were determined by AgSource, Menomonie, Wisconsin, using a near-infrared reflectance spectroscopy analyzer (MilkoScan 605; Foss Electric, Hillerød, Denmark).
Feed Analysis
Composite samples of all feeds, diets, and orts were obtained during each experimental period. Dried samples were ground to pass a 1-mm screen (Wiley Mill, Arthur H. Thomas, Philadelphia, PA). Analytical DM content of feeds was determined by oven-drying at 135°C for 2 h; OM was determined by ashing at 500°C for 16 h, and CP was determined by the micro-Kjeldahl method (AOAC, 1990). The NDF fraction was determined using 
-amylase (Sigma no. A3306: Sigma Chemical Co., St. Louis, MO) and sodium sulfite (Van Soest et al., 1991) and was not corrected for ash content; ADF was determined using the procedure described by Goering and Van Soest (1970). Starch was determined by a colorimetric assay including a pure cornstarch sample as described by Bal et al. (2000).
Particle size of forages and TMR was determined by dry-sieving using an oscillating screen particle size separator according to ASAE standard S424 (Am. Natl. Standards Inst., 1988). Particle size of the corn grain was determined by dry-sieving according to ASAE (1995) standards.
Digestibility
Lanthanum in solution (0.2 g/ml) was used as a marker to measure total tract digestibility (Hartnell and Satter, 1979) and was ruminally dosed at 12-h intervals for the last 14 d of each period to provide 0.8 g of La per cow per day. Fecal samples were collected at different times of the day during a 5-d interval concurrent with fecal sampling for rate of passage measurements. Sampling times differed such that the entire 24-h day was represented to account for possible diurnal variation. Fecal samples were dried, ground to pass a 1-mm screen, pooled by period for each cow and dry-ashed at 500°C for 16 h. Concentrations of La were determined by direct current plasma emission spectroscopy (Spectra Metrics, Inc., subsidary of Beckman Instruments, Inc., Andover, MA; Combs and Satter, 1992). Total tract nutrient digestibilities were calculated from fecal La concentration and nutrient concentrations in diets fed orts and feces.
In Sacco Measurements
Ruminal degradation of TMR was measured using large (25- x 35-cm), in situ bags made of Dacron polyester cloth with a pore size of 52 ± 5 µ. Approximately 80 g (as fed) of whole TMR samples were weighed into bags, soaked in warm water for 10 min, and placed in large mesh retaining sacs before being incubated ruminally for 0, 3, 4, 5, 6, 12, 24, 48, and 96 h. Only one bag per time point was incubated for 3, 4, 5, and 6 h, whereas other time points were done in duplicate. After removal from the rumen, bags were washed under cold, running tap water, and then machine-washed twice in cold water using a rinse cycle with 2 min of agitation. The 0 h time point bags were not placed in the rumen, but were subject to the same washing procedure. Bags were dried at 60°C for 72 h.
The kinetics of DM disappearance in sacco were estimated using nonlinear least squares (PROC NLIN of SAS; SAS, 1998). For each cow and period, the following model (McDonald, 1981) was fitted to the percentage of DM disappearance:
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where a = soluble fraction (%); b = slowly digestible fraction (%); kd = fractional rate of disappearance(%h–1); L = lag time (h); and t = incubation time (h). The indigestible fraction, referred to as ‘c’ in the results, was calculated by difference.
Rate of Passage
Lithium-Co-EDTA and Cr-mordanted fiber were prepared as described by Udén et al. (1980) and used as markers for liquid and solid passage rates, respectively. The Li-Co-EDTA was dried and ground using a mortar and pestle; Cr-mordanted fiber was prepared by mordanting wheat straw NDF ground through a 6-mm screen using a Wiley mill. Markers were placed in the rumen at the time of the morning feeding, and no attempt was made to manually mix markers with rumen contents. Fecal grab samples were taken at 0, 6, 10, 14, 18, 22, 26, 30, 36, 42, 48, 54, 60, 72, 84, 96, and 120 h after dosing to determine the rate of passage. Samples were dry-ashed, and fecal marker concentrations of Cr and Co were determined by direct current plasma emission spectroscopy (Spectra Metrics, Inc., subsidiary of Beckman Instruments, Inc., Andover, MA; Combs and Satter, 1992).
Fecal Cr and Co excretion curves were fitted to the double-compartment model represented by two exponential constants and a time delay (Grovum and Williams, 1973):
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where Y = marker concentration (ppm); A = scale parameter; k1 = ruminal rate of passage (%/h); k2 = lower digestive tract rate of passage (%/h); t = sampling time post dosing (h); and TT = transit time. Total mean retention time in the digestive tract was calculated as the sum of retention in the rumen (1/k1) and in the lower digestive tract (1/k2) plus the transit time (TT). Data were analyzed by nonlinear regression using the NLIN (iterative Marquardt method) procedure of SAS (SAS, 1998).
Microbial Protein Synthesis
Microbial protein synthesis was not measured directly. Instead, the urinary excretion of the purine derivatives allantoin and uric acid were used as an estimate of microbial N flow to the duodenum. On three consecutive days in each experimental period, total urine was collected using indwelling catheters (24
-, 75-cc balloon lubricious catheter, C. R. Bard, Inc., Covington, CA). Containers with 500 ml of 1.5 N H2SO4 were attached to each cow, and output of urine was measured twice daily. After recording the volume of urine excreted, acidified urine was mixed, and 20-ml samples were taken, diluted to 100 ml with tap water and frozen (–20°C) for later analysis. Concentration of allantoin in urine was determined colorimetrically using the method described by Chen and Gomes (1992) however, 1 M HCl was used instead of 0.5 M HCl in the assay in order to keep pH <3. Samples were diluted 10 times with tap water in the laboratory to a final dilution of 50 before analysis. Uric acid in urine was determined colorimetrically using a diagnostic uric acid reagent (procedure no. 685, Sigma Diagnostics). For the uric acid assay, 1 ml of reagent was used with 50 µl of urine diluted 25 times (samples were diluted five times with tap water in the lab before analysis). Purine absorption and intestinal flow of microbial N was calculated using the assumptions and equations given by Chen and Gomes (1992). The quantitative relationship between absorption of microbial purines (X mmol/d), and excretion of purine derivatives in urine can be described by the following equation:
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where W0.75 represents the metabolic BW (kg) of the animal. The slope of 0.85 represents the recovery of absorbed purines as purine derivatives in urine. The component within parenthesis represents the net endogenous contribution of purine derivatives to total excretion after correction for the utilization of microbial purines by the animal. The following factors were used to calculate intestinal flow of microbial N (g N/d) from the microbial purines absorbed (X mmol/d): Digestibility of microbial purines was assumed to be 0.83; the N content of purines was 70 mg N/mmol; and the ratio of purine-N to total N in mixed rumen microbes was taken as 11.6:100. Thus, microbial N was calculated as:
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This assumes that the purine to protein ratio in mixed rumen microbes was unchanged by dietary treatment.
Statistical Analysis
Data on all variables were analyzed using the mixed model procedure in SAS (SAS, 1998); period, level of RFC, particle size of forage, and the interaction of RFC and forage were fixed effects in the model, and period was used as a repeated measurement with first-order auto regressive covariance structure. The random statement included square and cow within square. The model used for intake and production variables, digestibilities and purine derivative excretion data is shown below.
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where µ = overall mean; Si = random effect of square (i = 1 to 2); Cj(i) = random effect of cow within square (j = 1 to 4); Pk = fixed effect of period analyzed as repeated measurements (k = 1 to 4); Ml = fixed effect of level of RFC (l = 1 to 2); Fm = fixed effect of forage particle size (m = 1 to 2); (M x F)lm = fixed effect of interaction of Ml and Fm; and eijklm = random residual error, assumed to be normally distributed. Significance was declared at P 
0.05. A trend was considered to exist if 0.05 < P 0.10. All reported values are least square means unless otherwise stated.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The geometric mean particle size of 13.6 mm was considered to be above the threshold of 6.4 mm that is reported to reduce rumination time and cause milk fat depression when dairy cows are fed chopped alfalfa hay as forage (Woodford et al., 1986). The mean geometric particle size of the finely chopped silage was below this threshold. All diets used in this study were relatively low in NDF (Table 2). Based on the low NDF content of the diets and the small particle size of the finely chopped forage, the diets based on finely chopped silage were predicted to be inadequate in level of effective NDF. Based on the diagonal diameter of the screens in the UW forage particle size separator (ASAE standard S424, American National Standards Institute, 1988), the two top screens of the UW forage particle size separator represent the top screen of the Penn State particle size separator, whereas screen number three represents the middle screen of the Penn State particle size separator and the last two screens the pan. Using this approximation, the forages and TMR had the following distribution on the top screen, middle screen, and pan of the Penn State particle size separator (% as fed): CS: 41, 35, 24; FS: 0, 18, 82; HMCFS: 0, 15, 85; HMCCS: 22, 24, 54; DCFS: 0, 11, 89; DCCS: 24, 22, 55.
Intakes of DM, OM, and nutrients are shown in Table 3. Intakes of DM and OM were higher for DC than for HMC diets. Other studies have found no difference in DMI when comparing dry corn versus high-moisture corn fed to dairy cows (Knowlton et al., 1998; Ying et al., 1998). However, the effect of corn moisture on DMI seems to depend on level of corn in the diet (Oba and Allen, 2000a). Intake of DM decreased when high-moisture corn replaced dry corn in diets containing 31% starch but not in diets containing 21% starch (Oba and Allen, 2000a). Diets fed in the current study were intermediate in starch content; 28.6% for HMC diets and 26.1% for DC diets.
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An interaction between RFC and forage particle size was observed for NDF, ADF, and starch intake. When cows were fed HMC, increasing forage particle size decreased NDF and ADF intake, whereas when cows were fed DC, increasing forage particle size increased NDF and ADF intake. Starch intake increased with increasing forage particle size when cows were fed HMC, but decreased with increasing forage particle size when cows were fed DC.
Milk Production
Cows fed HMC tended to produce more milk (P = 0.08) compared with cows fed DC (Table 4); however, this increase was observed only for the FS diets. In the literature, milk production responses to HMC are mixed. No significant difference in milk production was observed between cows fed HMC and DC in two studies (Chandler et al., 1975; Knowlton et al., 1998), whereas Clark et al. (1973) observed an increase in milk production when cows were fed HMC. A slight decrease in milk production was found by De Brabander et al. (1992) when HMC replaced a control concentrate.
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for nutrient intakes). Solids-corrected milk yield was unaffected by diet, but tended to increase with increasing corn fermentability (P = 0.08). Because of lower DMI of diets containing HMC, efficiency of milk production, expressed as SCM production per unit of DMI, was higher for diets containing HMC than for diets containing DC. Greater ruminal fermentation is associated with higher energetic efficiency, which could explain the higher efficiency of milk production for the HMC diets compared to the DC diets.
Level of ruminally fermentable carbohydrate (RFC) did not affect milk fat content or yield. Oba and Allen (2000a) reported a depression in fat percentage when HMC replaced DC in high-starch diets (31%), but no difference when diets contained 21% starch. Knowlton et al. (1998) found no difference in milk composition between DC and HMC when fed at 42.4% of diet DM. Despite the small particle size of the finely chopped silage, no effect of forage particle size was observed on milk fat percentage or fat yield. It was surprising that no reduction in milk fat percentage was observed when forage particle size was reduced to 3.7 mm, considering the consistent response to reducing forage particle size found in other studies (Woodford et al., 1986; Woodford and Murphy, 1988) and the significant reduction in time spent chewing when forage particle size was reduced (Krause et al., 2002). Milk fat percentage is the animal response that often is associated with effective NDF (eNDF) content of a ration (Mertens, 1997). Based on the milk fat percentages observed in this study, and the lack of diet effect on milk fat percentage, it can be concluded that all four diets provided adequate amounts of eNDF to sustain milk fat percentage. However, diets containing FS did have numerically lower milk fat percentages than CS diets. The number of animals used in this trial might not have been adequate for detecting differences in milk components caused by dietary treatments.
Milk protein percentage and protein yield were similar across diets. For milk protein percentage, level of RFC and forage particle size tended (P = 0.09) to interact; protein percentage increased when forage particle size was increased in HMC diets, whereas it decreased in DC diets. This interaction might have been linked to the interaction between level of RFC and forage particle size on starch and fiber intakes. A higher starch intake for HMC diets when forage particle size increased could support an increase in microbial protein synthesis, which might result in a higher milk protein percentage. Lactose percentage and yield were not affected by diet. As for percent protein, percentage of SNF increased with increasing forage particle size for diets containing HMC, but decreased with increasing forage particle size for diets containing DC. No effect of diet was found on yield of SNF.
Rate of Passage of Liquids and Solids
Increasing forage particle size increased liquid outflow rate from the rumen (Table 5), probably because of a higher saliva production for cows fed CS. Chewing activity, which is reported in a companion paper (Krause et al., 2002), was found to be higher for cows fed CS compared with FS; therefore, a higher saliva production would be expected for cows fed CS. Ruminal outflow rate of solids was not affected by level of RFC or forage particle size, but level of RFC tended (P = 0.06) to decrease ruminal retention. Transit time decreased when forage particle size was increased, and total mean retention time in the GI tract was decreased by increasing forage particle size and also decreased by increasing level of RFC. That mean retention time for solids was decreased with increasing forage particle size can seem surprising, but could be related to the fact that the marker used was ground wheat straw. The small particle size of the ground wheat straw might have allowed it to follow the liquid phase, which had an increased outflow rate when forage particle size was increased.
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). This finding is consistent with several studies (Knowlton et al., 1998; Ying et al., 1998), whereas Oba and Allen (2000b) found that only ruminal starch digestibility increased, while total tract digestibility remained unchanged when HMC replaced DC. Total tract digestibilities of NDF and ADF were not affected by diet and were relatively high considering the type of diets fed. Knowlton et al. (1998) found that HMC tended to decrease total tract fiber digestibility compared with DC.
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Kinetics of ruminal DM digestibility of the diets are shown in Table 7. The soluble DM fraction, a, was higher for HMC diets than for DC diets and higher for FS than for CS diets. The slowly digestible DM fraction, b, was higher for DC than for HMC diets and increased with increasing forage particle size. The indigestible DM fraction, c, was lower for HMC than for DC diets. Lag time was similar across diets. Ruminal rate of digestion, kd, was unaffected by level of RFC, but increased with increasing forage particle size. Effective ruminal digestibilty (ERD) of diet DM tended to be higher for HMC diets than for DC diets when calculated using the solid outflow rate found in the study. This was in agreement with the higher apparent total tract digestibility of DM observed for HMC diets compared with DC diets.
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. Daily excretion of the two purine derivatives, uric acid and allantoin, was not affected by level of RFC or forage particle size. However, total purine excretion increased when DC was replaced with HMC, as did absorption of microbial purines. Consequently, the calculated intestinal flow of microbial N increased by 49.7 g/d. Increasing the availability of ruminal fermentable energy increased ruminal microbial protein synthesis. The estimated increase in microbial N supply did not result in an increase in milk protein percentage or protein yield. However, microbial N supply increased numerically when forage particle size was increased in HMC diets and decreased numerically when forage particle size was increased in DC diets (P = 0.11). As mentioned earlier, cows fed HMC increased starch intake when forage particle size was increased, whereas cows fed DC decreased starch intake when forage particle size was increased. These numerical differences in microbial N supply supports the tendency for a forage particle size and corn-grain moisture interaction effect on milk protein percentage.
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No effect of forage particle size on microbial N yield was found despite the lower ruminal pH for FS diets compared with CS diets (340.9 g N/d for CS vs. 338.4 g N/d for FS; see Krause et al., 2002 for ruminal pH results). A similar lack of relationship between ruminal pH and efficiency of microbial N production in vivo in both beef and dairy cattle was reported by Beauchemin et al. (2000). No depression in fiber utilization was found at the lower pH values in this study, which could explain the similar microbial N yield for FS and CS diets.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Based on the results from this study, diets low in effective fiber and high in fermentable carbohydrates can be fed to midlactation cows without causing negative effects on diet digestibility and cow productivity.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication September 12, 2001. Accepted for publication February 4, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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