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1 Agriculture and Agri-Food Canada, Research Centre, Lethbridge, AB, Canada T1J 4B1
2 Department of Animal Science, Sahmyook College, Seoul, Korea
Corresponding author: Karen A. Beauchemin; e-mail: beauchemin{at}agr.gc.ca.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: mechanically processed forage • barley silage • digestibility • milk production
Abbreviation key: ERD = effective ruminal degradability, MPBS = mechanically processed barley silage, MPBS-TMR = TMR containing mechanically processed barley silage, pef = physical effectiveness factor, RBS = regular barley silage, RBS-TMR = TMR containing regular barley silage
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Although a wide variety of forage crops are successfully used in dairy production systems, barley silage is the main forage component for dairy cow diets in western Canada. Barley silage is typically harvested in an early to mid-dough stage of maturity, allowing the crop to be ensiled at about 30 to 40% DM (Baron et al., 2000). With advancing maturity, digestibility of the forage crop declines due to maturation of the hull, which provides a physical barrier to digestion of the whole kernel. Barley silage, like most small cereal silages, has a high proportion of rumen-fermentable carbohydrates such as starch. Mechanical processing of the barley crop before ensiling may disrupt the physical barriers and increase the surface area for digestion by microbial or host enzymes. Therefore, we hypothesized that an improvement in digestibility and milk production could occur if cows were fed mechanically processed barley silage (MPBS). The objective of this study was to determine the effects of feeding MPBS as the main forage source on digestibility and lactational performance of dairy cows.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Barley (semidwarf cultivar, Duke) was grown under irrigation near Lethbridge, AB, Canada. The crop received recommended amounts of fertilizer based on soil tests. The whole plant was harvested in a mid-dough stage of maturity in August 2002. Half of the material was left unprocessed, and the other half was mechanically processed during harvesting. A theoretical chop length of 0.95 cm was used in both cases. The mechanical processing was done using an automatic roller mill (Jaguar 900 model, Claas Ltd., Hansewinkel, Germany) designed for processing of corn plants, which was attached to the forage harvester. The rollers were adjusted to the smallest possible setting with 1-mm clearance. Both silages were stored in separate silo bags.
The experiment was conducted as a completely randomized design with covariate and treatment periods for a total of 56 d. For the first 2 wk, all cows received a TMR containing regular barley silage (RBS). This 2-wk phase was used as the covariate period, thus milk yield, milk composition, and DMI were determined. At the end of the covariate period, cows were divided into 2 groups, each with 12 animals, similar in milk yield (34.7 ± 8.0 and 35.1 ± 8.0 kg/d) and DMI (21.1 ± 2.2 and 22.2 ± 3.6 kg/d) and allocated to 1 of 2 treatment diets. Equal numbers of multiparous and primiparous cows were assigned to each treatment diet. The 2 treatment diets were: 1) TMR containing RBS (RBS-TMR), and 2) TMR containing (MPBS) (MPBS-TMR) (Tables 1
and 2). The RBS-TMR was the same diet that was fed to the cows during the covariate period. Diets were formulated to sustain 35 kg/d of milk production with 3.5% of fat and 3.3% of CP using the Cornell-Penn-Miner System (CPM Dairy, Version 2.0), a computer program based on the Cornell Net Carbohydrate and Protein System principles (Sniffen et al., 1992).
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Cows were cared for according to the Canadian Council on Animal Care guidelines (Ottawa, ON, Canada). Cows were housed in individual tie stalls fitted with rubber mattresses and bedded with wood shavings, and were milked twice daily at 0630 and 1630 h. They were turned outside in a dry-lot for exercise for at least 1 h daily in the morning after being milked.
Barley silages, chopped alfalfa hay, and concentrates were sampled weekly to determine DM content. Diets were adjusted weekly to account for changes in DM content. Samples of the barley silages and TMR fed were collected daily and composited weekly. Part of the weekly composited samples was saved for determining particle size distributions, and the rest were dried at 55°C, ground to pass a 1-mm screen (standard model 4; Arthur H. Thomas Co., Philadelphia, PA), and stored for subsequent analyses.
Particle size distributions of barley silages and TMR were measured by sieving using the Penn State Particle Separator (Lammers et al., 1996). The sum of the DM retained on the top and middle sieves was assumed equal to the physical effectiveness factor (pef) of feeds.
Milk was sampled on 2 consecutive days during the a.m. and p.m. milkings in each week throughout the experiment. Milk samples were preserved with potassium dichromate and stored at 4°C until sent to the Central Alberta Milk Testing Laboratory (Edmonton, AB, Canada). Milk was analyzed for fat, protein, and lactose (AOAC, 1990) using an infrared analyzer (Milk-O-Scan 605; Foss Electric, Hillerød, Denmark). Milk composition was corrected for differences in milk volume between the a.m. and p.m. milkings.
Mean BW of cows was calculated by weighing the cows weekly during the experiment at approximately 1300 h. Body condition score, ranging from 1 (thin) to 5 (obese), was recorded at the beginning and end of the covariate and the treatment periods according to the method of Wildman et al. (1982).
Digestibility
Apparent digestibilities of DM and nutrients in the total tract were determined using chromium as an external marker for the last 3 wk during the treatment period. Approximately 2.92 g of Cr2O3 (equivalent to 2 g of Cr) diluted using ground barley was top-dressed daily onto the feed at the p.m. feeding for 19 consecutive days. To ensure the cows consumed the entire marker each day, it was allocated as follows. Immediately after the orts were removed from the feeder, a small amount of TMR (approximately 1 kg) and the top-dressed marker were placed in front of the cows. Once the marker was consumed in its entirety, the additional TMR was allocated to the cows. After 7 d of dosing, fecal samples (approximately 100 g) were taken from the rectum of each cow once or twice daily at various times for the last 12 d. The fecal samples were pooled by individual cow, dried at 55°C, and ground to pass a 1-mm screen (Intermediate mill, Arthur H. Thomas Co.) for chemical analysis. Chromium was assumed to be completely indigestible and the digestibility of DM was calculated using the following equation: DM digestibility = 1 – [Cr fed (mg/d)/DMI (kg/d)]/Cr in feces (mg/kg of DM), where DMI was the average of DM consumed on the days that fecal samples were collected. Digestibility of nutrients was calculated using the same approach.
In Situ Measurements
Two multiparous cows were used for in situ measurements of the ruminal DM degradation kinetics of RBS, MPBS, RBS-TMR, and MPBS-TMR. The feeds were processed before incubation, thus the purpose of the in situ measurements was to verify whether the rate and extent of digestion was the same for both treatments once the physical differences between treatments were removed. Fresh barley silages and TMR were chopped for 5 s using a Knifetec 1095 sample mill (Foss Tecator, Höganäs, Sweden). Five grams of samples (as-fed) were weighed into bags (10 x 20 cm) made of monofilament PeCAP polyester screen (pore size, 51 ± 2 µm; B. & S. H. Thompson, Ville Mont-Royal, QC, Canada). Bags were heat-sealed and placed in large (20 x 30 cm) mesh retaining sacs with 3- x 5-mm pores that permitted ruminal fluid to percolate freely. Triplicate nylon bags were placed in the rumen for 0, 3, 6, 9, 12, 24, 48, 72, and 96 h. Upon removal, bags were washed under running tap water until the effluent was clear and then were dried at 55°C for 48 h. Bags and contents were weighed for the calculation of DM disappearance, and residues were ground to pass a 1-mm screen and stored for the analysis of fiber.
Chemical Analysis
Feed DM was determined by oven drying at 55°C for 48 h. Analytical DM content of the samples was determined by drying at 135°C for 3 h (AOAC, 1990). Organic matter content was calculated as the difference between DM and ash contents, with ash content determined by combustion at 550°C overnight. For the measurement of CP (N x 6.25), samples were ground using a ball mill (Mixer Mill MM2000; Retsch, Haan, Germany) to a fine powder. Nitrogen was quantified by flash combustion with gas chromatography and thermal conductivity detection (Carlo Erba Instruments, Milan, Italy). Chromium was determined by inductively coupled plasma emission spectrometry (SpectoCirosCCD, Specto Analytical Instruments, GmbH & Co., Kleve, Germany) after dry ashing at 550°C for 5 h and extraction of Cr according to the method of Williams et al. (1962), but dilution after digestion of sample was done with only water instead of adding calcium chloride. Neutral detergent fiber and ADF were determined in the ANKOM200 Fiber Analyzer (ANKOM Technology Corp., Fairport, NY) using heat-stable 
-amylase and sodium sulfite. Starch content of feed and feces was determined by a 2-step enzymatic method (Rode et al., 1999) with a microtiter plate reader (Dynatech Laboratories, Chantilly, VA) to read glucose release colorimetrically at 490 nm.
Statistical Analysis
Kinetics of DM degradation in situ was estimated by the nonlinear regression procedure of SAS (SAS Institute, 2001). For each cow and type of feed, the following model (Ørskov and McDonald, 1970) was fitted to the percentage of DM degradation:
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where a = soluble fraction (percentage), b = slowly degradable fraction (percentage), c = rate of degradation (per hour), L = lag time (hours), and t = time of incubation (hours).
Effective ruminal degradability (ERD) of DM was estimated using the following equation (Ørskov and McDonald, 1970):
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where k = rate of particulate passage (assumed to be 2.8 and 4.0%/h for barley silage and TMR, respectively).
Daily intake, milk yield, and milk composition values were reduced to weekly means before data analysis. Data for DMI, BW, and milk yield obtained during the covariate period were used as covariates for the corresponding measurements during the treatment period. ANOVA was conducted using the Proc Mixed procedure of SAS (SAS Institute, 2001) for a completely randomized design with repeated measures. The model included the effects of treatment, week, and the interaction between treatment and week, with the random variable being the cow within treatment. Simple, autoregressive one, and compound symmetry covariance structures were used in the analysis depending on low values for the Akaike’s information criteria and Schwartz’s Bayesian criterion. Least square means are reported throughout. Treatment effects were declared significant at P < 0.05 and trends were discussed at P < 0.15 unless otherwise noted.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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), suggesting that processing did not affect nutrient composition of barley silage. Barley silage typically contains 48 to 60% of NDF, 29 to 37% of ADF, and 12 to 14% of starch in the DM (Khorasani et al., 1993; Yang et al., 1997; Mustafa et al., 2000). The NDF (38%) and ADF (20%) concentrations of the barley silages used in this study were considerably lower, and starch concentration of the barley silage (24%) was higher than those reported in the literature for silages of similar maturity. In cereal silage, grain content may vary substantially. Baron et al. (1992) reported that barley grain content could range from 30 to 43% and from 37 to 52% at 30 and 35% of silage DM, respectively, implying that the grain content of barley may vary depending upon different stages of plant development and moisture contents at harvest. In addition, a semidwarf type of barley such as the Duke cultivar used in this study typically has lower fiber levels, higher grain content, and higher in vitro digestible OM than other barley types (Baron et al., 2000). The low fiber concentrations of the barley silages used in this study likely resulted from a dilution effect on whole plant fiber content caused by increased grain content and resultant increased starch concentration.
Analysis of particle size indicated that MPBS had a slightly lower proportion retained on the top screen compared with RBS (Table 3). Overall, mechanical processing of barley silage caused minimal changes in particle length, and the pef of both silages were similar. The same trend was observed for particle distributions of the diets.
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). The lack of effect of feeding MPBS on DMI was consistent throughout the experiment (Figure 1). Although there were no differences in intakes of starch, NDF, or N between treatments (Table 4), cows fed the MPBS-TMR had lower ADF intake than those fed the RBS-TMR, due to lower concentration of ADF in the MPBS-TMR than the RBS-TMR (14.2 vs. 15.9%, Table 1). The ratio of starch to NDF intake increased in cows fed the MPBS-TMR compared with cows fed the RBS-TMR due to numerically higher intake of starch and numerically lower intake of NDF in cows fed the MPBS-TMR.
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). Similarly, digestibilities of OM, ADF, NDF, and N did not differ between treatments. Although feeding the MPBS-TMR tended (P = 0.15) to increase starch digestibility, the increase was not as great as expected based on improvements observed for corn silage. For example, Weiss and Wyatt (2000) reported that processing corn silage increased digestibility of total dietary starch by 4.4 percentage units (93.2 vs. 97.6% for unprocessed vs. processed corn silage, respectively). With advancing maturity, barley grain kernels harden and develop complex matrices that can reduce starch digestion. It appears that mechanical processing of barley silage did not sufficiently disrupt kernel integrity to increase starch digestion. Digestibility of starch observed in the present study is in general agreement with previous reports (Yang et al., 1997, 2000, and 2001). In contrast, digestibilities of NDF and ADF in our study were considerably lower than those published for studies in which barley silage was fed as the main forage source. The lower fiber digestion may have resulted from a reduction in the numbers of fibrolytic bacteria caused by a shift from fibrous to nonfibrous carbohydrate fermentation. The high grain content of the barley crop together with the relatively high concentrate proportion in the diet may have contributed to this microbial shift. Mertens et al. (2003) suggested a significant negative effect of the ratio of starch to NDF intake on fiber digestibility, which is not supported by our results. However, the low NDF digestibility in this study may have minimized the negative effects of high starch to NDF intake ratio on NDF digestibility.
Milk production averaged 33.9 kg/d between treatments (Table 5), and was not affected by feeding the MPBS-TMR. Furthermore, neither milk composition nor milk component yield was influenced by feeding the MPBS-TMR. However, milk fat concentration was numerically (P = 0.32) higher for cows consuming the MPBS-TMR, which resulted in a numerical (P = 0.51) increase in FCM production. Differences in milk fat and FCM were only trends and not statistically significant because of the large variation associated with means. This variation indicates that not all cows responded in the same manner to the effects of mechanical processing of barely silage. It is possible that this variability would have been reduced had only multiparous cows been used in the study.
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). In spite of low fiber digestibility, the dairy efficiency in this study fell within the recommended range (Hutjens, 2002). The efficiency of N use, assessed as kilograms of milk N per kilogram of N intake, averaged 0.27 across treatments, and was not affected by feeding MPBS-TMR. Neither BW nor BCS differed between the treatments.
Determination of the effect of mechanical processing on in situ degradability requires that samples should not be dried or ground before in situ measurement to maintain potential treatment effects (Johnson et al., 1999). Because we ground the samples for in situ measurement of degradability, we did not expect any effect of mechanical processing of barley silage on in situ degradability. However, mechanical processing increased the soluble fraction, but decreased ruminal degradation rate of barley silage (Table 6). In contrast, ERD of DM was similar for RBS and MPBS, indicating that when the silages were ground to remove the effects of mechanical processing, the potential digestion was similar for both silages. In contrast to previous findings (Yang et al., 2000; 2001), the soluble fractions of the barley silages were considerably higher and the slowly degradable fractions lower than those reported in the literature, resulting in higher ERD. This result further confirms that the barley silages used in this study had higher grain content with concomitant reduction of fiber content, resulting in the greater ERD and lower fiber digestibility.
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Higher disappearances of NDF and ADF for RBS than for MPBS were observed at 48 h of incubation in situ (Table 7), but they did not differ when incubated as a TMR for 48 h. Similarly, Johnson et al. (2003) reported (for silages with short chop length of 0.95 cm), that NDF disappearance was greater for unprocessed corn silage than for processed corn silage measured at 24 and 48 h of incubation in the rumen. Because samples for in situ measurement were ground, it is not clear whether mechanical processing damaged the barley stem tissue sufficiently and increased fiber degradation. Given the similar in vivo fiber digestibility between treatments, however, it appears that mechanical processing did not sufficiently damage the fiber fraction of barley silage. We speculate that the lack of response to processing was likely caused by the conditions of the mechanical processing. In order for mechanical processing to be effective for barley silage, processing conditions should be modified from the ones used for corn silage. Alternatively, mechanical processing may be a more effective means of improving digestibility for barley silage harvested in a more advanced stage of maturity than the mid-dough stage used in this study.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication July 15, 2004. Accepted for publication September 9, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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