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Department of Dairy and Animal Science The Pennsylvania State University, University Park 16802
Corresponding author: A. J. Heinrichs; e-mail: ajh{at}psu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: rumen • eating behavior • pH • rumination
Abbreviation key: LG = long treatment containing long harvested corn silage, MLG = intermediate long treatment composed of 2 parts LG, 1 part SH (DM basis), MSH = intermediate short treatment composed of 1 part LG, 2 parts SH (DM basis), NDFI = neutral detergent fiber intake, peNDF = physically effective neutral detergent fiber, PSPS = Penn State Particle Separator, Sgm = geometric standard deviation, SH = short treatment containing rechopped corn silage, TC = total chewing activity, TCL = theoretical cut length, Xgm = geometric mean length
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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It is commonly believed that feeding forage of short particle size will decrease salivary buffer secretion, ultimately lowering rumen pH and milk fat percentage (Mertens, 1997). More specifically, the cascade of these events is believed to be precipitated by diets low in physically effective fiber, which is defined as that portion of the diet that stimulates chewing activity and salivary buffer production. Because fiber content of the diet is a primary factor affecting chewing activity, both are believed to be indicators of the diet’s effect on rumen health and function (Yang et al., 2001). Physically effective NDF (peNDF) is an estimate of physically effective fiber and is calculated by multiplying the proportion of feed greater than 1.18 mm in length by total ration NDF (Mertens, 1997). Although a number of studies have recently evaluated the relationships between peNDF, chewing activity, and rumen pH, information regarding corn silage-based diets remains limiting (Yang et al., 2001; Krause et al., 2002).
Mastication reduces particle size of ingested feed and is necessary for the consumption of feed, thus it follows that physical manipulation of feed may affect chewing activities. A number of studies have attempted to determine the effects of reducing forage particle size through grinding (Rodrigue and Allen, 1960), fine chopping (Bal et al., 2000) or pelleting (Woodford and Murphy, 1988) on chewing activities and feeding behavior. Although it is generally understood that both eating and ruminating time increase per unit of DM or NDF consumed when feeds of longer particle size are fed, data are lacking on the effects on consumption of total DM, NDF, or the proportion of particles consumed over time. Supplying the rumen with a uniform supply of nutrients throughout the day should lead to a more constant environment for bacterial and protozoa growth. Alternatively, if ingestion is rapid or selective, large diurnal variation in acid production may result in subacute ruminal acidosis (Van Soest, 1994).
The Penn State Particle Separator (PSPS) is a quick and cost-effective method to estimate forage and TMR particle size and was constructed as an on-farm alternative to the standard S424 of the American Society of Agricultural Engineers (2001) for particle size determination. The compact, manually operated sieving device is constructed of three sieves with pores measuring 19.0, 8.0, and 1.18 mm and a solid bottom pan (Kononoff et al., 2003). Though the apparatus has been widely accepted, few studies have attempted to describe effects of feeding forage of different particle size as measured by the PSPS.
The objective of the present experiment was to determine the effect of feeding corn silage of different particle size on eating behavior, chewing activities, and rumen fermentation in lactating dairy cows. It was hypothesized that rations of shortest particle size would be consumed in greatest amounts but result in lowest chewing activity and rumen pH. Alternatively, rations of longest particle size would be consumed in least amounts and result in greatest amounts of bunk sorting activity.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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and 2.
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Animals were housed in individual stalls and milked at 0700 and 1900 h. Cows were fed at 0800 h for ad libitum consumption to allow for approximately 5.0% refusal. Cows were cared for and maintained according to guidelines stipulated by the Pennsylvania State University Animal Care and Use Committee.
Experimental Measures and Sample Analysis
Samples of feed were collected daily, and composite samples were made for each week of the experiment. Orts from individual cows were collected daily for calculation of DMI and NDFI (NDF intake). Collected samples were immediately frozen (-20°C) and stored for further analysis. Samples were then dried at 55°C in a forced-air oven to determine moisture content and then ground (1-mm screen; Wiley Mill, Arthur A. Thomas Co., Philadelphia, PA). All TMR and forage samples were analyzed in duplicate for moisture; Kjeldahl nitrogen (CP) using a Kjetec 1030 auto analyzer; ether extract; calcium and phosphorus; magnesium; potassium; and ash (AOAC, 1990). Neutral detergent fiber, ADF, and acid detergent lignin were analyzed according to the procedure of Van Soest et al. (1991). Neutral detergent insoluble CP was analyzed on NDF residue. Heat stable alpha-amylase (number A3306; Sigma Chemical Co., St. Louis, MO) was included in the NDF procedure (100 µl/0.5 g of sample). Nonfiber carbohydrate was calculated by difference: 100 - (% NDF + % CP + % Fat + % ASH). Total nonstructural carbohydrate was determined according to the procedure of Smith (1981), but modified to use ferricyanide as a colorimetric indicator. The PSPS was used to measure particle size for both forage and TMR (Kononoff et al., 2003a).
On d 7 and 8 of each period, feed bunk contents of each animal were weighed and sampled at 0, 8, 16, and 24 h after a.m. feeding to determine particle size, DMI, and NDFI. All samples were sieved in duplicate and placed in a forced air oven at 55°C to determine DM of each sieved fraction.
Eating and ruminating activities were measured during d 12 and 13 of each period using Graze Jaw Movement Analysis Software of the IGER Behavior Recorder (Ultra Sound Advice, London, UK) as described by Rutter et al. (1997) and validated with TMR diets by Kononoff et al. (2002). Data were expressed as daily eating, ruminating, or total chewing activity (TC; sum of eating and ruminating time over 24 h). Activities per unit of DMI and NDFI were also calculated by dividing total minutes by the mean of each parameter measured.
On d 13 of each period, ruminal contents were collected from the dorsal, ventral, and caudal area of the rumen at 0.0, 1.5, 3.5, 5.5, 8.5, 11.5, 14.5, 18, 21.5, and 24.5 h after a.m. feeding (0.0 h). Collected digesta was mixed and filtered through four layers of cheesecloth. Rumen liquid pH was immediately determined using a handheld pH electrode (model M90, Corning Inc., Corning, NY). Approximately 15 ml of filtered liquid was then placed into bottles containing 3 ml of 25% metaphosphoric acid and 3 ml of 0.6% 2-ethly butyric acid (internal standard) and stored at -20°C. Samples were later centrifuged three times at 4000 x g for 30 min at 4°C to obtain a clear supernatant, which was analyzed for ammonia using a phenol-hypochlorite assay (Broderick and Kang, 1980) and VFA concentration using gas chromatography (Yang and Varga, 1989).
Milk production was recorded daily on d 13 to 15 of each period; samples were collected and preserved using 2-bromo-2-nitropropane-1,3 diol. Milk samples were analyzed for milk fat and true protein by the Pennsylvania DHIA milk testing laboratory using infrared spectrophotometry (AOAC, 1990; Foss 605B Milk-Scan; Foss Electric, Hillerød, Denmark).
Performance, intake, and chewing activity data were analyzed as a 4 x 4 Latin Square with model effects for cow, period, and treatment. The first order autoregressive covariance structure (AR(1)) and the MIXED procedure of SAS, Version 8.1 (1999) were used to analyze all data. Mean separation for TMR particle size, NDF content by particle size, and peNDF was determined using the PDIFF procedure. Repeated measurements of rumen ammonia, pH, and VFA concentration and ration and refusal particle size were analyzed by including a REPEATED model statement, as well as terms for time and interaction of treatment by time. All means presented are LSMEANS and for ammonia, pH, and VFA are an average over all time points. Linear, quadratic, and cubic orthogonal contrasts were tested using the CONTRAST statement of SAS (1999). Significance for all effects was declared at P 
0.10.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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and 2. Total mixed ration composition and particle size analysis are presented in Tables 3 and 4. Ration particle size reflected the amount of rechopped forage included in the treatments. Rechopping forage reduced the proportion of material on the 19.0-mm screen, but increased proportion on the 8.0- and 1.18-mm screens of the PSPS. Although the proportion of particles <1.18 mm increased with decreasing particle size, differences were not significant. Geometric mean length decreased linearly as the amount of rechopped forage in the TMR decreased. The peNDF value, as measured by the proportion of NDF >1.18 mm, decreased numerically with decreasing particle size, but these differences were small and not significant.
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. As particle size decreased there was a linear increase in DMI and NDFI. The largest difference in total DMI and NDFI was observed between SH and LG treatments (2.3 kg of DM and 0.8 kg of NDF). Feed refusal samples were analyzed for NDF to evaluate sorting in the feed bunk, and concentration of NDF in ort samples decreased with reduced particle size. The smallest numerical differences in NDF concentration between orts and TMR were observed in animals consuming the SH treatments and largest differences in animals consuming LG.
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. Differences in time spent eating or ruminating were not detected between treatments; however, TC exhibited a quadratic effect with highest chewing activities observed for animals consuming diets with shortest and longest particle size. Eating and ruminating times per kilogram of DMI were not affected by corn silage particle size, but TC per kilogram of DMI decreased linearly as particle size was reduced. In contrast, when expressed as minutes per unit of NDFI, ruminating, and TC were reduced linearly with reduced particle size.
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. A quadratic effect of particle size was observed 8 h postfeeding, but DMI increased linearly as particle size was reduced after 16 and 24 h. The concentration of NDF in feed contained in the bunk at 0, 8, 16, and 24 h postfeeding is presented in Figure 1. The concentration of NDF in feed remaining in the bunk at 8, 16, and 24 h decreased linearly with reduced particle size. Although no differences in NDFI were observed 8 h postfeeding, NDFI increased linearly with reduced particle size after 16 and 24 h (Table 7).
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. Reducing corn silage particle size linearly decreased the proportion of particles >19.0 mm and increased the proportion of particles 8.0 to 19.0 mm and 1.18 to 8.0 mm. Differences between treatments increased over time. Total mixed rations containing SH forage contained 12.6% less mass retained on the 19.0-mm sieve when compared to TMR containing LG forage, but this difference increased to over 50% after 24 h. In comparison, TMR containing SH forage contained 7.2% more mass retained on the 8.0 mm sieve, but this difference increased almost 30% after 24 h. Lastly, TMR containing SH forage contained 5.0% more mass retained on the 1.18-mm sieve but contained over 20% more after 24 h. Although the proportion of material <1.18 mm in original TMR was not different across treatments, by 24 h a linear effect of reducing corn silage particle size was observed for this fraction. Comparing original TMR particle size to orts remaining after 24 h, the proportion of DM retained on the 1.18-mm sieve and pan decreased significantly (16.0 and 3.1%) for treatment LR, but decreased only moderately (0.4 and 0.6%) for diet SH.
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). As particle size of the ration decreased, a linear decrease in consumption of particles >19.0 mm was observed at 8, 16, and 24 h postfeeding. In contrast, reducing forage particle size linearly increased consumption of particles 8.0 to 19.0 mm at each time point. A quadratic effect on intake of material 1.18 to 8.0 mm in length was observed at 8 and 16 h postfeeding, but after 24 h intake increased linearly with reduced particle size.
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. Treatment x time interaction effects were not significant for pH, VFA, and ammonia measurements. Rumen pH only tended to have a linear effect with reduced particle size even though total concentration of VFA increased linearly from 89.1 to 93.6 mM/L as particle size was reduced. Reducing particle size resulted in a quadratic effect on concentration of propionate (data not shown) with highest concentration observed in animals consuming diets of shortest particle size (23.0, 20.6, 20.5, 21.7 kd/d for SH, MSH, MLG, and LG, respectively). Molar proportion of acetate as well as acetate-to-propionate ratio exhibited a quadratic effect, with highest values observed on MSH and MLG treatments. Rumen ammonia concentration did not differ between treatments and averaged 8.7 mg/dl.
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. Milk yield was similar across diets and averaged 41.6 kg. However, a quadratic effect was observed for 3.5% FCM with highest production observed on the MSH and MLG treatments. Yield and percent of milk fat responded quadratically to reduced particle size; highest values were observed on MSH and MLG treatments. No effect was observed for yield or percent milk protein, which averaged 1.17 kg/d and 2.8% across all treatments.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Results of this study indicate that DMI is increased by reducing corn silage particle size. These results are similar to some previous studies (Stockdale and Beavis, 1994; Weigand et al., 1992); however, this effect has not been observed in others (Schwab and Shaver, 2002; Kononoff and Heinrichs, 2003b). Reducing particle size also linearly increased total NDFI and linearly decreased NDF content of orts. Although increased consumption of NDF on shorter diets was in part due to higher DMI, reducing particle size also resulted in less sorting and greater consumption of coarse, high fiber particles. Methu et al. (2001) reported similar effects when feeding corn silage of different particle size. Bal et al. (2000) also observed a decrease in sorting tendency when feeding cows kernel processed corn silage. In the present study, NDF content of feed remaining in the bunk after 8, 16, and 24 h was linearly reduced as corn silage particle size in the ration decreased (Figure 1). These results clearly demonstrate that sorting activity occurred during the first 8 h after feeding and continued over the entire 24-h period. Particle size differences between original TMR and feed remaining in the bunk 8, 16, and 24 h postfeeding increased, thus supporting the suggestion that reducing corn silage particle size will reduce sorting behavior. Clearly some sorting behavior was observed in diets of shortest particle size; DM > 19.0 mm increased from 2.8% in the original TMR to 9.5% in orts after 24 h. Highest sorting was observed on the TMR of longest particle size; DM > 19.0 mm increased from 15.5% in the original TMR to 60.2% in orts after 24 h. Diets of longer particle size resulted in greater consumption of small particles that measure 1.18 to 8.0 mm and < 1.18 mm than diets with shorter particle size. Practically, it may be important to consider that even though fine chopping or processing corn silage increases power requirements and harvest costs, reduced sorting behavior should be observed when these forages are fed to lactating dairy cattle.
Although a number of studies have evaluated the effect of corn silage particle size on eating and ruminating behavior, the current study is unique because it evaluates the intake, particle size, and fiber content of feed originally offered to the animals as well as that remaining over 24 h. As expected, animals consuming diets of shortest particle size consumed the fewest particles > 19.0 mm (0.8 kg/d). Even though animals consuming diets containing long corn silage refused the largest amount of longest particles, they consumed the greatest amount of material > 19.0 mm (3.1 kg/d). We originally hypothesized that reducing particle size would reduce TC (minutes per day), but a quadratic effect was observed with diets of intermediate particle size having lowest values. It is plausible that the observed response is a function of two competing effects, total intake and intake of long particles (>19.0 mm). Thus, although animals consuming the SH treatment consumed the least amount of particles >19.0 mm, this diet was consumed in greatest amounts. In comparison, animals consuming the LG treatment consumed the largest amount of particles >19.0 mm but had the lowest DMI. In support of this suggestion, when normalized for intake, reducing corn silage particle size was observed to result in a linear reduction in TC per unit of DM and NDF consumed. This result was similar to that observed in a study that evaluated effects of reducing alfalfa haylage particle size (Kononoff and Heinrichs, 2003a). In that study, it was suggested that increasing the proportion of particles > 19.0 mm may be a primary factor affecting chewing activity of dairy cattle. In the current study, both the proportion and total intake of feed particles > 19.0 mm decreased with reduced particle size, while the proportion and intake of particles 8.0 to 19.0 mm and 8.0 to 1.18 mm increased with reduced particle size. Thus the results of this study would further suggest that although particles >19.0 mm are most likely to be refused by dairy cattle, this particle fraction of the PSPS best reflects TC per unit of DM and NDF intake.
Differences between silage particle size and, presumably, eating behavior also resulted in differences in rumen fermentation. Similar to the effects on intake, total concentration of VFA increased with reduced particle size. These observations are consistent with other studies in which the reduction in particle size of both alfalfa haylage (Kononoff and Heinrichs, 2003a) and barley silage (Soita et al., 2002) resulted in increased DMI and rumen VFA concentration. In addition to these observations, highest molar proportion of acetate and lowest molar proportion of propionate was observed on diets of intermediate particle size and may have been a result of two competing effects namely, starch digestibility and fiber intake. We speculate that a lower proportion of acetate and higher proportion of propionate on diets of shortest particle size may have been due to increased starch digestibility, which may be associated with reducing corn silage particle size (Johnson et al., 2003). In comparison, due to sorting behavior diets of longest particle size resulted in the lowest NDFI, which is known to decrease rumen molar proportion of acetate and increase molar proportion of propionate (Van Soest, 1994).
Although it is commonly believed that increased silage particle size will result in increased chewing activity and salivary buffer production, rumen pH was not different across treatments. This observation is consistent with Yang et al. (2002). Although differences in rumen pH could be expected given the differences in VFA concentration, the observed results may illustrate a lack of effect of increased chewing activity on buffering the rumen environment. These results support the suggestion that increasing chewing activity by feeding forage of longer particle size does not substantially increase daily saliva output to affect rumen pH. The lack of effect may be due to the fact that saliva is also secreted while cows are resting and that rumen fermentability of starch may be more important in understanding the extent of subclinical ruminal acidosis (Yang et al., 2000).
Due to the short experimental periods and small number of animals used, milk production results of the current experiment should be interpreted with caution. Total milk yield, protein percentage, and yield were not affected by particle size reduction. A quadratic response was observed for 3.5% FCM, milk fat percentage, and yield. Highest milk fat levels were paired with highest ratio of acetate-to-propionate on intermediate treatments, and lowest fat levels were similarly paired with lowest acetate to propionate ratio on shortest and longest ration particle size treatments. These observations are consistent with observations of Grant et al. (1990) and Le Liboux and Peyraud (1998) in which diets of reduced particle size and lower consumption of fiber decreased the ratio of acetate to propionate and paired with lower milk fat. Although it is generally accepted that trans fatty acid formation in the rumen directly affects de novo milk fat synthesis (Baumgard et al., 2000), understanding of the impact of feeding diets with finer particle size on ruminal formation of trans fatty acid remains incomplete and should be further investigated.
Current NRC (2001) recommendations state that diets fed to lactating dairy cows must contain a minimum of 25% NDF but do not clearly specify guidelines for physical form. Because it is widely understood that forage particle size can influence rumen pH and fermentation, presumably by influencing chewing activity, a number of studies have been designed to investigate the effect of corn silage particle size on chewing activities and rumen pH of lactating dairy cattle. In the current study reducing corn silage particle size did not affect total time spent eating, which was similar to other reports (Bal et al., 2000; Schwab et al., 2002). However, a quadratic effect was observed in which longest time spent chewing was observed on the shortest and longest rations. When time spent chewing was normalized per unit of NDFI, total time spent chewing decreased as particle size was reduced. The results of this study are similar to those reported by Krause et al. (2002), in which physical effectiveness of diet NDF decreased with decreasing forage particle size. Given this observation, one may conclude that reducing corn silage particle size reduced effectiveness and should result in reduced chewing activity. However, in the current study, total time spent chewing was elevated, presumably due to the higher intake of fiber, which is also known to stimulate chewing activity. Practically, it is important to understand that corn silage of shorter particle size actually may be more effective in stimulating chewing activity based on the amount of NDF consumed and that reducing particle size may also minimize sorting behavior.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication January 16, 2003. Accepted for publication March 30, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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