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J. Dairy Sci. 2007. 90:2355-2366. doi:10.3168/jds.2006-609
© 2007 American Dairy Science Association ®
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Effects of Chop Length of Alfalfa and Corn Silage on Milk Production and Rumen Fermentation of Dairy Cows

S. K. Bhandari, K. H. Ominski, K. M. Wittenberg and J. C. Plaizier1

Department of Animal Science University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

1 Corresponding author: plaizier{at}ms.umanitoba.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Effects of chop length (shorter = 10 mm or longer = 19 mm) of alfalfa silage and corn silage were determined in 16 midlactation Holstein cows using a 4 x 4 Latin square design with a 2 x 2 arrangement of treatments. Experimental periods were 21 d long and consisted of 14 d of adaptation and 7 d of sampling. Cows received total mixed ration containing (dry matter basis) 44.0% barley grain-based energy supplement, 12.6% protein supplement, and 21.7% longer chop or shorter chop alfalfa silage and 21.7% longer chop or shorter chop corn silage. Reducing the chop length of alfalfa silage and corn silage reduced the average geometric particle length from 14.4 to 11.0 mm and from 14.2 to 10.4 mm, respectively. Reducing the chop length of both silages reduced the proportion of the diets retained by the 8-and 19-mm screen of the Penn State Particle Separator from 55.0 to 46.0% of dry matter. Reducing the alfalfa chop length increased total rumen volatile fatty acids at 4 to 5 h after feeding but did not affect rumen pH at 4 to 5 h after feeding, feed intake, and milk production. Reducing the corn silage chop length increased dry matter intake from 22.3 to 23.2 kg/d, increased rumen pH at 4 to 5 h after feeding from 6.12 to 6.20, but did not alter rumen volatile fatty acids at 4 to 5 h after feeding or milk production. Daily milk yield, milk fat percentage, and milk protein percentage averaged 38.2 kg/d, 2.62%, and 3.29%, respectively, across all diets. The low milk fat percentages suggest that all diets induced subacute ruminal acidosis (SARA), whereas the rumen pH did not indicate SARA. This discrepancy could be due to a difference in the time of rumen pH measurement and the time of the lowest rumen pH. Hence, the pH data need to be interpreted with caution. Diets could have induced SARA, because for all experimental diets the content of forage neutral detergent fiber was lower than recommended for barley grain-based diets.

Key Words: forage chop length • rumen fermentation • feed intake • milk production


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
On many dairy farms forages used for ensiling are chopped short, because long forage chop lengths can result in poor silage fermentation due to difficulties in packing and maintaining anaerobic conditions (Johnson et al., 2003). However, short forage particle lengths can result in a reduction in chewing, saliva production, and rumen buffering, which increase the risk of subacute ruminal acidosis (SARA; Mertens, 1997; Stone, 2004).

The particle size distribution of forages and TMR can be determined with the Penn State Particle Separator (PSPS; Kononoff et al., 2003a). Recommendations for the PSPS distributions of silages and TMR were given by Heinrichs and Kononoff (2002). A recent survey on Manitoba dairy farms showed that alfalfa silage and corn silage were the most commonly used forages and that barley grain was the most common concentrate (Plaizier et al., 2004). This survey also showed that the particle size distributions of silage varied widely among farms. Only 35% of alfalfa silages and 30% of corn silages had PSPS distributions that conformed to the recommendations of Heinrichs and Kononoff (2002), and 35% of alfalfa silages and 30% of corn silages had average particle lengths that were lower than these recommendations (Plaizier et al., 2004). However, the impact of deviating from the recommendations of Heinrichs and Kononoff (2002) were not obvious (Plaizier et al., 2004).

Results from earlier studies on the effects of forage particle length in dairy cows are inconclusive. A reduction in forage particle length has been shown to decrease rumen pH (Krause et al., 2002b; Beauchemin et al., 2003) or to have no effect on rumen pH (Yang et al., 2001; Kononoff and Heinrichs, 2003a; Kononoff et al., 2003b). Also, reducing forage particle length has increased DMI (Kononoff and Heinrichs, 2003b; Kononoff et al., 2003a), not affected DMI (Yang et al., 2001; Beauchemin et al., 2003; Kononoff and Heinrichs, 2003a), or decreased DMI (Krause and Combs, 2003). Milk fat was reduced by a reduction in forage particle length in some studies (Kononoff and Heinrichs, 2003a; Krause and Combs, 2003), but was not affected by forage particle length in other studies (Krause et al., 2002a; Beauchemin et al., 2003; Kononoff and Heinrichs, 2003b). The results of these earlier studies are difficult to compare because many different forage sources, concentrate sources (i.e., corn grain vs. barley grain), particle length ranges, forage to concentrate ratios, and forage NDF contents were used. Also, results of earlier studies are not fully representative for the effects of variation in forage particle length on commercial dairy farms because this variation was obtained by rechopping and grinding forages, which are not common practices on these farms.

The large variation in forage particle size observed by Plaizier et al. (2004) highlights the need for determining the impact of deviating from the current guidelines for the particle size distribution and forages and diets fed to dairy cows. This impact cannot be conclusively determined from earlier studies. The objectives of our study were therefore to determine the effects of chop length of alfalfa silage and corn silage on feed intake, rumen fermentation, milk production, and blood parameters in lactating dairy cows fed diets based on alfalfa silage, corn silage, and barley grain.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Experimental Procedures
Sixteen multiparous lactating Holstein cows, housed in a tie-stall barn at the Glenlea Research Station, University of Manitoba, were used in a 4 x 4 Latin square design with a 2 x 2 arrangement of treatments. Each experimental period consisted of a 14-d adaptation period, followed by a 7-d collection period. Animals were cared for in accordance with the Canadian Council on Animal Care guidelines (CCAC, 1984). Upon commencement of the experiment, cows averaged 78 ± 37 DIM (mean ± SD), had an average BCS of 3.55 ± 0.49 on a 5-point scale (Edmonson et al., 1989), and had an average BW of 756 ± 37 kg.

Second-cut alfalfa (Pickseed Custom Forage Mix, Pickseed Canada Inc., Winnipeg, Manitoba, Canada) was harvested in the late bud stage. Corn (Pioneer 39T71, Pioneer Hi-Bred International Inc., Johnston, IA) was harvested at the second/third milk line stage of maturity. Corn and alfalfa were chopped at 10 mm (shorter chop) or 19 mm (longer chop) using a New Holland Forage Harvester, model 790 (New Holland Inc., New Holland, PA). The corn silage was not kernel processed. Each forage was cut from the same field on the same day. These silages were ensiled and stored in plastic covered piles of approximately 30 tons without additives or inoculants for 3 mo before the beginning of the experiment. Cows were assigned to 1 of 4 diets during each of the 4 experimental periods (Table 1Go). Diets were fed as TMR and contained (DM basis) 44.0% barley grain-based energy supplement (containing 42% pellets) and 12.6% protein supplement (containing 58% pellets; Tables 1Go and 2Go), 21.7% of longer or shorter chop of alfalfa, and 21.7% of longer or shorter chop of corn silage. The concentrate to forage ratio was 57:43 on a DM basis.


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  		Table 1. Ingredients and nutrient composition of experimental diets containing shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages (±SD)
 

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  		Table 2. Ingredient composition of energy supplement and protein supplement (% of DM)
 
Diets were mixed using a Data Ranger mixer (American Calan, Northwood, NH) with a Weigh Tronix weigh head (model 1000, American Calan). The TMR were fed once daily for ad libitum consumption allowing 5 to 10% orts. Cows had unlimited access to water.

DMI and Feed Analyses
During the collection periods, the amounts of TMR offered and refused were recorded daily for each cow. Diet samples and individual ort samples were collected daily. Forages were sampled once per collection period and pooled across collection periods. The DM content of pooled diets, forages, and ort samples were determined by drying at 60°C for 48 h. Dried feed samples were ground using a Wiley mill through a 1-mm screen (Thomas-Wiley, Philadelphia, PA) and stored at –20°C until analyzed. Prior to subsequent analysis, analytical DM were determined (method 934.01; AOAC, 1990). All feed samples were analyzed for CP using the CuSO4/TiO2 mixed catalyst Kjeldahl procedure (method 988.05; AOAC, 1990), NDF (Van Soest et al., 1991) using {alpha}-amylase (Sigma No. A3306, Sigma Chemical Co., St. Louis, MO), and sodium sulfite and corrected for ash concentration adapted for Ankom 200 Fiber Analyzer (Ankom Technology, Fairport, NY), ADF (method 973.18; AOAC, 1990), ether extract (method 920.39; AOAC, 1990), and ash (method 942.05; AOAC, 1990). Calcium, P, K, Mg, and Na were measured by inductively coupled plasma emission spectroscopy (method 968.08; AOAC, 1990) using an Atom Scan 25 Plasma Spectrometer (Thermo Jarrell Ash Corp., Grand Junction, CO) after acid digestion.

Particle size distribution was determined for all TMR, pooled refusals, and forage samples using the PSPS (Kononoff et al., 2003a). Approximately 150 g of wet sample was placed on the top screen of the PSPS. The PSPS was shaken 40 times (5 times in each direction, twice; Kononoff et al., 2003a). The contents of each fraction were weighed and analyzed for DM and NDF as described earlier. Physically effective fiber was determined using 3 techniques: 1) as the proportion of the dietary NDF retained by the PSPS sieves (peNDFNDF); 2) as the proportion of dietary DM retained by the PSPS sieves multiplied by the dietary NDF content (peNDFPS); 3) using tabular values (peNDFM) for the physical effectiveness of feeds as recommended by Mertens (1997). The geometric mean (Xgm) and standard deviation (Sgm) of feed samples were obtained by fitting PSPS distribution with a lognormal equation, as described by Kononoff et al. (2003a).

Particle size distributions of TMR were also measured by dry sieving using a vertical oscillating test sieve shaker (EFL 1 KII, Endecotts Ltd., London, UK) equipped with a stack of 6 brass sieves and a bottom pan with a 200-mm diameter (ASTM E11, Endecotts Ltd.) arranged in descending mesh size. Sieve mesh sizes were 19, 9.5, 6.3, 4.0, 1.18, and 0.6 mm. Approximately 200 g was placed on the top screen, and the stack of sieves was shaken until the distribution of materials did not change (approximately 15 min). The peNDF>1.18 was determined as the proportion of DM retained on and above the 1.18 mm screen multiplied by dietary NDF.

Milk Yield and Composition Analysis
Cows were milked twice daily in their stalls and milk production was measured using Tru Test regulation meters (Westfalia Surge, Mississauga, Ontario, Canada) that are accurate within 5% or 0.3 L. Milk samples were collected from 4 consecutive milkings in 50-mL vials in each collection period and preserved with 2-bromo-2-nitropropane-1,3 diol. Milk samples were stored at 4°C until analyzed for fat and protein at the laboratory of the Dairy Farmers of Manitoba (Winnipeg, Manitoba, Canada) by near-infrared analysis using the Milk-O-Scan 303AB (Foss Electric, Hillerød, Denmark). This infrared analysis was calibrated by the Babcock method for milk fat analysis (AOAC 989.04, 1990) and the Kjeldahl method for nitrogen/protein nitrogen analysis in milk (AOAC 991.22, 1990).

Rumen pH Measurement and Blood Sample Collection
Rumen fluid and peripheral blood were sampled between 4 and 5 h after feeding on d 1 and 3 during each 7-d collection period. The rumen fluid collection was conducted only once daily because more frequent sampling was considered to jeopardize the health of the cows. Rumen fluid was aspirated using an oral probe (Duffield et al., 2004). The first 200 mL of collected rumen fluid was discarded, and the subsequent 50 mL of rumen fluid was kept for subsequent analysis and processing. Rumen fluid pH was measured using an Accumet Basic 15 pH meter. Rumen fluid samples were centrifuged at 1,900 x g for 10 min and the supernatant was frozen immediately and stored at –20°C until further analysis. Blood samples were collected by coccygeal venipuncture in heparinized 10-mL vacutainers, and centrifuged at 1,900 x g for 10 min. Subsequently, the plasma was aspirated and stored at –20°C until further analysis.

VFA, Ammonia, and Blood Plasma Analysis
Frozen rumen fluid samples were thawed at room temperature and 1 mL of 25% meta-phosphoric acid solution was added to 5 mL of rumen fluid. The tubes were vortexed and placed in a –20°C freezer for 17 h. Thawed samples were centrifuged for 10 min at 1,900 x g. Approximately 2 mL of supernatant were decanted and placed into the autosampler device (model 8100, Varian, Walnut Creek, CA) for analysis. Concentrations of VFA were determined by gas chromatography (model 3400 Star, Varian) using a 1.83-m packed glass column (model 2-1721, Supelco, Oakville, Ontario, Canada; Erwin et al., 1961). The injector and detector temperatures were set at 170 and 195°C, respectively, with initial and final column temperatures set at 120 and 165°C, respectively. The run time was 4 min followed by a 2-min thermal stabilization period.

Ammonia nitrogen concentration of rumen fluid samples was determined using the method described by Novozamsky et al. (1974). Absorbance was read at 630 nm on a Pharmacia Biotech Ultraspec 2000 UV/visible spectrophotometer (Biochrom, Cambridge, UK).

Blood plasma was analyzed for glucose, urea, and lactate using a Nova Stat Profile M blood gas and electrolyte analyzer (Nova Biomedical Corporation, Waltham, MA). Glucose and urea were analyzed according to Gourmelin et al. (1990), and lactate was analyzed as described by Kruse (1995).

Silage Fermentation Profile
Alfalfa silage and corn silage were analyzed for VFA, lactate, and ethanol using HPLC with the method described by Siegfried et al. (1984). This method uses 0.015 N H2SO4 and 0.25 mM EDTA (free acids) as the mobile phase. The temperature of the column (Bio-Rad Aminex ion exclusion HPX-87H [300 x 7.8 mm], Bio-Rad, Hercules, CA) was kept at 42°C, and the flow was maintained at 0.6 mL/min.

Forage pH was measured using an Orion 710 pH/ISE meter calibrated with pH 4.0, 7.0, and pH 10.0 buffer solutions. Pooled silage samples were mixed with distilled water in a plastic vial and kept for 10 min before measurement (Buchanan-Smith and Yao, 1981). Ammonia nitrogen concentration of silage samples was determined according to AOAC method 920.03 (AOAC, 1990).

Statistical Analysis
The ANOVA for weekly averages of rumen fluid, blood plasma variables, milk, and intakes was conducted using the SAS MIXED procedure (SAS Institute, 1990). The effects of alfalfa chop length and corn chop length were considered fixed. The effects of square, cow within square, and period were considered random. For dependent variables that had repeated measurements (i.e., rumen measurements), the repeated measurement option within the SAS MIXED procedure (SAS Institute, 1990) was used. Statistical significance was set at a P ≤ 0.05. Differences among treatment means were tested for significance using Tukey’s multiple range test (SAS Institute, 1990). Reported standard errors are those used for the comparison of treatment means.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 ACKNOWLEDGEMENTS
 REFERENCES
 
Chemical and Physical Compositions of Forages and Diets
The nutrient compositions of all diets were similar (Table 1Go). The diets used in this experiment exceeded the NRC (2001) minimum recommended NDF content for barley grain-based diets (34% DM; Beauchemin, 1991). Forage NDF met the minimum dietary content recommended by NRC (2001; 19% of DM) but was lower than the minimum recommended content for barley grain-based diets recommended by Yang and Beauchemin (2005; 22 to 23% of DM). Chop length did not affect chemical composition of the silages, with the exception that the DM content of the longer chop alfalfa silage was 5.3 percentage points higher than that of the shorter chop alfalfa silage (Table 3Go). Kung and Shaver (2000) recommended target levels for the pH, ammonia-N content, and lactate content of legume silage of 4.0 to 4.3, <12% of CP, and 4.0 to 6.0% of DM, respectively. These authors recommended target levels for the pH, ammonia-N content, and lactate content of corn silage of 4.0 to 4.5, <8% of CP, and 5.0 to 10.0% of DM, respectively. Hence, the pH and ammonia-N content of all silages were higher and the lactate content of silages in our study (Table 1Go) were lower than the recommended concentrations in silages. Kononoff and Heinrichs (2003b) reported a pH, ammonia-N content, and lactate content of alfalfa silage of 4.7, 12.7% of CP, and 5.9% of DM, respectively. Kononoff and Heinrichs (2003a) reported a pH, ammonia-N content, and lactate content of corn silage of 3.7, 5.4% of CP, and 5.1% of DM, respectively. Hence, the pH and ammonia-N contents of silages in our study were higher and lactate contents of all silages were lower than those observed in these earlier studies. Reducing the chop length of alfalfa silage and corn silage reduced their proportions retained by the 8- and 19-mm screens of the PSPS from 84.4 to 75.0% of DM and from 89.0 to 75.9% of DM, respectively (Table 4Go). Reducing the chop length of alfalfa silage and corn silage also reduced the average particle length from 14.4 to 11.0 mm and from 14.2 to 10.4 mm, respectively. Reducing the chop length of both silages reduced the proportion of the diet retained by the 8- and 19-mm PSPS screens from 61.6 to 51.0%, the average geometric length of the dietary feed particles from 8.5 to 6.7 mm, and the dietary peNDFPS from 19.1 to 16.4% of DM (Table 5Go) but did not decrease peNDF>1.18 mm. The decrease in peNDFNDF resulting from these reductions in chop length was similar to the reduction in peNDFPS.


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  		Table 3. Nutrient composition and silage fermentation profile of the silages included in the experimental diets (±SD)
 

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  		Table 4. Penn State particle size (PSPS) analysis, geometric mean (Xgm), and SD (Sgm) of shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages (±SD)
 

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  		Table 5. Penn State Particle Separator (PSPS) analysis, physical effective fiber content, geometric mean (Xgm), and SD (Sgm) of experimental diets containing shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages (±SD)
 
A comparison of the PSPS distributions of the diets and their respective orts showed that diets contained fewer particles that were retained by the 19-mm PSPS screen than their orts (Figure 1Go). This showed that cows selected against large feed particles. This finding agrees with Leonardi and Armentano (2003), who reported similar selective eating behavior of dairy cows. Although orts were only between 8.5 and 9.9% of the feed provided, this selective behavior reduced the intake of feed particles longer than 19 mm and the intake of physically effective fiber. Hence, the physically effective fiber content of the diet was higher than that of the ingested feed.


Figure 1
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  		Figure 1. Penn State Particle Size distribution of diets and orts (as-fed basis) containing shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages. ALCL = alfalfa longer chop and corn longer chop; ALCS = alfalfa longer chop and corn shorter chop; ASCL = alfalfa shorter chop and corn longer chop; ASCS = alfalfa shorter chop and corn shorter chop; AF = as-fed basis.

 
DMI
Alfalfa chop length did not affect DMI (Table 6Go). However, decreasing the chop length of corn silage increased DMI. This increase in DMI was higher in the diets containing longer chop alfalfa than in the diet containing shorter chop alfalfa. The absence of an increase in DMI due to a reduction in the chop length of alfalfa silage agrees with Krause et al. (2002a). However, Kononoff and Heinrichs (2003b) found that a reduction in alfalfa silage particle length increased DMI. Also, in contrast with our results, Krause and Combs (2003) observed a reduction in DMI when particle length of alfalfa silage decreased. The increase in DMI due to a reduction in the chop length of corn silage agrees with Kononoff et al. (2003b) but contrasts with Kononoff and Heinrichs (2003a) and Beauchemin and Yang (2005), who did not observe such an effect, and with Krause and Combs (2003), who observed that a reduction in particle length of corn decreased DMI. Chopping at 19 mm failed to fully fractionate corn cobs and left unpalatable chunks. These chunks were poorly ingested and resulted in higher refusals for the diets containing the longer chop alfalfa silage-longer chop corn silage and shorter chop alfalfa silage-longer chop corn silage compared with the longer chop alfalfa silage-shorter chop corn silage and shorter chop alfalfa silage-shorter chop corn silage diets, as well as higher DMI with diets containing the shorter chop corn silage (Table 6Go and Figure 1Go). These results confirm the findings of Kononoff et al. (2003b) that sorting increases when the chop length of corn silage increases.


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  		Table 6. Dry matter intake and milk production of experimental diets containing shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages
 
The study of Kononoff and Heinrichs (2003b) used a wider range of chop length of alfalfa silage (i.e., a theoretical cut length of 22.3 mm followed by rechopping at 4.8 mm) compared with the current study, which might explain the discrepancy between these studies. The discrepancy between the study from Beauchemin and Yang (2005) and the current study could be due to the kernel processor that was used in their study, but not in our study, because the use of this processor would have contributed to the fractionation of corn cobs. The reduction in DMI caused by a reduction in forage particle length in the study of Krause and Combs (2003) might be explained by the short average dietary particle lengths of this study, ranging from 2.5 to 4.2 mm, and the induction of SARA that resulted from a further reduction in forage particle length (Kleen et al., 2003).

In the current study, reducing the chop length of corn silage had a larger effect in diets containing the longer chop alfalfa silage than in diets containing the shorter chop alfalfa silage. A reduction in dietary particle length could have a larger effect on DMI in diets with long feed particle lengths (e.g., diets with an average particle length longer than 7.7 mm), compared with diets with short feed particle lengths (e.g., diets with an average particle length shorter than 7.3 mm) because in the former diets physical fill of the reticulorumen might limit voluntary feed intake (Allen, 2000).

As mentioned earlier, Kononoff and Heinrichs (2003a,b) fed TMR containing silages with better fermentation characteristics, compared with the silages used in our experiment, to lactating dairy cows and observed that DMI ranged from 3.1 to 4.1% of BW, whereas in the current study DMI ranged from 2.9 to 3.1% of BW. This shows that the fermentation characteristics of the silages used in the current study might have reduced DMI.

Rumen Fermentation
The chop length of alfalfa silage did not affect rumen pH, whereas reducing the chop length of corn silage increased the rumen pH (Table 7Go). The effects of day of sampling and the interactions of day of sampling with the chop length of alfalfa silage and with the chop length of alfalfa silage on rumen pH were not significant. The average rumen pH of all diets were higher than 6.12. A pH below 5.90 in rumen fluid samples collected with an oro-ruminal probe is abnormal and indicates SARA (Duffield et al., 2004). However, a pH of these samples between 5.9 and 6.2 is marginal (Duffield et al., 2004). Hence, the rumen pH data do not provide a strong suggestion any of the diets induced SARA. However, the low milk fat percentages of all diets suggest that SARA was induced (Kleen et al., 2003).


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  		Table 7. Rumen fluid composition for experimental diets containing shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages
 
Rumen fluid samples were collected at 4 to 5 h after feeding because earlier studies with corn grain-based diets (Krause et al., 2002b; Duffield et al., 2004) showed that the rumen pH is the lowest at that time. However, Beauchemin et al. (2003) and Yang and Beauchemin (2005) found that the lowest rumen pH were obtained between 12 and 16 h after the first feeding of a barley grain-based diet. Hence, it is possible that the time of rumen fluid sample collection in the current study did not coincide with the lowest rumen pH and that SARA occurred despite the rumen pH at 4 to 5 h after feeding being higher than 6.12. Hence, the pH data need to be interpreted with caution.

As mentioned earlier, the forage NDF content of all diets was lower than what has been recommended for barley grain-based diets by Yang and Beauchemin (2005). These authors concluded that the diets in their study induced SARA due to insufficient intake of forage NDF and excessive intake of starch. The forage NDF contents of the diets in their study and in our study were similar, but the diets in our study contained more peNDFPS. However, because cows in our study selected against long feed particles, the peNDFPS content of the ingested feed was lower than the dietary content of peNDFPS. As a result, the intake of forage NDF might have been insufficient to provide sufficient rumen buffering, resulting in SARA. As mentioned earlier, the DMI expressed as a percentage of BW was lower than in earlier studies on the effects of the particle length of alfalfa silage and corn silage in dairy cows (Kononoff and Heinrichs, 2003a,b; Kononoff et al., 2003b). It was suggested earlier that the lower DMI in our study might be due to the poorer fermentation characteristics of the silages used in our study compared with the other studies, but SARA could also have contributed to the low DMI.

Results obtained in other studies on the effect of forage chop length on rumen pH have been contradictory. Krause et al. (2002b) found that a reduction in the particle length of alfalfa silage reduced rumen pH. Kononoff and Heinrichs (2003a), Kononoff et al. (2003b), and Beauchemin and Yang (2005) did not observe an effect of particle length of corn silage on rumen pH. However, Krause and Combs (2003) demonstrated an interaction between forage particle length and type of grain on rumen pH because a reduction in forage particle length increased the time spent below pH 5.8 for diets containing high-moisture corn, but not for diets containing dry cracked shelled corn. The same authors also demonstrated an interaction between the particle length and the source of the forage on rumen pH because a reduction in forage particle length increased the time spent below pH 5.8 when alfalfa was the only forage but decreased the time below pH 5.8 when the diet included both alfalfa silage and corn silage. Earlier studies have shown that increased forage particle length and physically effective fiber enhance chewing activity but do not result in increased saliva production and rumen buffering; increased chewing during eating and ruminating subsequently reduced saliva production during time periods when chewing activity was absent (Maekawa et al., 2002). This could explain the limited effects of forage particle length on rumen pH seen in earlier studies (Beauchemin et al., 2003; Kononoff and Heinrichs, 2003a; Beauchemin and Yang, 2005) and why the alfalfa silage particle length did not affect rumen pH in our study.

With the exception of Krause and Combs (2003), no earlier study reported that a reduction in forage particle length can increase rumen pH. A factor that could have contributed to the higher rumen pH of diets containing the shorter chop corn silage compared with the diets containing the longer chop corn silage is that the shorter chop corn silage had a higher pH than the longer chop corn (Thomas and Wilkinson, 1975). Krause and Combs (2003) showed that the particle length of corn silage and alfalfa silage can affect the time of the lowest rumen pH. Hence, the time relative to feeding at which the lowest rumen pH was obtained might have differed between the diets containing the longer chop corn silage and the shorter chop corn silage, and if rumen fluid had been collected at another time relative to feeding, the rumen pH of the diets containing the shorter chop corn silage might not have been higher than that of the diets containing the longer chop corn. This would suggest the need for more frequent sampling of rumen fluid. Gozho et al. (2005) also concluded that the duration of rumen pH below 5.6 is preferred as an indicator of SARA than a single daily rumen pH sample. However, because more frequent sampling of rumen fluid with the oro-ruminal probe was considered to create health risks to the cows, this sampling was restricted to once daily. However, because of the once daily sampling of rumen fluid, the rumen pH data must be interpreted with caution.

Reducing the chop length of alfalfa silage increased the concentrations of total VFA and the molar proportion of acetate in rumen fluid but did not affect the molar proportions of propionate and butyrate and the acetate to propionate ratio (Table 7Go). The chop length of corn silage did not affect concentrations of total VFA, the molar proportions of VFA, and the acetate to propionate ratio in the rumen. The effects of day of sampling and the interactions of day of sampling with the chop length of alfalfa silage and with the chop length of alfalfa silage on the concentrations of rumen VFA were not significant. Reducing the forage particle length may increase ruminal rate of digestion and VFA production due to increased surface area for microbial attachment (Krause et al., 2002a). Also, reducing forage particle length can reduce saliva production and liquid passage rate (Krause et al., 2002a), thereby increasing the concentrations of VFA in the rumen. However, a reduction of forage particle length can also reduce VFA production in the rumen due to increased particulate passage rate (Soita et al., 2003). Despite this, Kononoff and Heinrichs (2003a,b) and Beauchemin and Yang (2005) did not find that a reduction of the particle length of alfalfa silage and corn silage affected the outflow rate of liquid and particulate digesta from the rumen.

Similar to our study, Krause et al. (2002b) and Kononoff and Heinrichs (2003b) found that a reduction of the particle length of alfalfa silage increased the concentration of total VFA in rumen fluid. However, in contrast to our study, in these earlier studies, concentration of propionate increased more than that of acetate, which reduced the acetate to propionate ratio. Studies have reported conflicting results on the effects of the particle length of corn silage on rumen VFA. Similar to our study, Kononoff and Heinrichs (2003a) found that the particle length of corn silage did not affect rumen VFA. However, Kononoff et al. (2003b) and Beauchemin and Yang (2005) found that reducing the particle length of corn silage increased the concentration of total VFA in the rumen. Beauchemin and Yang (2005) also observed that reducing the corn silage particle length tended to decrease the acetate to propionate ratio. The NDF contents of the corn silages in our experiment were 49.4 and 51.7% of DM for the shorter chop and the longer chop corn silage, respectively (Table 3Go). These NDF contents are higher than the NDF contents of the corn silages of the studies of Kononoff et al. (2003b; 39.6% of DM) and Yang and Beauchemin (2005; 45.8% of DM). This suggests that the effect of particle length on VFA production may be lower in corn silage with a high NDF and lower rumen digestion content compared with corn silage with a low NDF content and higher rumen digestion.

Reducing the particle length of corn silage and alfalfa silage did not affect the rumen ammonia concentration (Table 7Go). This agrees with the studies from Kononoff and Heinrichs (2003a,b), Kononoff et al. (2003b), and Beauchemin and Yang (2005), suggesting that reduction of forage particle length may not have affected the protein digestion in the rumen.

Blood Metabolites
The plasma concentrations of glucose, urea, and lactate were not significantly affected by the chop lengths of corn silage and alfalfa silage (Table 8Go) and were similar to those reported by Plaizier (2004). The plasma concentrations of glucose and urea were within the normal ranges from 2.4 to 4.2 mmol/L and 2.8 to 8.8 mmol/ L, respectively (Merck and Co., 2006). Most earlier studies on the effect of forage chop length did not monitor blood parameters. Plaizier (2004) found that reducing dietary particle length by replacing alfalfa silage with chopped hay increased blood glucose and decreased blood urea, but this was due to differences among these forages other than particle length.


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  		Table 8. Concentrations of metabolites in blood plasma for experimental diets containing shorter (10 mm) or longer (19 mm) chop lengths of alfalfa and corn silages
 
Milk Production and Composition
The chop length of alfalfa silage and corn silage did not affect milk yield, milk fat yield, milk protein yield, and the percentages of milk fat and milk protein (Table 6Go). The milk fat percentages of all diets were low, and an inversion of the milk fat percentage and milk protein percentage occurred for all diets. Milk fat percentages were lower than normal for this herd because the average milk fat percentages before and after the study were 3.3 and 3.2%, respectively, as reported by Canwest Dairy Herd Improvement (2004). Low milk fat percentages are regarded as a sign of SARA (Kleen et al., 2003). Although the rumen pH obtained in our study was not indicative of SARA, they cannot rule out SARA because the timing of rumen fluid collection may not have coincided with the lowest rumen pH. Yang and Beauchemin (2005) found similarly low milk fat percentages in their study on the effect of corn silage particle length and physically effective fiber on milk production and explained that the milk fat depression by low rumen pH was due to the dietary contents of forage NDF and starch that were, respectively, lower and higher than recommended for barley grain-based diets. The dietary contents of forage NDF in our study (18.7 to 19.2% of DM) were also below the minimum requirements for barley grain-based diets suggested by Yang and Beauchemin (2005; 22 to 23% of DM), which might have compromised rumen buffering. Selection against long feed particles might have further compromised rumen buffering.

Similar to our study, earlier studies (Krause et al., 2002a; Kononoff and Heinrichs, 2003a,b; Yang and Beauchemin, 2005) also did not find that the particle lengths of alfalfa silage and corn silage affected milk yield. The absence of an effect of the particle length of alfalfa silage and corn silage on milk fat yield agrees with Krause et al. (2002a), Kononoff and Heinrichs (2003a,b), and Yang and Beauchemin (2005), although Kononoff and Heinrichs (2003a) reported that a reduction in the particle length of corn silage tended to reduce milk fat percentage. Krause and Combs (2003) found that a reduction of forage particle length reduced milk fat percentage in diets containing both corn silage and alfalfa silage, but not in diets containing alfalfa silage as the sole forage. The interaction of forage particle length and diets composition might be explained by differences in rumen pH and starch contents among diets. Mertens (1997) concluded that a reduction in physically effective fiber causes a greater reduction in milk fat percentage in diets with a low physically effective fiber and low rumen pH compared with diets with a high physical effective fiber content and a higher rumen pH. The differences in the effects of the particle length of corn silage and alfalfa silage on milk fat percentage between the study from Krause and Combs (2003) and our study might be explained by the relative shorter dietary particle lengths and the relative wider range in particle lengths in the study by Krause and Combs (2003). The milk fat percentages in our study were lower than those in studies that demonstrated a reduction in milk fat percentage due to a reduction in forage particle size. The milk fat depression found in our study could be one of the reasons that increased forage particle size failed to increase the milk fat percentage.

The absence of a particle length effect of alfalfa silage and corn silage on milk protein agrees with Krause et al. (2002a), Krause and Combs (2003), and Yang and Beauchemin (2005). In contrast, Kononoff and Heinrichs (2003b) demonstrated a quadratic effect of the particle length of alfalfa silage on milk protein percentage, but not on milk protein yield, with the lowest milk protein percentage for the diets containing the longest alfalfa silage particles. Kononoff and Heinrichs (2003a) found that increasing the particle length of corn silage reduced milk protein percentage and milk protein yield. This discrepancy between the studies from Kononoff and Heinrichs (2003a,b) and our study might be related to the relatively low milk protein percentages in the earlier studies. In the studies of Kononoff and Heinrichs (2003a,b), milk protein percentages ranged from 2.82 to 2.93% and from 2.63 to 2.68%, respectively, among diets, whereas in our study milk protein percentage ranged from 3.29 to 3.30% among diets. These differences in milk protein percentages may imply that these findings of Kononoff and Heinrichs (2003a,b) are not representative for cows with high milk protein contents.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
ACKNOWLEDGEMENTS
 REFERENCES
 
Reducing the chop length of alfalfa and corn silage harvested using a New Holland forage harvester (model 790) from 19 to 10 mm reduced the geometric mean particle length from 14.4 to 11.0 and from 14.2 to 10.4 mm, respectively. The fermentation profile of all silages suggested less than optimal silage quality. Incorporation of either 21.7% of DM longer chop or shorter chop alfalfa silage and either 21.7% of DM longer chop or shorter chop corn silage in TMR of lactating dairy cows that contained 56.6% of DM of barley grain-based concentrate showed that reducing the chop length of alfalfa silage did not affect DMI, milk production, rumen pH, and rumen ammonia but increased the concentration of total rumen VFA in rumen fluid. Reducing the corn silage chop length increased DMI and rumen pH but did not affect rumen VFA, rumen ammonia, and milk production. Milk fat was lower than 2.72% for all diets, which was below average for the herd and suggests that diets induced SARA. Rumen pH at 4 to 5 h after feeding were all higher than 6.0, which does not suggest SARA. However, this rumen pH data does not exclude SARA because the time of rumen pH measurement might not have coincided with the lowest rumen pH. The pH data, therefore, need to be interpreted with caution. Induction of SARA might have occurred due to the lower than recommended dietary content of forage NDF and the selection against long feed particles in favor of short feed particles.


   ACKNOWLEDGEMENTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 ACKNOWLEDGEMENTS
REFERENCES
 
The staff of the Glenlea Research Station, University of Manitoba, Terri Garner, and Janice Haines are thanked for their technical assistance. This study was supported by grants from Dairy Farmers of Canada (DFC), the Agri-Food Development Research Initiative (ARDI), and the Natural Sciences and Engineering Research Council of Canada (NSERC).

Received for publication September 18, 2006. Accepted for publication January 17, 2007.


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


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