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Effects of Rumen Acid Load from Feed and Forage Particle Size on Ruminal pH and Dry Matter Intake in the Lactating Dairy Cow

B. Rustomo^*, O. AlZahal^*, N. E. Odongo^*, T. F. Duffield and B. W. McBride^*,1

^* Department of Animal and Poultry Science, and
Department of Population Medicine, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

¹ Corresponding author: bmcbride{at}uoguelph.ca

	ABSTRACT

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

 
The objective of this study was to evaluate the effects of level of concentrate acidogenic value (AV) and forage particle size on ruminal pH and feed intake in lactating dairy cows. Two isoenergetic (net energy for lactation = 1.5 ± 0.01 Mcal/kg) and isonitrogenous (crude protein = 17.4 ± 0.1% dry matter) concentrates with either a low AV or high AV were formulated and fed in a total mixed ration with either coarsely or finely chopped corn silage and alfalfa haylage ad libitum. Four rumen-fistulated cows (114 ± 14 d in milk) were randomly assigned to 1 of the 4 treatments in a 4 x 4 Latin square with a 2 x 2 factorial treatment arrangement. Each period consisted of 3-wk (14-d treatment adaptation and 7-d data collection). Increasing the concentrate AV decreased the mean pH (from 6.07 to 5.97) and minimum pH (from 5.49 to 5.34). Cows fed high-AV diets spent a longer time below pH 5.6 (135.1 vs. 236.7 min/d; low-AV diet vs. high-AV diet, respectively) and pH 5.8 (290.0 vs. 480.6 min/d; low-AV diet vs. high-AV diet, respectively) than cows fed low-AV diets. Increasing forage particle size had no effect on the mean and minimum ruminal pH. There was an interaction between concentrate AV and forage particle size on maximum ruminal pH. Increasing forage particle size increased the maximum pH for cows fed the high-AV concentrate (6.69 vs. 6.72; low-AV diet vs. high-AV diet, respectively) and had no effect on the maximum pH for cows fed the low-AV concentrate (6.98 vs. 6.76; low-AV diet vs. high-AV diet, respectively). Increasing the concentrate AV did not affect dry matter intake but reduced neutral detergent fiber intake from 9.7 to 8.8 kg/d. Milk fat content was negatively correlated with time and area below pH 5.6 (time below, r = –0.51; area below, r = –0.56) and pH 5.8 (time below, r = –0.42; area below, r = –0.54). These results suggest that coarse forage particle size can attenuate drops in ruminal pH. However, the ameliorating effects of forage particle size on drops in ruminal pH were more apparent for high-AV diets than for low-AV diets. The AV approach combined with physically effective neutral detergent fiber would therefore improve the formulation of diets and help to mitigate subacute ruminal acidosis in dairy cows.

Key Words: acidogenic value • dry matter intake • forage particle size • ruminal pH

	INTRODUCTION

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

 
Rumen acidosis is related to the amount of acid produced from fermented feed and the capacity of the feed to support salivary buffer production. Subacute rumen acidosis (SARA) is characterized by repeated bouts of depressed ruminal pH between 5.2 and 5.6, often resulting from a large intake of rapidly fermentable carbohydrates, which leads to an increase of organic acids in the rumen (Owens et al., 1998). The length of time per day when ruminal pH is below 5.6 (Keunen et al., 2002) or below 5.8 (Krause et al., 2002b; Krause and Combs 2003) is a more important determinant of rumen acidosis than the mean daily ruminal pH.

Previously, Rustomo et al. (2006a) evaluated the acidogenic value (AV) of feeds using an in vitro laboratory technique. The AV varied with the protein, starch, NFC, and fiber contents of feedstuffs, being highest for NFC-rich feeds, intermediate for forages, and lowest for feeds high in protein. The rate at which rumen fluid pH changed followed a pattern similar to changes in the AV (Rustomo et al., 2006a), and the differences in AV and pH changes likely were associated with the fermentability of the feeds (de Smet et al., 1995). A subsequent in vivo study evaluated the effect of concentrate AV on in vivo ruminal pH (Rustomo et al., 2006b). The ruminal pH of cows fed low-AV (LA) diets remained below pH 5.6 for 170 min/d during the day compared with 342 min/d for the high-AV (HA) cows, suggesting that the LA diets possessed higher buffering capacity.

Ruminal pH is determined by the equilibrium between acid production and acid disappearance from the rumen (neutralization and absorption). Fiber intake affects acid production and saliva secretion (Allen, 1997). Physically effective NDF (peNDF) has been defined as the fraction of feed that stimulates chewing and saliva production (Mertens, 1997), and the peNDF value indicates the physical characteristic of the fiber, such as particle size. Although the peNDF concept was developed to reduce the problems associated with an increased rumen acid load from feeding highly fermentable feeds, the peNDF value does not take into account acid production. The effect of forage particle size (FPS) on intake and ruminal pH is well documented (Kononoff and Heinrichs 2003a,b; Kononoff et al., 2003a; Beauchemin and Yang, 2005). Although works have been done to evaluate the effects of feed fermentability (Krause et al., 2002b; Krause and Combs, 2003) or AV (Dewhurst et al., 2001; Rustomo et al., 2006b) on intake, to our knowledge, no work has evaluated the relationship between concentrate AV and FPS on ruminal pH and performance in lactating dairy cows. The present study was conducted to test the hypothesis that the effect of increasing the concentrate AV on ruminal pH would depend on the degree of fineness of the FPS. Therefore, the objective of the present study was to determine the effects of the concentrate AV and FPS on the ruminal pH and DMI of dairy cows.

	MATERIALS AND METHODS

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

 
Animals
Four multiparous Holstein dairy cows (114 ± 14 DIM) were used in this trial. The cows were fitted with a ruminal fistula and averaged 600 ± 66 kg of BW at the beginning of the experiment and 652 ± 92 kg at the end of experiment. The experiment was conducted at the Elora Dairy Research Center (University of Guelph, Guelph, Ontario, Canada) from June 17 to September 8, 2005. Animals were cared for and handled in accordance with the Canadian Council on Animal Care regulations, and the University of Guelph Animal Care Committee approved their use for this experiment.

Cows were fed ad libitum, allowing 10% refusals, and the feed was offered twice daily at 0900 and 1300 h in equal portions. Orts from individual cows were weighed each morning prior to feeding. Representative feed and ort samples were taken 3 times per week and stored at –20° C for later analysis. The samples were then composited over the recording week of each period. Feed intake and milk yield were monitored daily throughout the experiment. The animals had unlimited access to fresh water and were milked in their stalls twice daily at 0500 and 1500 h. Milk samples were collected 3 times per week from morning and afternoon milkings and preserved using 2-bromo-2-nitropropane-1-2-diol. The milk samples were pooled 3 times each week based on milk yield, and the pooled samples were immediately submitted to the Central Milk Testing Laboratory (Laboratory Services Division, University of Guelph) for compositional analysis.

Experimental Design
The experiment was conducted as a 4 x 4 Latin square with a 2 x 2 factorial treatment arrangement. Each period was 3 wk (14-d treatment adaptation and 7-d data collection) in duration. Feed ingredient AV was used to formulate 2 isoenergetic (NE_L = 1.5 ± 0.01 Mcal/kg) and isonitrogenous (CP = 17.4 ± 0.1% DM) concentrate diets (LA vs. HA) fed ad libitum in a TMR alongside a coarse or finely chopped corn silage and alfalfa haylage. The concentrate ingredients were ground in a Schutte pulverizing rotary hammer mill (model 44-24-301B; Schutte Pulverizer Company, Inc., Buffalo, NY) through a 3-mm screen and pelleted (4-mm diameter). First-cut wilted alfalfa haylage and corn silage harvested (John Deere harvester, model 7200; John Deere, Moline, IL) at 1.3 and 1.9 cm theoretical length of cut, respectively, provided the coarse fraction of the diets (CS). The finely chopped silage and haylage (FS) was obtained by rechopping CS at 1,600 rpm for 36 min daily in a mixer (model 1350; Taylor, Orton, Ontario, Canada). The geometric particle sizes of the mixed forages are given in Table 1. The 2 levels of FPS (CS and FS) were combined with the 2 levels of concentrate AV (HA and LA) to prepare 4 TMR in a data ranger (American Calan, Northwood, NH): HACS, HA concentrate + CS; HAFS, HA concentrate + FS; LACS, LA concentrate + CS; and LAFS, LA concentrate + FS. All diets were formulated to meet or exceed the requirements of a 620-kg multiparous cow producing 30 kg/d of milk using the Cornell–Penn–Miner dairy model, version 3.0 (CPM-Dairy). Diets were fed as a TMR with a forage-to-concentrate ratio of 50:50. The DM of the feeds and orts was determined by oven-drying at 60° C for 48 h using standard AOAC procedures (AOAC, 1990), and the diet ingredients were adjusted weekly to account for changes in the DM content. Diet composition and chemical analyses are shown in Tables 3 and 4.

Ruminal pH Measurements
Ruminal pH was measured continuously for 4 d in each period using an industrial electrode as described by AlZahal et al. (2005). Briefly, a heavy-duty pH probe designed for industrial applications (PHE-7352-15-PT100; Omega Engineering Inc., Stamford, CT) was placed through the cannula into the anterior region of the ventral sac of the rumen. A stainless-steel weight (approximately 0.5 kg) was attached to the proximal end of the probe via an ultrahigh molecular weight (food-grade plastic) connector to maintain the probe in place and the probe was connected directly to a compact lightweight (120 g) data logger (pH Temp101; Monarch Instrument, Amherst, NH) mounted on the animal’s back. Ruminal pH readings were taken every minute and were downloaded onto a computer for subsequent analysis. The position of the pH electrode was examined 3 times each day (before the morning feeding, 3 to 4 h after the morning feeding, and 3 to 4 h after the afternoon feeding) by palpation through the rumen fistula to ensure that the probe remained in place. Electrodes and the pH transmitter were calibrated at least once during each recording period using standard pH 4 and pH 7 buffer solutions (Fisher Scientific, Fairlawn, NJ). Rumen fluid was also sampled 3 times each day (before the morning feeding, 3 to 4 h after the morning feeding, and 3 to 4 h after the afternoon feeding) for pH measurement using a handheld pH meter (pH 310; Oakton Instruments, Vernon Hills, IL) to verify that the continuous pH recordings were accurate. The daily ruminal pH data were recorded using data-recording software (version 2.0; Monarch Instruments) that automatically calculates the average, minimum, and maximum pH and standard deviation. The continuous ruminal pH data were summarized for each 24-h period by calculating the minimum, maximum, and mean pH; the amount of time below pH 5.0 to 6.8; and the area (time x pH) below pH 5.0 to 6.8. Using this data set, mean, minimum, and maximum pH; time below pH 5.0 to 6.8; and area below pH 5.0 to 6.8 for each cow were analyzed. The area below pH 5.0 to 6.8 was calculated as the product of time (min) and deviation (0.1 pH unit) from the designated pH value (Keunen et al., 2002).

Chemical Analysis
Dried feed samples were ground to pass through a 1-mm screen (Wiley mill; Arthur H. Thomas, Philadelphia, PA) and chemical composition was determined in duplicate using standard AOAC procedures (AOAC, 1990) at a commercial laboratory (Agri-Food Laboratories, Guelph, Ontario, Canada). The analytical DM contents of feeds were determined by oven-drying at 135° C for 2 h; OM was determined by ashing at 500° C for 16 h; and CP, soluble protein, and nonprotein nitrogen were determined by using the macro-Kjeldahl method. Starch was analyzed using the starch gelatinization and hydrolysis method (Hall, 2000), and Ca and P were analyzed by inductively coupled plasma spectroscopy. The samples were also analyzed for ether extract, ADF, and lignin (AOAC, 1990) and NDF (AOAC, 2002) using -amylase (A-3306; Sigma-Aldrich, St. Louis, MO). Nonfiber carbohydrates were calculated as NFC = 100 – (% NDF + % CP + % fat + % ash). The AV (mg of Ca/g of DM) of the concentrate and TMR were determined as previously described by Rustomo et al. (2006a). Briefly, 1-g samples (DM basis) were weighed directly from the freeze-drier into 100-mL incubation tubes held at 39° C in a water bath, and the samples were incubated in duplicate with 30 mL of buffered rumen liquor comprising 60% buffer and 40% rumen liquor, as described by Wadhwa et al. (2001). Apparent AV was calculated as the product of Ca concentration from the analysis and fluid volume (30 mL) divided by the sample weight (1 g; Wadhwa et al., 2001). Milk samples were analyzed for CP, fat, and lactose using a near-infrared analyzer (Foss System 4000; Foss Electric, Hillerød, Denmark).

Statistical Analysis
Data on the response variables were analyzed using PROC MIXED of SAS (v. 9.1; SAS Institute Inc., 2004) using the model

where Y_ijkl is the dependent variable, µ is the overall mean, C_i is the effect of cow (_i = 1, 2, 3, 4), P_j is the effect of period (_j = 1, 2, 3, 4), A_k is the effect of concentrate AV (_k = 1, 2), F_l is the effect of FPS (_l = 1, 2), (A x F)_kl is the effect of the interaction of A_k and F_l, and _ijkl is the random residual error. The effects of concentrate AV and FPS were analyzed as fixed effects. For the repeated measurements of continuous ruminal pH data, day of sampling was analyzed as a fixed effect with repeated measurement. The correlations between ruminal pH and either rumen acid load or nutrient intakes were determined using PROC CORR of SAS. Effects were considered significant at P < 0.05.

	RESULTS AND DISCUSSION

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

 
Particle Size Distribution
Analysis of the FPS showed that the alfalfa haylage and corn silage were coarsely chopped (Table 1). Ingredient composition and AV of the concentrates and the dietary composition, chemical analysis, and AV of the TMR are presented in Tables 2 and 3, respectively. Rechopping the forages reduced the geometric particle length, reduced the percentage of DM retained on the top sieve ( > 19 mm), and increased the percentage of DM retained on the bottom sieve (1.18 mm) and in the pan (Table 1). The percentage of DM retained on the second sieve was not reduced with rechopping (60.6 vs. 59.7). Rechopping the mixture of alfalfa haylage and corn silage reduced the DM of the TMR retained on the 19- and 8.0-mm screens and increased the DM of the TMR retained on the 1.18-mm screen and in the pan. This reduced the geometric mean particle size and pef of the mixed fine alfalfa haylage and corn silage diets (Table 1) and the pef and geometric particle size of the fine diets (HAFS and LAFS; Table 4). The pef _{> 8} of both the coarse and fine diets were lower than the pef _{> 1.18} because a high proportion of particles of both diets was retained on the bottom screen ( > 1.18 mm). Thus, the peNDF _{> 8} of both the coarse and fine diets were lower than the peNDF _{> 1.18} (Table 4).

As expected, all diets had similar energy and protein contents. The NDF contents of the HA diets were lower than those for the LA diets (Table 3). However all diets were relatively high in NDF compared with the minimum requirement of 25% (NRC, 2001). The geometric mean particle sizes of 9.8 and 7.5 mm for the CS and FS diets, respectively, were above the threshold of 6.4 mm, which has been reported to decrease rumination and the milk fat content (Woodford et al., 1986). Despite a higher proportion of the CS diets being retained on the top (19 mm) and middle screens (8.0 to 19.0 mm) compared with the FS diets (6.6 vs. 3.7%, and 53.9 vs. 46.4%, CS vs. FS, top and middle screens, respectively), both the CS and FS diets met the standards of 3 to 7% of the TMR particles being retained on the top sieve and 30 to 50% of the TMR being retained on the second sieve, as recommended by Heinrichs and Kononoff (2002). Additionally, the peNDF _{> 1.18} for all diets, whether estimated based on the total dietary NDF (peNDF_{T > 1.18} of 37.8 to 41.2% DM) or based on the fractional NDF (peNDF_{F > 1.18} of 34.2 to 40.6% DM), were greater than the critical value of 22% DM recommended to maintain a ruminal pH of 6.0 and were greater than the critical value of 20% DM needed to maintain milk fat at 3.4% (Mertens, 1997). Thus, all diets were expected to be adequate in effective fiber.

Ruminal pH
Diurnal ruminal pH fluctuations for the 4 treatments are shown in Figure 1. All diets resulted in a similar biphasic pattern related to feed intake. The diurnal patterns of ruminal pH changes shown by the diets are typical for grain-fed dairy cows (Keunen et al., 2002). Ruminal pH declined shortly after feeding, after which it gradually increased and then returned to normal pH ( > 6.0) overnight. However, the rate and extent of pH decline after feeding increased with the HA diets. The higher and faster rate of decrease in ruminal pH after feeding exhibited by the HA diets is in agreement with our previous study (Rustomo et al., 2006b). The LAFS and HAFS cows maintained a lower ruminal pH longer than the LACS and HACS cows, suggesting that at the same level of concentrate AV, increasing the FPS mitigated the depression in ruminal pH. However, the effect of increasing the FPS on ameliorating ruminal pH depression was more apparent for cows fed the HA concentrate than those fed the LA concentrate. The lower effect of increased FPS on attenuating ruminal pH drops in the LA diets may have been from the higher intrinsic buffering capacity of the LA concentrate compared with the HA concentrate. The LA concentrate was predominantly composed of 46.1% wheat bran (DM basis; Table 2), which had a relatively low AV and could maintain a relatively high rumen fluid pH after incubation compared with other energy feeds (Rustomo et al., 2006a).

View larger version (24K): [in this window] [in a new window]   Figure 1. Mean daily ruminal pH pattern for the dietary treatments [HACS = high acidogenic value concentrate, coarse forage; HAFS = high acidogenic value concentrate, fine forage; LACS = low acidogenic value concentrate, coarse forage; LAFS = low acidogenic value concentrate, fine forage].

Dietary Intake
The intake of DM, OM, and other nutrients is presented in Table 6. Dry matter intake was not affected by increasing the concentrate AV (21.9 vs. 21.2 kg/d; LA vs. HA concentrate, respectively). This is in agreement with our previous study (Rustomo et al., 2006b). Decreasing the FPS had no effect on DMI (21.4 vs. 21.7 kg/d). Allen (2000) suggested that the effect of physical distention in the reticulorumen could limit feed intake at the low-concentrate inclusion, but the effect would not be expected on the high-concentrate diet. Beauchemin and Yang (2005) found that reducing the peNDF _{> 8} of corn silage-based diets by a high inclusion of concentrate (60% of ration DM) did not affect DMI. The concentrate level of the current study was high (50% in ration DM; Table 3); hence, the effect on DMI of reducing FPS was not observed. There was no interaction between concentrate AV and FPS on DMI (Table 6). This result is in agreement with other studies in which reducing the particle size of alfalfa silage (Krause et al., 2002a) or corn silage (Kononoff and Heinrichs, 2003a) did not affect DMI. Conversely, positive effects of reducing the corn silage particle size on DMI have been observed (Kononoff et al., 2003a). Although reducing the FPS is commonly believed to reduce chewing activity and salivary buffer secretion and therefore reduce ruminal pH (Mertens, 1997), the effects of FPS on DMI have remained less clear.

The intake of OM decreased from 20.5 to 19.5 kg/d and that of NDF decreased from 9.7 to 8.8 kg/d with increasing concentrate AV. Forage particle size had no effect on OM and NDF intakes. The intake of ADF, starch, NFC, and CP was not affected by diet (Table 6). The nonsignificant differences in the intake of starch, NFC, and CP were expected because the diets were formulated to have an isoenergetic and isonitrogenous content. The difference in NDF intake may have contributed to the ruminal pH response, but the effect may be smaller than that of feed acidogenicity. This is because all diets were above the critical NDF value of 22% DM needed to maintain a ruminal pH above 6.0 (Mertens, 1997). Thus, the effects of dietary treatments on ruminal pH depression we observed were likely related to differences in feed fermentability reflected by differences in the estimated total acid load from feed fermentation. Decreasing the FPS decreased the peNDF_{T > 1.18} from 40.4 to 38.5% DM but not the acid load (199.6 vs. 202.8 g Ca/d; CS vs. FS diet, respectively). Increasing the concentrate AV increased the estimated acid load from 190.7 to 211.8 g Ca/d. However, the increased acid load could not explain the nonsignificant response in DMI. This is consistent with the report by Rustomo et al. (2006b), in which increasing the acid load from feed fermentation did not affect the DMI. However, Dewhurst et al. (2001) reported a significant effect of concentrate AV on decreasing the DMI in corn silage-based diets as compared with grass silage-based diets (17.4 vs. 22.2 kg DM). These authors suggested that the depression in DMI of the corn silage-based diets was more likely affected by the acidogenicity of the forage than the effect of peNDF of the diets on DMI. Because we used mixed (50:50) alfalfa haylage and corn silage in the TMR, the effect of forage acidogenicity on DMI was expected to be lower than that reported by Dewhurst et al. (2001) when corn silage was fed as the only forage. Feeding methods (TMR or components) could also be a factor influencing DMI; hence, DMI depression in the current study was expected to be lower than that found by Dewhurst et al. (2001).

Milk Yield and Composition
Milk yield and composition are presented in Table 7. Milk yield, milk composition, and 4% FCM were not affected by dietary treatment. Increasing the concentrate AV did not affect the milk fat content or fat yield (Table 7), which is consistent with our previous study (Rustomo et al., 2006b). Milk fat content and fat yield were not affected by decreasing the FPS, which is in agreement with the results of Kononoff and Heinrichs (2003b), Krause et al. (2002a), and Krause and Combs (2003), although their ranges of particle size were wider than those in the present study. All diets were above the critical peNDF value of 20% DM needed to maintain milk fat at 3.4% (Mertens, 1997), and therefore would be expected to sustain the milk fat content. There was a tendency for the concentrate AV to interact with FPS on the milk fat percentage. Increasing the FPS for cows fed the HA concentrate resulted in a 0.32% increase in the fat content (3.87 vs. 4.19%) compared with cows fed the LA concentrate, in which a relatively constant fat content (4.01 vs. 4.03%) was observed. Dietary treatments had no effect on the milk protein percentage, protein yield, lactose percentage, and lactose yield, in agreement with Krause et al. (2002a). As noted in our previous study (Rustomo et al., 2006b), milk yield and composition may not be explained by acid load from feed fermentation but are more likely because of complex interactions among nutrients in a diet.

	CONCLUSIONS

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

	ACKNOWLEDGEMENTS

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

 
We gratefully acknowledge the Dairy Farmers of Ontario (Mississauga, Ontario, Canada); the Ontario Ministry of Agriculture, Food and Rural Affairs (Guelph, Ontario, Canada); and the Natural Sciences and Engineering Research Council (BWM; Ottawa, Ontario, Canada) for their financial support, without which this work could not have been done. Thanks also go to the staff of the Elora Dairy Research Centre (University of Guelph) and John Las for their assistance during the experiment.

Received for publication March 13, 2006. Accepted for publication June 29, 2006.

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

 


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