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Agriculture and Agri-Food Canada, Research Centre, Lethbridge, Alberta, T1J 4B1, Canada
2 Corresponding author: beauchemink{at}agr.gc.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: physically effective fiber • chewing • ruminal pH • acidosis
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Physically effective NDF (peNDF) measures the physical characteristics of fiber by accounting for particle length and NDF content, which promote chewing and the flow of salivary buffers to the rumen (Mertens, 1997). The term peNDF is used in diet formulation to provide fiber of adequate particle length to reduce acidosis. A number of studies have been conducted recently to determine the effects of peNDF on chewing activity and ruminal pH in dairy cows (Krause et al., 2002; Kononoff and Heinrichs, 2003a; Beauchemin and Yang, 2005). However, the responses obtained from these studies have been somewhat inconclusive. In some studies, increased chewing activity as a result of increased peNDF intake increased ruminal pH, helping to minimize ruminal acidosis (Krause et al., 2002; Beauchemin et al., 2003; Yansari et al., 2004), whereas in other studies, peNDF intake was poorly related to chewing activity and ruminal pH (Kononoff and Heinrichs, 2003b; Yang and Beauchemin, 2006a). Chewing activity can increase with increasing intake of peNDF without elevating ruminal pH, particularly when diets contain highly fermentable carbohydrate sources (Kononoff et al., 2003b; Kononoff and Heinrichs, 2003a; Beauchemin and Yang, 2005) because the concept of peNDF does not account for differences in ruminal fermentability of feeds, which can have a major effect on ruminal pH (Yang et al., 2001a; Krause et al., 2002). Beauchemin and Yang (2005) concluded that the effects of dietary peNDF content on chewing activity and rumen function of dairy cows are variable because peNDF content of diets can be increased by increasing the forage proportion of the diet and by increasing the particle length of forages. Increased forage proportion affects intake of fermentable OM as well as intake of fiber, which could have a significant effect on peNDF requirements. There is little information available documenting the influence of ruminally fermentable carbohydrates on the effects of dietary peNDF levels.
The main objective of this study was to determine the effects of increasing the peNDF content of dairy cow diets on decreasing the risk of acidosis in dairy cows. Dietary peNDF content was varied by adjusting the proportion of forage in the diet and the particle length of silage. We hypothesized that the effects of increased dietary peNDF content depend on the means of increasing dietary peNDF content because forage proportion affects ruminal fermentability of carbohydrates in addition to peNDF, whereas adjusting forage particle length (FPL) only affects peNDF.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). Particle size distribution of the silage was determined using the Penn State Particle Separator (PSPS; Kononoff and Heinrichs, 2003a) consisting of 3 screens (19, 8, and 1.18 mm) and a pan. The DM content was determined by oven drying at 55°C for 48 h. The third portion of the samples was composited by experimental period and retained for determination of chemical composition. Fermentation characteristics of the silage were determined commercially by Cumberland Valley Analytical Service, Inc. (Maugamsville, MD) with a single representative sample from the silo before starting the experiment.
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Cows were offered 1 of 4 diets that consisted of the short and long AS, combined with low (35:65) and high (60:40) forage:concentrate (F:C) ratios (DM basis; Table 2). Thus, intake of peNDF was increased through FPL and proportion of forage in the diet. The peNDF contents ranged from 9.6 to 19.8% DM and 28.6 to 34.0% DM determined using the PSPS with 2 sieves (peNDF8.0) or 3 sieves (peNDF1.18), respectively (Table 3). The diets were formulated using the NRC model (2001) to supply sufficient energy and protein for a 650-kg cow to produce 35 kg/d of milk containing 3.5% fat and 3.2% protein.
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Meal Duration and Feeding Behavior
Feeding behavior was monitored for 48 h on d 14 to 16 of the period. Mangers attached to load cells (Omega Engineering Inc., Stamford, CT) and connected to a computer were used to measure the feed weight over time. An average weight was obtained every 11 s and stored using Collect software (Labtronics, Inc., Guelph, Ontario, Canada). During feeding activity, the weight of the manger decreased. Feeding activity was separated into meals using a meal criterion of 27 min as outlined by DeVries et al. (2003). A meal criterion is the minimum time interval between 2 meals and is used to determine meal frequency and meal duration. A meal was defined as eating activity > 30 s and > 300 g of feed being removed from the feeder. Rate of DMI was calculated as the ratio of DM ingested and duration of the meal.
Chewing Activity
Chewing activity was measured concurrently with feeding behavior. Cows were fitted for 48 h with leather halters that measured jaw movements. Each halter contained a piezo disk (Edmund Scientific Company, Barrington, NJ), which was inserted within the halter and positioned under the jaw. The chewing action places stress on the disk generating an electrical signal, which is then processed and counted as a single jaw movement. A datalogger (Campbell Scientific Inc., Logan, UT) was used to receive the output signal from each cow. The number of jaw movements was summed each minute and stored for subsequent analysis. The jaw movements were then designated as eating or ruminating chews based on several criteria. Jaw movements were considered as eating chews if they occurred during a meal (i.e., duration of the eating activity was > 30 s and > 300 g of feed was removed from the feeder during the meal). Jaw movements were designated as ruminating chews if they did not occur during a meal and rate of chewing exceeded 30 chews/min. All other jaw movements were considered to be associated with licking, drinking, and grooming and were not included in the eating or ruminating categories. Total number of chews was calculated as the sum of eating and ruminating chews. Total time spent eating, ruminating, and chewing (eating + ruminating) was based on the duration of each chewing activity.
Ruminal pH and Fermentation
Ruminal pH was monitored for 48 h concurrent with measurements of feeding behavior and chewing activity. The pH was measured using an industrial electrode (model S650-CDHF, Sensorex, Garden Grove, CA) linked to a pH controller (model PHCN-37, Omega Engineering Inc.) as described in detail by Penner et al. (2006). The electrodes were suspended approximately 60 cm in the rumen using a cable that was anchored to the ruminal cannula plug. The electrode was weighted down to ensure submersion within the rumen contents. This indwelling continuous ruminal pH measurement device results in values that are about 0.05 units lower than manual measurements of pH taken from the same location within the rumen (Penner et al., 2006). The electrodes were covered with a perforated guard to prevent them from coming into direct contact with the rumen wall. The pH electrodes were removed from the rumen for 20 min daily between 1400 and 1430 h for calibration using pH 4.0 and 7.0 standards. Continuous measurements from the indwelling probe were sent to a datalogger (Campbell Scientific Inc.) every 5 s and averaged every 5 min. Ruminal pH data were summarized daily for each cow as mean, minimum, and maximum pH, area between the observed pH and a line drawn at pH 5.8 or 5.5, and time (h) under pH 5.8 or 5.5. The minimum and maximum pH value for each day and each cow was obtained from the raw input data using PROC MEANS (SAS Institute, 1996). The area was calculated by adding the absolute value of negative deviations in pH from 5.5 or 5.8 for each 5-min interval. A ruminal pH of 5.8 was chosen as a benchmark because the corresponding loss in fiber digestion below this pH threshold has negative effects on milk production. The pH of 5.5 was used to further categorize subclinical ruminal acidosis in terms of its severity.
Ruminal fluid was collected on d 17 at 0900, 1300, and 1600 h from multiple sites in the rumen for VFA and NH3-N determination. Samples were immediately squeezed through 4 layers of cheesecloth with a mesh size of 250 µm. Five milliliters of filtrate was preserved by adding 1 mL of 25% HPO3 and used to determine VFA, and 5 mL of filtrate was preserved by adding 1 mL of 1% H2SO4 and used to determine NH3-N. The samples were subsequently stored frozen at –20°C until analyzed.
Chemical Analyses
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). The OM content was calculated as the difference between DM and ash contents, with ash determined by combustion at 550°C overnight. The NDF and ADF contents were determined using the methods described by Van Soest et al. (1991) with amylase and sodium sulfite used in the NDF procedure. Starch was determined by enzymatic hydrolysis of 
-linked glucose polymers as described by Rode et al. (1999). Content of N in the samples was determined by flash combustion (model 1500; Carlo Erba Instruments, Milan, Italy). Ruminal VFA were separated and quantified by gas chromatography (Varian 3700; Varian Specialties Ltd., Brockville, Ontario, Canada) using a 15-m (0.53-mm i.d.) fused silica column (DB-FFAP column; J&W Scientific, Folsom, CA). The ammonia content of ruminal samples was determined using the method described by Weatherburn (1967) modified to use a plate reader. Particle size distributions of AS and TMR were determined using the PSPS. Physical effectiveness factors (pef) for silage and TMR were calculated as the sum of the proportion of DM retained on 2 screens: 19 and 8 mm (pef8.0; Lammers et al., 1996) or on 3 screens: 19, 8, and 1.18 mm (pef1.18; Kononoff et al., 2003a). The peNDF8.0 and peNDF1.18 contents of the AS and TMR were calculated by multiplying NDF content of the feed by the pef8.0 and pef1.18, respectively.
Calculations and Statistical Analyses
Data for feeding behavior, chewing activity, and ruminal pH were summarized by day and analyzed using the mixed model procedure of SAS (PROC MIXED; SAS Institute, 1996). The model included treatments (FPL and F:C) and the interaction between FPL and F:C as a fixed effect and square, period within square, and cow within square as random effects with day included as a repeated measure using compound symmetry. Estimation method was restrictive maximum likelihood (REML) and the degrees of freedom method was Kenward-Rogers. Similarly, data for VFA and ammonia were analyzed by sampling time using repeated measures techniques. Data for particle distribution, pef and peNDF of forages and diets were averaged by period and analyzed by including particle length variables as fixed effects and period as a random effect. Pearson correlation coefficients were estimated using the CORR procedure of SAS. Effects of the factors were declared significant at P < 0.05 unless otherwise noted and trends were discussed at P < 0.10.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The proportion of particles retained on the 8- and 19-mm sieves increased as the TCL of AS was increased from 7.9 to 19.1 mm, and as a result, pef8.0 and peNDF8.0 were increased by up to 40%. Changes in pef1.18 and peNDF1.18 were minimal because increased proportion of long particles (19 mm) was compensated for by reduced proportion of short particles (1.18 mm).
Low and high F:C diets differed in chemical composition as expected (Table 3). In addition, increased F:C ratio increased the proportion of particles on the 19- and 8-mm sieves, pef8.0, peNDF8.0, and peNDF1.18, but decreased the proportion of particles on the 1.18-mm sieve and pef1.18. Increased dietary FPL increased the proportion of particles on 19- and 8-mm sieves, and pef8.0 and content of peNDF8.0. However, contents of pef1.18 and peNDF1.18 were not affected by the dietary FPL because the proportion of particles retained on the 1.18-mm sieve was reduced with increasing dietary FPL.
Thus, measuring pef and peNDF using 3 sieves was insensitive to changes in proportion of forage and FPL. There were interactions between F:C and FPL for proportion of particles on the 19- and 1.18-mm sieves, pef8.0, and peNDF8.0; pef and peNDF measured using 2 sieves were all increased to a greater extent by increasing FPL of the high F:C diet compared with increasing the FPL of a low F:C diet, which reflected the difference in silage proportion between these 2 diets.
Intake, Milk Production, and Composition
Increased F:C ratio decreased DMI but increased intake of NDF (Table 4), whereas intakes of DM and NDF were not affected by FPL. The effects of increasing FPL on intake of peNDF8.0 depended upon F:C ratio; the increase in peNDF8.0 intake with increasing FPL was greater for the high F:C diet (58%) than for the low F:C diet (35%). Actual milk yield were decreased with increasing F:C ratio. Content of milk fat was increased, whereas contents of milk protein and milk lactose were decreased with increasing F:C ratio. Milk production and composition were not affected by the FPL.
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). However, FPL did not affect eating activity. Increasing the proportion of F:C also increased number of ruminating chews (chews/d and chews/kg of DMI) and ruminating time per unit of DMI, but FPL only increased these variables when the proportion of forage in the diet was high. The amount of rumination promoted per unit of peNDF1.18 was consistent among diets, whereas rumination per unit of peNDF8.0 intake varied with F:C and FPL. Total chewing (eating + ruminating) measured as number of chews (/d and /kg of DM) and chewing time (min/d and min/kg of DM) increased or tended to increase with increasing F:C. This increase was due to increased NDF intake, because chews per kilogram of NDF and minutes per kilogram of NDF were constant among diets. Thus, increasing the proportion of forage in the diet increased chewing activity due to the increase in NDF consumed. The effects of FPL on chewing activity depended upon F:C; increasing FPL only increased chewing (chews/kg of DM and min/kg of DM) when diets contained a high proportion of forage. In addition, each unit of peNDF8.0 was more effective in promoting chewing in low-forage diets containing short-chopped silages. Total chewing time per unit of peNDF8.0 intake was decreased by 74 or 94 min, respectively, with increasing F:C ratio or FPL. However, chewing time per unit of peNDF1.18 in-take was constant across the treatments.
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). The highest eating activity was observed after the 1500 h feeding followed by activity at the 0600 h feeding. However, cows did not exhibit peak eating activity at the third feeding (1800 h) due to high eating activity between 1500 and 1700 h. Ruminating time was equally distributed throughout the day except during eating time. Cows fed a high F:C consumed more meals per day of smaller size (kg of DM/meal) with slower eating rate than cows fed low F:C diets (Table 5). Meal pattern was not affected by dietary FPL.
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). Diurnal variation in ruminal pH was greater for the low F:C diet than for the high F:C diet, especially when low F:C was combined with long FPL. For example, between 1400 and 2100 h, the average hourly pH declined by 0.8 units (from 6.3 to 5.5) for cows fed low F:C combined with long FPL, whereas pH for cows on the other diets dropped 0.42 to 0.49 units during this time. Mean, minimum, and maximum ruminal pH, area between the curve and a horizontal line at pH 5.8 or 5.5, and duration that pH < 5.8 or pH < 5.5 were elevated by increasing F:C ratio (Table 6). Increased dietary FPL increased only mean and maximum ruminal pH. There was a numerical (P < 0.11) interaction between F:C and FPL for mean ruminal pH; increasing F:C ratio increased mean ruminal pH to a greater extent when dietary FPL was short (0.60 units) compared with when dietary FPL was long (0.38 units) and increasing FPL increased ruminal pH to a greater extent with low F:C diets (0.31 units) vs. high F:C diets (0.09 units).
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). Increasing the F:C ratio decreased total concentration of VFA, increased the molar proportion of acetate and branched-chain VFA, decreased the proportion of propionate, and increased the molar proportion of ammonia. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The wide range in particle size distributions of AS and TMR used in our study reflect the variability observed on commercial dairy farms. For example, Plaizier et al. (2004) used the PSPS to measure the particle length of AS and TMR from 40 dairy farms across Manitoba, Canada, and found that the particles (proportion by weight) retained on the 19- and 8-mm screens and peNDF content (% of DM) ranged from 6 to 70%, 26 to 48%, and 15 to 49%, respectively, for AS, and from 3 to 69%, 18 to 50%, and 8 to 37%, respectively, for TMR. The recommended proportions for AS are 10 to 20% on the 19-mm sieve and 45 to 75% on the 8-mm sieve, and, for TMR are 2 to 8% on the 19-mm sieve and 30 to 50% on the 8-mm sieve (Kononoff and Heinrichs, 2007). These recommendations are based on the assumption that feed particles retained on these sieves promote chewing activity that contributes to rumen buffering. The diets used in our study were within the recommended range, except for the low F:C–short AS diet, which was slightly lower, and the high F:C–long AS diet, which was slightly higher.
One of the limitations of increasing peNDF through increased F:C ratio is the negative effect on DMI, which is caused by increased rumen fill (Allen, 2000). Lack of effect of FPL on DMI was due to the resulting particle length of the diets. In studies in which increasing peNDF lowered DMI, either a high-forage diet (> 50%; Kononoff and Heinrichs, 2003a; Yansari et al., 2004) or very long FPL (TCL = 19 mm; Einarson et al., 2004) was used.
The increase in chewing activity, both rate (chews/kg of DMI) and duration, when peNDF intake was increased through F:C ratio was expected (Kononoff and Heinrichs, 2003a). In other studies, total chewing time increased by 11% as NDF content of the diet increased from 31 to 37% (Beauchemin, 1991) or by 21% as the NDF content of the diet increased from 29 to 38% (Oba and Allen, 2000). Furthermore, the increase in chewing activity with increasing F:C ratio was the resulting combination of increased ruminating and eating activities, which confirms results from our previous study (Yang et al., 2001b).
Increasing peNDF intake through FPL had inconsistent effects on chewing time, with only increased chewing activity (chews/kg of DMI and min/kg of DMI) when diets were high in F:C ratio. The fact that FPL only promoted chewing in high-forage diets is somewhat surprising given that positive effects of FPL on chewing have been reported in cows fed diets with a wide range of F:C; from low (39:61; Krause et al., 2002) to high (69:31; Le Liboux and Peyraud, 1999). Increased chewing activity due to FPL with the high F:C diet was associated with the larger increase in peNDF8.0; 1.5 kg/d more peNDF8.0 with the high F:C diet compared with only 0.7 kg/d more with the low F:C diet with increased FPL. Thus, low chewing time associated with very low peNDF diets is not easily corrected by simply increasing FPL, because the overall increase in peNDF intake due to FPL alone is relatively small when the proportion of forage in the diet is low.
Furthermore, the increase in chewing time with increased FPL was solely due to increased ruminating activity. Similar observations have been reported for AS (Beauchemin et al., 2003), corn silage (Kononoff et al., 2003b), and barley silage (Yang and Beauchemin, 2006a), whereas other studies (Kononoff and Heinrichs, 2003a; Yansari et al., 2004) showed that both eating and ruminating activities were improved by increasing the particle length of alfalfa forage or corn silage. Whether increased chewing results from ruminating alone or from both eating and ruminating seems to depend on the final particle length of the diet. Diets with relatively long particle length reduce the ease of prehension and mastication, and therefore increase eating activity in addition to increasing ruminating activity (Kononoff and Heinrichs, 2003a; Yang and Beauchemin, 2006a). For example, De Boever et al. (1993) reported that eating time increased when the TCL of corn silage was increased from 8 to 16 mm, whereas only ruminating time increased when the TCL of corn silage was increased from 4 to 8 mm.
Eating time (min/kg of DMI; r = 0.54, P < 0.04) and total chewing time (min/d; r = 0.49, P < 0.05) were positively correlated to NDF intake from forage sources but not total NDF intake (Table 7), indicating that it is primarily the NDF from long particles that affects chewing. In fact, total chewing time (min/d) and eating and ruminating time (min/kg of DMI) were more highly correlated to the particles on the 19-mm screen and dietary peNDF than to dietary NDF content (Table 7). A linear increase in chewing time (min/d, Table 5; analysis not shown) in response to peNDF content of the diet (Table 3) was also detected. These results suggest that peNDF is a better predictor of chewing activity than particle length or NDF content alone because peNDF accounts for both particle length and NDF content. Previous studies report linear increases in chewing time with increasing dietary peNDF8.0 from 10.1, 12.0, and 13.3 to 15.2% of DM for cows fed AS-based diets (Kononoff and Heinrichs, 2003a), or from 8.9 and 10.3 to 11.5% of DM for cows fed corn silage-based diets (Beauchemin and Yang, 2005).
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). However, Zebeli et al. (2006) reviewed 99 published treatment means and found that chewing index varied from 54 to 160 min/kg of NDF intake, indicating that NDF intake does not reliably predict chewing duration. Chewing index based on intake of peNDF (min/kg of peNDF) was relatively constant among diets with peNDF1.18 (114 to 127), but not with peNDF8.0 (235 to 402). However, other studies report considerable variation in chewing index based on peNDF1.18, ranging from 71 to 644 for 87 published treatment means (Zebeli et al., 2006). Therefore, chewing time is generally increased with increasing NDF and peNDF content, but the amount of chewing per unit intake is inconsistent because it is affected by other factors such as fermentability of the diet (Krause et al., 2002). Increased ruminal pH with increasing F:C ratio was consistent with the decrease in ruminal VFA concentration and the increase in chewing activity. Ruminal pH reflects the balance between acid production in the rumen and acid removal through neutralization and absorption within the rumen. Feeding the high F:C diet would have reduced the rate of acid production in the rumen because less starch was fermented in the rumen compared with feeding the low F:C diet (our unpublished data). Furthermore, the smaller meal size and slower eating rate would have helped reduce the diurnal fluctuations in ruminal pH. The increased chewing could have also contributed to increased buffering capacity within the rumen due to higher salivary secretion. Consequently, the extent of subacute ruminal acidosis was minimal for cows fed the high F:C diet and extensive (i.e., more than 10 h/d of pH <5.8) for cows fed the low F:C diet even when FPL was increased.
Increasing peNDF intake through increased FPL increased the mean and maximum ruminal pH, but the increase did not completely alleviate the acidosis in cows fed the low F:C diet. Despite similar peNDF intakes, the degree of acidosis was greater in cows fed low F:C–long AS than cows fed high F:C–short AS, indicating that increasing F:C ratio is a more effective means of reducing acidosis than increasing forage chop length. Increasing FPL increases pH but the elevation in pH for cows fed a low F:C ratio diet is not mediated through increased chewing, a change in meal patterns, or fermentability of the diet, as is the case with increased F:C ratio. Our study clearly demonstrates that the degree of acidosis in cows fed diets with similar peNDF levels but different F:C ratios varies because the ruminal fermentability of feeds is not accounted for by the peNDF concept. Thus, formulating diets on the basis of peNDF without accounting for the fermentability of the diet may not totally eliminate acidosis when diets are rapidly fermented, as was the case in this study for the low F:C ratio diet. Thus, current recommendations for particle size distributions (Kononoff and Heinrichs, 2007) appear to be adequate for high-forage diets low in starch, but these recommendations will not eliminate subacute acidosis in cows fed low F:C diets.
When examined across all diets and both means of increasing peNDF intake, mean ruminal pH was highly (P < 0.01) correlated to intake of dietary particles retained on the 19-mm sieve (r = 0.78), peNDF8.0 of the diet (r = 0.73), and intake of peNDF8.0 (r = 0.84) from forage sources (Table 7). The duration that pH remained below 5.8 was inversely correlated to the peNDF8.0 of diet (r = –0.46, P < 0.10) and intake of peNDF8.0 (r = –0.77, P < 0.01) from forage sources, indicating that increasing peNDF8.0 intake reduced acidosis. However, the correlation between chewing and ruminal pH was not significant (data not shown). Therefore, increasing the intake of long forage particles and the intake of peNDF by feeding higher forage diets of longer chop length improves ruminal pH, but the improvement is not necessarily due to increased chewing time. Long forage particles create a floating mat in the rumen that stimulates reticuloruminal contractions. Without these mixing motions the rumen becomes a less dynamic pool, and removal of VFA via absorption and fluid passage from the rumen declines, thereby increasing the risk of acidosis. Feeding long-particle fiber can also shift the site of starch digestion from the rumen to the intestine, which reduces the potential for ruminal acidosis (Yang and Beauchemin, 2006b).
Our results demonstrate the importance of forage NDF over total NDF for maintaining the ruminal pH within a healthy range. The correlation coefficient between mean ruminal pH and forage NDF content (r = 0.88, P < 0.01) was twice as high as for ruminal pH and total NDF (r = 0.41, P < 0.11; Table 7). The correlation for duration of pH below 5.8, an indication of the degree of acidosis, and total NDF was not significant, whereas its correlation with forage NDF (r = 0.75, P < 0.01) was high. Use of peNDF from forage sources further refines the prediction of acidosis. The correlation of dietary peNDF8.0 with the duration of pH <5.8 tended to be significant (r = 0.46, P < 0.07), but the correlation was much higher when forage peNDF8.0 (r = 0.76, P < 0.01) was considered. These results confirm that forage NDF and peNDF, particularly peNDF from forage sources, are more reliable methods of formulating diets to prevent ruminal acidosis than total NDF content of the diet. For example, NRC (2001) recommends a minimum of 25% NDF in the diet, with 75% of this fiber coming from forage sources (i.e., 19% NDF from forages). The recommended amount of NDF from forage sources can be decreased to as low at 15% if total dietary NDF is increased and the NFC levels are lowered from 44 to 36%. Although these recommendations help prevent ruminal acidosis, incorporating the peNDF system would result in further improvement.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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This study shows that intake of dietary peNDF, particularly forage peNDF, is a good indicator of the ruminal pH status of dairy cows. Increasing forage proportion and the chop length of forages should be considered as strategies to reduce the risk of acidosis in dairy cows. However, for low-forage diets, the increase in peNDF intake resulting from increased FPL is relatively small, and thus is insufficient to fully alleviate subacute acidosis. In that case, increasing the forage proportion can be a very effective means of reducing the risk of acidosis.
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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, 2007. Accepted for publication February 19, 2007.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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; low-long, 
; high-short, 
; and high-long, 
).