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Agriculture and Agri-Food Canada, Research Centre, Lethbridge, AB, T1J 4B1, Canada
Corresponding author: Karen A. Beauchemin; e-mail: beauchemin{at}agr.gc.ca.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: physically effective NDF • chewing • rumen pH • dairy cows
Abbreviation key: CS = corn silage, FPL = forage particle length, pef = physically effectiveness factor, peNDF = physically effective NDF, PSPS = Penn State Particle Separator containing 2 sieves (19 and 8 mm) and a pan, TCL = theoretical cut length
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The concept of physically effective fiber (peNDF) was introduced to account for the physical characteristics of NDF (primarily particle size) that affect chewing activity (saliva secretion). This concept is based on the hypothesis that the fiber in long feed particles (>1 cm) promotes chewing and saliva secretion, which helps neutralize the acids produced during ruminal digestion of feeds. The fiber that promotes chewing is considered physically effective. The peNDF content of the diet can be determined by multiplying the NDF content of the diet by its physical effectiveness factor (pef). Although various methods are available to measure particle size of diets, the Penn State Particle Separator (PSPS) has become widely accepted as a quick and practical method for routine use on farm to evaluate particle size of forages and TMR (Lammers et al., 1996). Using the PSPS, a particle distribution is determined from 3 fractions: proportion of particles retained on the 19.0-mm sieve, proportion of particles that pass through the 19.0-mm sieve but are retained on the 8.0-mm sieve, and proportion of particles that pass through the 8.0-mm sieve (Lammers et al., 1996). The pef (ranging from 0 to 1) is calculated as sum of the proportion of particles retained on both 19.0 and 8.0-mm sieves.
Although a number of studies using the PSPS technology have been conducted in recent years to determine the effects of dietary peNDF on feed intake, chewing time, and ruminal pH (Yang et al., 2001; Krause et al., 2002; Beauchemin et al., 2003; Kononoff et al., 2003; Plaizier, 2004), the results obtained in these studies are not conclusive. In some studies, increasing intake of peNDF increased chewing activity but there was no effect on rumen pH for corn silage- (CS) (Kononoff et al., 2003) or for alfalfa haylage-based diets (Kononoff and Heinrichs, 2003a). Some studies (Yang et al., 2001; Kononoff and Heinrichs, 2003b) showed that peNDF was a poor predictor of chewing time and rumen pH. In contrast, Beauchemin et al. (2003) reported that the peNDF was a reliable indicator of chewing activity and subclinical ruminal acidosis.
The objectives of the present study were to determine the effect of increasing the peNDF concentration of a diet containing CS on feed intake, chewing activity, and ruminal pH and fermentation in lactating dairy cows. The peNDF concentration of the diet was increased by using CS differing in particle length and the peNDF content of the diets was determined using the PSPS.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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. These analyses were conducted commercially by Cumberland Valley Analytical Service, Inc. (Maugamsville, MD).
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Cows were offered 1 of 3 diets, which were chemically identical with approximately 58% of concentrate and 42% of CS (Table 2) but differed in peNDF level: high, medium, and low. The 3 dietary peNDF levels were obtained using CS differing in particle length: 100% long silage (high), 25% long silage + 75% medium silage (medium), and 100% fine silage (low). The diets were formulated using the Cornell-Penn-Miner System (CPMDairy, Version 2.23; Cornell University, Ithaca, NY; University of Pennsylvania, Kennett Square, PA; and William H. Miner Agricultural Research Institute, Chazy, NY) to supply adequate metabolizable energy and metabolizable protein for a 600-kg cow producing 35 kg/d of milk with 3.5% fat and 3.2% protein. Each period consisted of 11 d of adaptation to diets and 10 d of experimental measurements.
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). The DM content was analyzed by oven drying at 55°C for 48 h. The samples were then composited by experimental period and retained for determination of chemical composition.
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Chewing Behavior
Cows were fitted with leather halters for 72 h that were equipped to measure jaw movements during d 12 to 15 of the period. Each halter contained a piezo disk (Edmund Scientific Co., Barrington, NJ), which was inserted within the halter and positioned under the jaw. 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 until retrieval. The jaw movements were then designated as eating or ruminating chews. Jaw movements were considered eating chews if they occurred during a meal (i.e., duration of the eating activity was greater than 30 s and more than 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 associated with licking, drinking, and grooming, and 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 chewing activity. In the event of electrical or mechanical damage, corrupted data were deleted and measurements were made at a later time.
Ruminal pH and Fermentation
Ruminal pH was monitored for 72 h concurrently with measurements of chewing and eating behavior during d 12 to 15 of the period. The pH was measured using an industrial probe (model PHCN-37, Omega Engineering Inc.) that was modified for use in the rumen environment. The probe was anchored approximately 60 cm from the electrode, minimizing the opportunity for the electrode to relocate itself in the reticulum. Probes were weighted to ensure submersion within the rumen contents; however, the extent of their movement within the rumen was not determined. Thus, it should be recognized that these measurements of rumen pH might not correspond to manual measurements of pH taken from precise locations within the rumen. The probes were covered with a mesh guard to prevent them from coming into direct contact with the rumen wall. Continuous measurements from the indwelling probe were sent to a datalogger (Campbell Scientific, Inc.) every 5 s and were averaged every 5 min. Ruminal pH data were summarized daily for each cow as mean pH, minimum pH, maximum pH, area under the curve, area between the curve and a horizontal line at pH 5.8 or 5.5, and time (h) under pH 5.8 or 5.5. The area was calculated by adding the absolute value of negative deviations in pH from 5.5 or 5.8 for each 15-min interval. In the event of equipment malfunction all corrupted data were deleted and measurements were repeated at a later time during the period. The ruminal pH 5.8 was chosen as benchmark because a pH below 5.8, but above 5.0, is often used to indicate subclinical ruminal acidosis and the incidence of subclinical acidosis increases when ruminal pH falls below 5.8.
Ruminal fluid was collected on d 13 at 1000, 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.
Rate of Passage
Passage rate of digesta from the rumen or the postruminal tract was measured using Cr-mordanted NDF and CoEDTA as forage and liquid markers, respectively. Fiber from long, medium, and fine chopped silage was separately prepared by washing in a washing machine twice with detergent and then boiled for 4 h in diluted detergent solution until the NDF content of the material exceeded 85%. Methods used to mordant Cr to plant cell walls and to prepare CoEDTA were those of Udén et al. (1980). Two hundred fifty grams of Cr-mordanted NDF and 300 mL of solution containing 15 g of CoEDTA were introduced in the rumen via the ruminal cannulas. Fecal samples were collected from the rectum at 0, 6, 9, 12, 15, 18, 24, 30, 36, 48, 72, 96, and 120 h after dosing with the markers. A double compartmental model represented by 2 exponential constants and a time delay (Grovum and Williams, 1973) was used:
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where Y = marker concentration (mg/kg); A = scale parameter; k1 = ruminal rate of passage (%/h); k2 = lower digestive tract rate of passage %/h); t = sampling time post-dosing (h); and TT = transit time. Total mean retention time in the digestive tract was calculated as the sum of retention in the rumen (1/k1) and in the lower digestive tract (1/k2) plus the transit time (TT).
Data were analyzed by nonlinear regression using the NLIN (iterative Marquardt method) procedure of SAS (SAS Institute, 1996).
Chemical Analyses
Ingredient (silage, grain, concentrate mix) and diet DM was determined by oven-drying at 55°C for 48 h. Analytical DM content of the dietary and fecal 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 µ-linked glucose polymers as described by Rode et al. (1999). Contents of Cr and Co in the samples were determined using inductively coupled plasma optical emission spectroscopy according to the AOAC method (1990) modified such that no calcium chloride was used during sample digestion. Content of N in the samples was determined according to the method of Smith and Tabatabai (2004) by flash combustion, chromatographic separation, and thermal conductivity detection (Carlo Erba Instruments, Milan, Italy). Ruminal VFA were separated and quantified by gas chromatography (Varian 3700; Varian Specialties Ltd., Brockville, ON, Canada) using a 15-m (0.53-mm i.d.) fused silica column (DB-FFAP column; J and W Scientific, Folsom, CA). Ammonia content of ruminal samples was determined using the method described by Weatherburn (1967) modified to use a plate reader. Particle size distributions of CS, TMR, and orts were determined using the PSPS (Lammers et al., 1996). Physical effectiveness factors for silage, TMR, and orts were calculated as the sum of the proportions of the materials retained on the 19.0- and 8.0-mm sieves of the PSPS. The peNDF of the diets was determined by multiplying the pef of the TMR by the NDF content (DM basis) of the diet.
Statistical Analyses
Data were analyzed using the mixed model procedure of SAS (Proc Mixed; SAS Institute, 1996) to account for effects of square, period within square, cow within square, and treatment. The treatment was considered a fixed effect; square, period within square, and cow within square were considered random effects. Estimation method was restrictive maximum likelihood (REML) and the degrees of freedom method was Kenward-Rogers. Data for meal, chewing activity, and ruminal pH were summarized by day and then analyzed using the same mixed model but with day included as a repeated measure and using compound symmetry. 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 as a fixed effect and period as a random effect. Linear and quadratic orthogonal contrasts were tested using the CONTRAST statement of SAS. 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 < 0.15.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). Consequently, pef was linearly decreased with rechopping. However, reduction of pef was more pronounced by chopping the original silage once to produce the medium silage (pef was reduced by 14%) than by further rechopping to produce the fine silage (pef was reduced by 7%).
For the diets offered, the proportion of particles on the first and second sieves, and pef linearly decreased because of decreasing particle length of CS (Table 4). The pef of the TMR (range of 0.28 to 0.35) was significantly lower than that of the CS (range of 0.67 to 0.84) because nearly 60% of TMR was concentrates that mostly passed through the 8.0-mm sieve. The peNDF value of the TMR linearly decreased with decreasing particle length of CS and reflected the pef of the TMR.
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Intake, Chewing Activity, and Meal Patterns
Intakes of DM and NDF were not affected by dietary peNDF (Table 5). Intake of peNDF followed the same trend as pef contents of the diets and linearly decreased with reducing dietary peNDF.
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). Number of eating and ruminating chews per unit of NDF was not affected by dietary peNDF content. Pattern of diurnal eating activity of cows was similar among the treatments (Figure 1a). The highest eating activity was observed after the 1500 h feeding followed by activity at the 2 other feeding times (0800 and 1800 h). Eating activity was also high at 0600 h, which was the feeding time of these cows before the experiment started. Eating time (min/d) was linearly (P < 0.08) affected by dietary peNDF with highest eating time for cows fed the medium peNDF and lowest eating time for cows fed the low peNDF diet. As expected, duration of ruminating (min/d, P < 0.10) and duration of total chewing (min/d) were linearly increased with increasing dietary peNDF. In contrast, when expressed as minute per unit of peNDF, total chewing time (P < 0.10), as well as eating (P < 0.07) and ruminating time (P < 0.12) tended to decrease with increasing dietary peNDF.
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). However, duration of meals was not significantly affected by dietary peNDF. Reducing dietary peNDF did not affect amount of DM per meal but linearly reduced amounts of NDF (P < 0.13) and peNDF consumed per meal. Intake rate (g of DM/min) was similar among the treatments. Diurnal variation in amount of feed consumed by cows generally followed the pattern of eating time (Figure 1). An exception was observed for cows during the period from 0100 to 0300 h, in which feed intake was higher for cows fed the low peNDF diet.
Rate of Passage
Rumen liquid dilution rate and rumen solid passage rate averaged 10.9 and 3.1%, respectively, and were not affected by dietary peNDF (Table 7). No effects of the treatments were observed for mean retention time of liquid or solids in the total tract, although unexpectedly, postruminal transit time of solids numerically increased with decreasing dietary peNDF.
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). Ruminal pH was also lower (P < 0.15) after the 0800 and 1500 h feedings for the low peNDF diet compared with the medium peNDF diet. Mean rumen pH, area under pH 5.8 or 5.5, and duration of pH < 5.8 or pH < 5.5 were not affected by dietary peNDF (Table 8). The highest pH was 6.29 and lowest 5.01, but no differences were observed across treatments.
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). The same effect was observed for molar proportion of propionate, which increased from 28 to 32 mol/100 mol, but the opposite was observed for molar proportion of butyrate, which decreased from 12 to 10 mol/100 mol, and for acetate to propionate ratio, which reduced from 2.04 to 1.69 with reducing dietary peNDF. There was no effect of dietary peNDF on ammonia concentration in the rumen. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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When measured using the PSPS with 2 sieves (19 and 8 mm), the pef content of barley grain-based diets is mainly dependent on the proportion of forage in the diet and its particle length, because processed barley-based concentrates mostly pass through the 8-mm sieve. Nevertheless, changes in peNDF content of this barley-based diet by altering forage particle length (FPL) were not as great as expected. For instance, an increase of one percentage unit in pef of CS increased peNDF content of the diets by only 0.13 units (Table 4). The peNDF values (range of 8.9 to 11.5%) of the diets offered in the present study were lower than those (range of 20 to 22%) reported by Kononoff et al. (2003) because a higher proportion of CS (57.4%) was used in that study.
Sorting in favor of long feed particles was observed in the present study. Furthermore, the finer the diet, the more pronounced the effect. The present findings are in contrast to the reports by Calberry et al. (2003) and Leonardi and Armentano (2003), who observed that cows actually selected against coarse feed particles in favor of fine feed particles when offered alfalfa hay or alfalfa silage-based diets. The discrepancy among these studies may be attributed to the coarser particles used in those studies than in the present study. The peNDF contents of diets ranged from 20.1 to 23.3% in the study of Calberry et al. (2003). In fact, Plaizier (2004) did not find a greater proportion of particles on the 19- and 8-mm PSPS screens compared with the TMR when peNDF contents of the diets ranged from 9.2 to 12.5%. These findings suggests that dairy cows may intentionally select long feed particles to meet their need for physically effective fiber, especially when ruminal pH is low (Table 8).
The lack of effect of dietary peNDF on intake of DM and NDF is consistent with the results for passage rate from the rumen and mean retention time in the total tract for liquid and solids (Table 7). The present results are in agreement with some studies (Bal et al., 2000; Beauchemin et al., 2003) but in contrast to other studies in which positive effects on DMI and NDF intake from reduced particle size have been reported (Schwab et al., 2002; Kononoff et al., 2003). Feed intake is influenced by digestion rate and passage rate of feeds from the rumen. Lack of effect of dietary peNDF on passage rate of feeds out of the rumen was likely due to increased chewing activity (i.e., number of chews and duration of chewing) with increasing dietary peNDF. Hence, particle size reduction was not a rate-limiting step for particulate passage. Furthermore, effect of dietary peNDF on DMI may depend upon forage level or type of concentrate in the diets of dairy cows. Belyea et al. (1989) reported that decreased FPL increased intake of cows fed only alfalfa hay, but FPL had no effect on intake of cows fed a diet containing alfalfa hay (60% of diet, DM basis) plus concentrate because ruminal fill was not a limiting factor for DMI. There was no effect of dietary peNDF on DMI for diets containing about 40% of forage (Beauchemin et al., 2003; present study), whereas an effect on DMI was observed for diets containing about 60% forage (Schwab et al., 2002; Kononoff et al., 2003).
Although effect of CS particle length on chewing behavior has been investigated in a number of studies, there are no studies to our knowledge that investigated effect of dietary peNDF measured using the PSPS on chewing activity for diets based on CS and barley grain. Increased (P < 0.10) number of chews per unit of DM during the eating period with increasing dietary peNDF was in agreement with the observation of Kononoff and Heinrichs (2003a) for cows fed an alfalfa silage-based diet. In contrast, the effect was not observed for cows fed CS-based diets (Kononoff and Heinrichs, 2003b). As the number of chews per unit of DM during ruminating was not affected in the present study, cows efficiently reduced long particles by increasing chewing rate (i.e., chews/min) during eating. The chewing rate was linearly increased from 54.4 to 58.2, and then to 61.9 for high, medium, and low peNDF diet, respectively. The results may partially explain the similar passage rates of solids out of the rumen across treatments.
In the present study, increased chewing time due to increased eating and ruminating time with increasing dietary peNDF was consistent with the reports of Krause et al. (2002) and Yang et al. (2001). However, lack of effect of dietary peNDF on chewing time per unit of DM and NDF was in contrast to other reports (Beauchemin et al., 2003; Kononoff et al., 2003), and may be attributable to the linearly increased number of chews with increasing dietary peNDF. These results suggest that increased dietary peNDF may affect chewing activity either through prolonging chewing time or increasing chewing rate. Dietary peNDF content was moderately correlated with number of chews during eating (r = 0.41, P < 0.09) and with total chewing time (r = 0.39, P < 0.11) (Table 9). In addition, it was reported that the particles >19.0 mm may be a primary factor affecting chewing activity in dairy cattle fed diets containing alfalfa silage (Kononoff and Heinrichs, 2003a) or CS (Kononoff et al., 2003). However, in the present study there was no correlation of proportion of particles >19.0 mm with chewing activity because the proportion of particles >19 mm was similar after adjustment of particle size of orts, but proportion of particles >8.0 mm was moderately correlated (r = 0.64, P< 0.01) to chewing time. The correlation between particles >8.0 mm and chewing time is in agreement with previous observations that the fraction retained on the second (i.e., 8-mm) rather than the top (i.e., 19.0-mm) sieve of the PSPS was correlated (r = 0.56) to chewing time (Yang and Beauchemin, 2002). The discrepancy in correlation of particle fractions of the PSPS with chewing activity between the studies of Kononoff et al. (2003) and ours is likely due to the differences in particle distribution. In fact, in the studies of Kononoff and Heinrichs (2003a) and Kononoff et al. (2003), the proportion of particles >19.0 mm was increased, but the proportion of the particles >8.0 mm was decreased with increasing FPL. The present results indicate that increasing peNDF content is an effective means of stimulating chewing activity, whereas the effect of individual particle fractions of the PSPS on chewing activity may depend on the contribution of each particle fraction to the pef.
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It is commonly believed that increasing particle length promotes chewing activity and thus increases buffering capacity within the rumen due to increased saliva secretion. Hence, chewing time has been measured as an indirect indication of the potential of the diet to maintain high rumen pH. Unfortunately, this concept is not consistently supported by published results. No effect of FPL on rumen pH was reported (Schwab et al., 2002; Kononoff and Heinrichs, 2003a, b) for CS-based diets although chewing time was affected by FPL. However, those studies only measured mean ruminal pH, which does not reflect the extent of variation in pH among cows or the extent of diurnal fluctuations in rumen pH for individual cows. Several studies using continuous indwelling electrodes demonstrated that although mean ruminal pH was similar, duration of pH below 5.8 was reduced as particle length of alfalfa silage was increased (Yang et al., 2001; Krause et al., 2002). Beauchemin et al. (2003) further reported that decreased alfalfa FPL reduced mean ruminal pH by only 4%, but area of the curve under pH 5.8 and duration of pH < 5.8 were almost tripled or doubled, respectively. The incidence of subclinical acidosis increases when ruminal pH falls below 5.8. Beauchemin et al. (2003) observed that diets with higher peNDF concentrations resulted in higher ruminal pH values and longer rumination time. In the present study, increased chewing activity due to increased dietary peNDF did not significantly change ruminal mean pH or the diurnal pH variation. The correlation of particle fractions or peNDF concentration with ruminal pH was not significant (Table 9). However, it is worthwhile noting that ruminal pH was numerically higher (P < 0.15) for the period from 1400 to 1700 h for cows fed the high peNDF diet than for cows fed the low peNDF diet (Figure 1). The higher (P < 0.15) pH was likely associated with longer (P < 0.15) eating time (Figure 1). The increase in saliva output during eating could enable the cow to buffer the large quantity of fermentation acids produced soon after the feed is consumed. This is particularly beneficial for barley grain-based diets because barley grain is rapidly degradable in the rumen following ingestion.
Rumen pH is directly affected by the factors that lead to the production of fermentation acids. Therefore, the inconsistent effects of FPL on rumen pH reported in the literature may be related to other dietary factors including DMI, fermentability of the diet, and feeding management. A poor relationship (r2 = 0.13) between rumen pH and intake of fiber whether measured as NDF, NDF from forage, or effective NDF was reported in numerous publications in which dairy cows were fed a variety of diets (Beauchemin, 2000). In a study (Yang et al., 2001) to investigate the effects of grain processing, fiber content, and FPL on rumen pH and digestion, we observed that rumen pH was significantly affected by the grain processing but not by FPL. It is suggested that additional dietary factors that could affect rumen pH must be considered when determining the requirement of dietary peNDF by dairy cows. In fact, the concept of peNDF does not account for differences in fermentability of feeds, and does not predict differences in chewing and rumen pH due to grain fermentability.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The Penn State Particle Separator is a useful device that can be used on the farm to measure particle size of forages and TMR. The values obtained using this device can be used to determine the physical effectiveness of fiber, which is a good indication of the rumination potential of the feed.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication September 16, 2004. Accepted for publication December 20, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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; medium, •; and low, 
) on diurnal variation of eating time, amount of feed consumed in each hour, and rumen pH. Arrows indicate feeding times.