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Digestive processes of dairy cows fed silages harvested at four stages of grass maturity¹

MTT Agrifood Research Finland, Jokioinen, Finland

² Correspondence:
FIN-31600 Jokioinen (phone: +358 3 4188 3660, fax +358 3 4188 3661, E-mail:
marketta.rinne{at}mtt.fi).

	Abstract

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The objective of this experiment was to quantify ruminal digestive processes that could help to identify factors limiting DMI when silages differing in grass maturity were fed to dairy cows. Four silages were harvested at 1-wk intervals from a primary growth of a timothy-meadow fescue sward, resulting in feeds with digestible OM content in DM (D-value) of 739, 730, 707, and 639 g/kg in the order of succeeding harvest date. Four ruminally cannulated dairy cows were given ad libitum access to these silages supplemented with 7 kg concentrate per day in a 4 x 4 Latin square design. Rumen function was clearly affected by decreasing digestibility of silage fed. Passage rate of digestible NDF (DNDF) and indigestible NDF (INDF) increased, but it could not prevent the accumulation of DM, NDF, DNDF, and INDF into the rumen when silages of progressing grass maturity were fed. The greatest proportional increases in rumen pool were found in INDF and in medium particles (separated by wet sieving and measuring 315 to 2,500 µm). The passage of medium INDF particles decreased (P < 0.01) linearly (from 0.0365/h to 0.0281/h) with increasing maturity of grass ensiled, and it was slower than passage of small (80 to 315 µm) particles (on average 0.0524/h). Particle size reduction of large INDF particles to medium INDF particles was slower (P < 0.001) in the early cut silages (0.0216/h to 0.0484/h) but reduction of medium INDF particles to small INDF particles was faster (P < 0.001) in early cut silages (0.0436 to 0.0305). Passage of medium size particles and(or) rate of medium particle breakdown to small particles were potential intake-constraining properties of low digestibility forages, whereas large particle reduction to medium particles seemed not to be limiting. The increased feed intake of the early-cut silages was accompanied by decreased rumen fill, suggesting that rumen fill was not at least solely responsible for feed intake control.

Key Words: Dairy cows • Digestibility • Digestion Kinetics • Feed Intake • Particle Size Reduction • Rumen

	Introduction

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In dairy cow diets, voluntary silage intake has regularly increased with increasing digestibility of silage. Thus, physical control of feed intake seems to play an important role even in these circumstances. Huhtanen (1994) reported that silage DMI increased by 0.151 kg per 10 g/kg increase in the concentration of digestible organic matter in DM (D-value) derived from eight experiments, which resulted in a 0.30-kg increase in daily milk output. The rumen fill has, however, decreased in connection with increasing digestibility of silage (Gasa et al., 1991; Bosch et al., 1992), indicating that rumen capacity is not fully utilized when high digestibility forages are fed. The physical and metabolic satiety factors may actually not be mutually exclusive, but additive (Forbes, 1995; Ellis et al., 1999).

Feed intake and digestibility depend on rates of digestion, particle passage, and particle size reduction in the rumen. Feed particles in the rumen can be divided into non-escapable and escapable pools (Allen and Mertens, 1988). In an earlier study, Rinne et al. (1997a) found that the transfer of particles from the nonescapable to the escapable pool was prolonged when cattle were fed silages from progressively more mature grass. The results were obtained from fecal excretion of Yb and calculated with a two-pool model with gamma time dependency in the first pool. In the present experiment, we studied the particle size distribution and particle size reduction of ruminal digesta from cows fed similar diets to elucidate their potential role in the delayed transfer of particles to the escapable pool.

The objective of the present study was to identify possible feed intake-limiting changes in the digestive processes in the rumen when lactating cows were allowed to consume silages differing in harvest date on an ad libitum basis. Preliminary results of the present experiment have been presented by Rinne et al. (1996). The same silages were fed in a milk production trial, which has been reported earlier (Rinne et al., 1999b).

	Materials and Methods

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Diets, Animals and Design of the Experiment
Four silages were harvested with a flail-harvester from a primary growth of a mixed timothy (Phleum pratense var. Iki) and meadow fescue (Festuca pratensis var. Kalevi) grass on June 13, June 21, June 28, and July 4, 1994, in Jokioinen, Finland (60°49'N, 23°30'E). The sward was practically pure of other plant species and the proportion of timothy in the herbage was 0.59, 0.54, 0.54, and 0.60 in the order of harvest date. The silages were preserved in bunker silos with a formic acid-based additive (4.0 L formic acid/1,000 kg fresh grass). The same silages were used in a milk production experiment, and details of silage production were reported by Rinne et al. (1999b). The silages are referred to as I, II, III, and IV in the order of harvest.

The experimental silages were offered to four ruminally cannulated Finnish Ayrshire cows, which were, on average, 295 (SE = 11.6) d into their third lactation and nonpregnant in the beginning of the experiment. The silages were fed in a balanced 4 x 4 Latin square design with 21-d periods. Cows were housed in individual stalls equipped with a gate to the feeding table. The gate was closed daily from 0830 to 1230. Silage refusals of 10% of the daily consumption were allowed. Silages were supplemented with 7 kg (as fed) of a concentrate mixture daily. Concentrates were offered in two equal meals at 0530 and 1230. The concentrate contained crushed barley, crushed oats, molassed sugarbeet pulp, rapeseed meal, and minerals (31.8, 31.8, 16.0, 16.4, and 4.0%, respectively).

The cows were managed according to Finnish legislation documented in the order of using vertebrate animals for scientific purposes (1076/85) implemented under the auspices of the local animal use and care committee.

Experimental Procedures and Calculations
Feed intake and milk yield were recorded and feeds sampled as reported in Rinne et al. (1999b). The diet apparent digestibility was measured by using AIA as an internal marker. Fecal grab samples (approximately 200 g) were taken on the last 5 d of each period at 0700 and 1500. A single dose of 12 g of CoEDTA (Udén et al., 1980) diluted in 0.5 L of water was introduced into the rumen of the cows at 0930 on d 18 of each period. Later that day, ruminal fluid was sampled before feeding at 1230 and thereafter six times at 1.5-h intervals. The pH of the ruminal fluid was measured immediately; the filtered samples were acidified with H₂SO₄ for ammonia determination and treated with HgCl₃ + NaOH for VFA determination before freezing and storage at -20°C. Samples for Co determination were centrifuged, and the liquid was diluted with a 2.25 N HNO₃ + HCl solution containing 1 mg/mL K as KCl. Rumen liquid outflow rate was calculated as a slope of the regression of the natural logarithm of the Co concentration against time. Rumen protozoa were counted using a hemocytometer from a pooled sample by each cow and period. The samples were stored in formalin.

The rate and extent of ruminal NDF degradation of the four experimental silages was assessed by nylon bag incubations in situ. Fresh chopped (2 cm) silage samples were incubated in nylon bags (external dimensions 60 x 120 mm, pore size 38 µm, open surface area 28%; Swiss Silk Bolting Cloth Co., Zurich, Switzerland) for 3, 6, 12, 24, 48, 72, and 96 h in the rumen of the four cows. During incubations, the cows consumed a nonexperimental grass silage as a basal feed, and the proportion of concentrate in diet DM was 40%. After removal from the rumen, the bags were washed in cold water for 25 min using a household washing machine and dried at 60°C for 48 h. The NDF concentration was determined by incubating the bags for 1 h in boiling NDF solution and washing and drying the bags as previously described. The disappearance values of NDF were fitted to the nonlinear equation of Ørskov and McDonald (1979). Metabolic fecal OM in relation to DM intake was calculated as (fecal OM - fecal NDF)/kg DM intake.

Rumen evacuations were conducted on d 19 of each period at 1530 (3 h after feeding) and on d 21 at 1230 (prior to feeding). The times were chosen to represent the minimal and maximal rumen fill. Rumen contents were emptied manually through the cannula, mixed thoroughly, and sampled. To collect samples of silages after primary mastication, the cows were offered the experimental silages on d 21, when the rumen was empty. In 12 of 16 cases the cows started to eat silage, and ingestive masticate was manually collected through ruminal cannula directly after it entered the rumen. Finally, the total digesta were returned into the rumen.

The fractional rates (k, per hour) of intake, passage and digestion were calculated as follows:

Fecal output of NDF was used to represent the ruminal outflow of NDF. Some digestion of NDF occurs also in the lower digestive tract and it may vary for the different dietary treatments. The role of hindgut fermentation of NDF is, however, rather small and certain methodological problems are associated also with duodenal sampling, so this method was considered adequate for the purposes of this experiment.

Meaningful rate constants can only be calculated for homogeneous pools so that digestible NDF (DNDF) and indigestible NDF (INDF) were separated for these calculations. Concentration of INDF was determined by a 288-h rumen incubation of samples in nylon bags with a pore size of 6 µm and open surface area of 5%. This small pore size was used to minimize particle loss. The DNDF concentration was calculated as NDF - INDF. For INDF, only k_p is presented, because by definition it is indigestible. Fecal output was estimated from DM digestibility. Total fecal DNDF and INDF concentrations were derived from results from particles of different sizes, because a larger particle loss from nylon bags was found for total fecal samples. This could be caused by drying and grinding (6-mm sieve) of the total fecal samples prior to incubation, whereas the particles of different sizes were incubated fresh.

The particle size distribution of fresh silages, concentrate mixture, ingestive masticate, ruminal digesta (separately for the two sampling times), and feces was determined with a Retsch AS200 Digit wet sieving apparatus (Retsch GmbH, Haan, Germany) using sieve apertures of 2,500, 630, 315, 160, and 80 µm. Two replicates of 60 g fresh weight were sieved for each sample. Samples were sieved for 10 min using a water flow of 3.5 L/min. After sieving, samples from each sieve were quantitatively collected to preweighed nylon bags (pore size 38 µm) and dried at 60°C for 48 h. A single nylon bag of each sample was incubated in the rumen of a dairy cow for 288 h prior to a NDF determination to measure the potential digestibility of NDF. The other replicate was directly subjected to NDF determination. The proportion of different-sized particles was calculated based on the total particulate matter retained (i.e., excluding DM passing the 80-µm sieve). The mean particle size was calculated similarly to that reported in Ahvenjärvi et al. (2001).

We did not use a sieve measuring between 1,000 and 2,000 µm in pore size, which has commonly been used in the literature (see Lechner-Doll et al., 1991), because in an earlier study conducted at our Institute (Ahvenjärvi et al., 2001), little DM was retained on a sieve measuring 1,250 µm (e.g., 1.3% of DM in ruminal digesta), whereas the other sieves were as currently used. On average, 3% of fecal particles measured >2,500 µm, and these particles are defined as the "large" particles (L). To simplify the results, particles collected from 630- and 315-µm sieves (i.e., measuring 2,500 to 315 µm) were combined to yield the "medium" size particles (M), and particles collected from 160 and 80 µm sieves (i.e., measuring 315 to 80 µm) were combined to yield the "small" particles (S). Rates of passage were then calculated separately for L, M, and S.

The rate of particle size reduction (k_r) was calculated only for INDF, because for DNDF, k_d and k_r cannot be distinguished from one another. The k_r from L to M was calculated as k_i of L - k_p of L. For M, k_i included the material entering directly with feed, and from L through particle size reduction (Figure 1).

View larger version (24K): [in this window] [in a new window]   Figure 1. Schematic presentation of calculation of the rates of intake, passage, digestion, and particle size reduction.

Chemical and Statistical Analyses
Chemical analyses from feeds, ruminal fluid, digesta, and feces were made using procedures reported by Huhtanen and Heikkilä (1996). The D-values are based on the determined OM digestibility in sheep fed at maintenance level (Rinne et al., 1999a) and calculated as OM concentration (g/kg) x OM digestibility (g/kg)/1,000. Cobalt was analyzed from the ruminal fluid with an atomic absorption spectrophotometer (Bodenswerk Perkin-Elmer GmbH, Ueberlingen, Germany) (Williams et al., 1962).

The data were analyzed with an analysis of variance (GLM procedure of SAS; SAS Inst. Inc., Cary, NC) with cow, period, and diet in the model. For ruminal fermentation and evacuation data, the results were analyzed by a split-plot analysis of variance with sampling time as a subplot. Because very few time x treatment interactions were found, results averaged over sampling times are presented in the tables, and relevant interactions are reported in the text. Sums of squares for treatment effects were separated using orthogonal contrasts into single degree of freedom comparisons of linear, quadratic, and cubic effects of timing of grass harvest. The probabilities of these statistical significances are expressed as P_L, P_Q, and P_C. Generally, cubic effects were not found to be significant except for ruminal fermentation, so that they are not presented in the tables.

	Results

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The experimental silages are characterized in Table 1. The concentration of CP decreased 2.8, NDF concentration increased 7.6, and D-value decreased 4.8 g/kg DM per day with delayed harvest. The changes were, however, nonlinear for NDF and the D-value. The greatest change in NDF concentration was between harvests II and III (14.2 g/kg DM per day) and in D-value between harvests III and IV (11.3 g/kg DM per day). The quality of all the silages was good, as indicated by low pH (mean 4.03) and ammonia N concentration (mean 43 g/kg N).

The decline of particle size as the feed proceeded through the digestive tract is presented in Figure 2. No grass maturity effects were found in the particle size distribution of intact and masticated silages (data not shown). Diet effects on particle size in different parts of the digestive tract and ruminal pool sizes of differently sized particles are presented in Table 7. The median particle size clearly decreased as digestion progressed (1,909, 1,479, and 341 µm averaged over treatments for ruminal digesta 3 h after feeding, prior to feeding, and in feces, respectively). Dietary effects on mean particle size were detected in ruminal digesta; particle size decreased (P_L < 0.01) at both sampling times the later the silage was harvested, but mean particle size of fecal samples was similar on all diets. The pool sizes of ruminal digesta remained similar for L and S (P > 0.10), but M increased (P_L < 0.001, P_Q < 0.05) and DM not retained on the sieves decreased (P_L < 0.05) with advancing maturity of silage. No sampling time x diet interactions were found in the pool sizes of particles retained on different sieves.

View larger version (49K): [in this window] [in a new window]   Figure 2. Proportion of large (> 2500 µm), medium (< 2500 µm but > 315 µm), and small (< 315 µm but > 80 µm) particles in feeds, in silage after ingestive mastication (masticate), from rumen 3 h after feeding (Rumen A) and before feeding (Rumen B), and in feces of dairy cows (averaged over all diets).

Results from feeding behavior of the cows are presented in Table 9. Time spent ruminating and in total mastication increased absolutely (P_L < 0.05) and per kilogram of DMI (P_L < 0.001) with increased maturity of grass ensiled, but they remained constant (P_L > 0.05) when expressed per kilogram of NDF intake. When expressed per kilogram of INDF intake, times spent ruminating and in total mastication decreased (P_L < 0.001). The efficiency of INDF particle size reduction both from L to M and from M to S increased markedly (P_L < 0.001) with postponed harvest of grass. The values increased most between harvests III and IV, leading to a quadratic effect (P_Q < 0.05 from L to M and P_Q < 0.01 from M to S).

	Discussion

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Ruminal Fermentation
The changes in ruminal fermentation pattern caused by progressive development of ensiled grass were similar to those noted in many earlier studies; the proportion of acetate increased and that of butyrate decreased, but changes in the proportion of propionate were less consistent (for more detailed discussion, see Rinne et al., 1997b). There are some indications of more intensive lactate formation during fermentation in early-cut silages. Intake of lactate has led to increased proportion of propionate in the rumen (van Vuuren et al., 1995). In this experiment, silage lactate concentration and proportion of propionate in the rumen were closely correlated (R² = 0.90). The decrease in the proportion of butyrate with progressing maturity of ensiled grass was correlated with a decline in the protozoal population (R² = 0.98) in accordance with results of Rinne et al. (1997b), suggesting a strong relationship between them.

Feed intake by ruminants may be restricted not only by physical but also by acidic, chemical, and(or) osmotic load of the rumen (Forbes, 1995). These loads have obviously been greater when early-cut silages were fed, because the rate and extent of digestion of them was greater. The average pH of ruminal fluid was lower and the VFA concentration higher the earlier the silages fed were harvested. The lowest dietary mean of pH was observed 4.5 h after the 1230 feeding, being 5.82, 5.87, 5.85, and 6.07 for the silages in the order of harvest date.

Rumen Contents
The changes in grass associated with progressing stage of growth have led to increasing rumen contents of animals consuming such feeds (Aitchison et al., 1986; Bosch et al., 1992; Rinne et al. 1997a), in accordance with the results of the present study. When animals are offered ad libitum access to feeds, decreased feed intake is often observed simultaneously, suggesting that rumen fill limits feed intake of low- but not of high-digestibility forages.

Rumen fill may not be the only factor limiting intake. Intake may be an integrated outcome of different physical and metabolic signals (Forbes, 1995; Ellis et al., 1999). If rumen fill had been the only factor limiting intake in the present experiment, the cows consuming the more digestible silages should have been able to increase their intake clearly more than was observed, because they had lower rumen fill than cows feeding on the less-digestible silages.

The contribution of different digesta components to the fill effect may not be similar. In the following discussion, changes in the pool size of different components of the rumen contents are compared to identify the components, which responded mostly to changes in feed quality. Rumen contents of fresh matter, DM, NDF, and DNDF were moderately increased with increasing maturity of grass ensiled (1.12, 1.10, 1.28, and 1.14 greater pool on diet IV than on diet I, respectively), but the pool of INDF increased much more (1.60). Accumulation of indigestible fiber in the rumen has also previously been reported with declining digestibility of forage fed (Aitchison et al., 1986; Gasa et al., 1991; Rinne et al., 1997a), suggesting that it may be an important intake-limiting factor.

The pool of large particles in the rumen has increased when low-digestibility forages were fed (Pond et al., 1987). This conclusion is very susceptible to the definition of large particles. Bosch et al. (1992) and Bosch and Bruining (1995) reported that the proportion of large particles was greater when less-digestible forages were fed, when a sieve mesh of 1,250 µm was used as the divisor. In contrast, the proportion of large particles was smaller when a sieve mesh of 5,000 µm was used as the divisor. In the present experiment, the DM pool of L did not respond to postponed harvest of grass, but the pool of M did (1.02, 2.25, and 1.19 times greater pool on diet IV than on diet I for L, M, and S, respectively). It seems that with progressing maturity of grass, the proportion of large particles and small particles + soluble DM are affected less, but the proportion of medium particles increases. In the present data, the most profound increase in rumen contents was found in INDF of M (2.61 times greater pool on diet IV than on diet I).

Digesta Kinetics
The rate of NDF digestion decreased linearly with postponed harvest of grass measured both in situ and with rumen evacuations. A marked discrepancy was found in the level of values obtained with these methods. Applying the rate of NDF digestion determined in situ would result in unrealistically low estimates of ruminal NDF digestibility, as discussed in Rinne et al. (1997a). Our results are based on the assumption of no postruminal digestion of NDF (Rinne et al., 1997a), which could cause slight underestimation of k_p and overestimation of k_d. However, unless the proportion of total-tract NDF digestibility occurring in the rumen was affected significantly by treatment, the current treatment trends shown in Table 8 should represent actual trends satisfactorily.

The liquid outflow rate has generally not been affected by decreasing forage digestibility, although passage of particulate matter has increased (Gasa et al., 1991; Bosch et al., 1992; Bosch and Bruining, 1995). However, the increase of k_p was not large enough to prevent the accumulation of material in the rumen and(or) the decrease in feed intake, as was also observed in the present experiment.

Maturity of ensiled grass had different effects on k_p of differently sized particles. Basically, no L escaped the rumen, but passage of M was slower the later the grass was harvested, which caused the accumulation of M in the rumen. The passage of S was not significantly affected by dietary treatments.

The increase of k_p with decreasing particle size in the present experiment and in data reported by Poppi et al. (1980), McLeod et al. (1990), and Huhtanen et al. (1993a) support the use of particle size as a rough estimate of the escapability of particles from the rumen. The k_p of S was still clearly slower than that of liquid outflow rate, indicating that some mechanism, possibly capture by larger particles in the rumen, assists them in preventing passage from the rumen.

The potential digestibility of NDF in different particle size groups changed curvilinearly in the present data (Figure 3) as well as in data sets of McLeod et al. (1990) and Huhtanen et al. (1993a, b), but Pond et al. (1987) and Ahvenjärvi et al. (2001) found that the proportion of potentially digestible material declined successively the smaller the particles were. Higher potential digestibility of NDF in L than in M is in accordance with the theory that, during particle breakdown and microbial digestion, particle functional specific gravity and, subsequently, probability of escape from the rumen, increases. The high potential digestibility of NDF in S is, however, deviates from this. These particles might originate from the more digestible tissues of the grass or from the concentrate used. The NDF concentrations of L, M, and S averaged over all diets and both sampling times were 894, 879, and 700 g/kg DM, suggesting that S may differ from L and M in origin or may include metabolic comtamination (protozoa).

View larger version (29K): [in this window] [in a new window]   Figure 3. Potential NDF digestibility of large (> 2500 µm), medium (< 2500 µm but > 315 µm), and small (< 315 µm but > 80 µm) particles of ruminal digesta 3 h after feeding (a) and prior to feeding (b), and of feces (c) of dairy cows fed silages harvested at four stages of grass maturity (date of harvest for I = June 13, II = June 21, III = June 28, and IV = July 4).

Particle Size Reduction
The concept of critical particle size (the size that particles need to reach before they can escape from the rumen) has not proven very useful, because, generally, the majority of the rumen particulate matter is below the indicated size (Poppi et al., 1980; Ulyatt et al., 1986; Lechner-Doll et al., 1991). The escapability of the particles from the rumen seems to be a function of specific gravity or, inversely, buyonancy (i.e., from the digestibility of the particles rather than the size). The particle size reduction may still play a significant role in facilitating the access of microbial enzymes to potentially digestible substrates within the feed particles, which is a prerequisite for increasing functional specific gravity (Mertens, 1993; Kennedy and Doyle, 1993).

The delayed transfer of particles from the nonescapable pool to the escapable pool with increasing maturity of ensiled in the data of Rinne et al. (1997b) seems not to have been limited by the particle size reduction of L. Because the particles actually leaving the rumen are much smaller than the "critical size," the reduction of M to S may also constrain DMI. According to the results of the present experiment, k_r from L to M was faster but k_r from M to S was slower the later the grass was harvested. Hence, k_r from M to S seems to be a potential constraint to feed intake in low-digestibility forages. On the other hand, low k_r from L to M could limit DMI of the cows receiving high-digestibility silages, which in terms of rumen capacity could have been higher.

The largest sieve we used retained 0.953 and 0.822 of intact and masticated silage DM, respectively. This distribution did not allow the calculation of the mean particle size for these samples. The mean particle size values for ruminal digesta were slightly underestimated based on subjective comparison of material retained on different sieves, but values for fecal samples corresponded reasonably well to the observed particle size distribution. With this method, a marked decrease in mean particle size of ruminal digesta was detected with increasing maturity of grass ensiled at both sampling times.

The faster particle size reduction of L in more mature forages may be explained by greater fragility or brittleness of the more lignified particles. Grenet (1989) found fewer large particles (on a DM basis) in boluses of cattle fed late-cut rather than early-cut ryegrass (Lolium multiflorum), and Wilson et al. (1989) observed that particle size reduction of leaves from less-digestible tropical grass (Panicum maximum var. trichoglume) declined more in size during mastication than more-digestible temperate grass (Lolium multiflorum) leaves. Poppi et al. (1981) reported that 12-wk regrowths of tropical grasses were more prone to particle size reduction than 6-wk regrowths.

Concomitantly with increasing maturity of grass, the morphological composition of herbage changes dramatically. The proportion of leaves and stems in the ensiled herbage was not determined in the current experiment but was estimated from similar material (correlation between D-value and proportion of leaves in timothy; Rinne et al., 1999b); the proportion of leaves was approximately 0.60, 0.57, 0.50, and 0.29 in silages I to IV, respectively. Leaf and stem fractions differ clearly in chemical composition as well as in digestion characteristics. They may also contribute differently to different particle size groups in the rumen (Kennedy and Doyle, 1993). Stems were more resistant than leaves to particle size reduction (Poppi et al., 1981 using tropical grasses; McLeod and Minson, 1988 using ryegrass). However, according to Wilson and Kennedy (1996), breakdown of stems vs leaves and tropical vs temperate grasses was greater in an in vitro device mimicking ingestive mastication.

The particle size distribution of feces can be used to represent the particle size of particles leaving the rumen, because very little, if any, particle size reduction takes place in the lower tract (Deswysen et al., 1989; Ahvenjärvi et al., 2001). The equal particle size distribution in feces reveals that particle size distribution of digesta flowing from the rumen was similar across the diets. In some cases cattle have been able to increase the particle size of feces in response to decreased digestibility of grass (McLeod and Minson, 1988; Bosch et al., 1992), but Pond et al. (1987) found no compromises in fecal particle size consistent with the results of the present study.

Ingestive and ruminative mastication are the major processes responsible for particle size reduction. Absolute mastication times and time spent masticating per kilogram of DMI have increased when low-quality forages have been fed, but, if expressed per kilogram of NDF intake, mastication times have been relatively constant (Aitchison et al., 1986; Bosch et al., 1992; Beauchemin and Iwaasa, 1993), consistent with the results of the present study and those obtained from the milk production experiment utilizing the same silages (Rinne et al., 1999b). The efficiency of particle size reduction in INDF particles (g) reduced in size per hour spent masticating increased dramatically with advancing maturity of grass silage. The previous speculations on origins and properties of different fractions may have contributed to it.

	Implications

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Passage rate of medium-sized particles (defined as smaller than the critical size to passage) and further degradation rate of them into small particles decreased with increasing maturity of grass fed, so that they are potential constraints to intake. The degradation of large particles (greater than the critical size) to medium particles did not seem to limit intake of low-digestibility forages. Because rumen fill increased but metabolic load decreased with decreasing quality of silage fed, neither physical nor metabolic intake regulating factors seem to have solely controlled feed intake in the present experiment, but rather it may have been a combination of both. Understanding the mechanisms controlling DMI are necessary to develop intake prediction models.

	Footnotes

 
¹ We gratefully acknowledge the contribution of Terttu Heikkilä during harvesting of silages and that of Kaisa Kaustell to feeding behavior observations and calculations. We thank Aino Matilainen and the staff at the Experimental Stables of MTT for careful processing of the samples.

Received for publication September 18, 2000. Accepted for publication January 18, 2002.

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