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An approach was developed for describing live weight gain and the contribution of wet gut fill gain to live weight gain in growing cattle. In a continuous growth study, energy densities of winter:spring diets were used to define four treatment groups of cattle. Concentrates and forages were the major ingredients of higher and lower energy-density diets, respectively. Cattle receiving high and low energy sequences (HH and LL) were designated as control groups and were compared with two change-over groups: high to low (HL) and low to high (LH). Switches involved simultaneous changes in several feed characteristics. Every 2 wk for 4 mo, 39 heifers and 19 steers were weighed. Switching young cattle to another diet affected growth during the 2 wk immediately after the switch. After being switched to the lower energy-density diet (i.e., pasture), HL and LL groups lost (P< .001) from 3.5 to 27.4 kg in both fed and fasted weight. Neither fed nor fasted weights of HH and LH steers changed during the switch to the higher energy-density diet (i.e., concentrates); HH and LH heifers continued to gain (P< .001). Throughout the remainder of the study, growth rates of cattle were similar for HH and LH groups and for HL and LL groups. In spring, cattle consuming concentrate diets (HH and LH) had greater fill (P< .001) than pasture-fed contemporaries (HL and LL). Less frequent measurement of growth characteristics would have obscured important facets of growth. A loss of weight followed by continuous gain is not equivalent to a reduced growth rate.

Effects of ingestive mastication on physical degradation of Coastal bermudagrass hay (CBH), grazed Coastal bermudagrass (GCB) and grazed ryegrass (GRG) were studied macro-scopically, microscopically and histochemically. The masticate of each forage was collected from cattle fitted with 5-cm id esophageal cannulae and separated into fragments retained on sieves of 20 to 1,600-µm. Masticated particles passing a l,000-µm sieve made up 15.5, 51.9 and 25.8% (by weight) of the consumed CBH, GCB and GRG, respectively. Particles retained on sieves of 500, 800, 1,000 and 1,600 µm appeared as irregular sized particles of leaf and stem characterized by ends formed from totally crushed vascular bundles (VB) and edges defined by cleavage between VB. The central areas of these fragments contained many sites of rupture of the cuticle, partial fracturing of the VB and exposure of the epidermis and mesophyll. Observed only in GRG were large isolated sheets of cuticle with attached and readily exposed mesophyll or epidermal cells. Particles of 100 to 500 µm appeared to be derived largely by further reduction in VB length and longitudinal fracturing between the VB. Particles <100 µm (<10% of the masticate) were represented by epidermis, cuticle, layers of parenchyma bundle sheath cells and isolated VB. Based on their anatomy and histochemistry, they appeared higher in indigestible tissue (lignin staining) components than most larger particles. Mastication decreased particle size and thereby exposed more surface area. Additionally, the crimping and crushing action of the mastication process exposed more area of digestible tissue within a given particle size. In view of the protective nature of intact plant structures, ingestive mastication appears important in disrupting these anatomical barriers to subsequently allow increased microbial enzyme accessibility.

Samples of forage fragments were obtained from the upper (RUS) and lower (RLS) strata of the reticulorumen and feces (F) of four Brahman × Jersey steers grazing Coastal bermudagrass (CB) of two maturities with dry matter digestibilities (DMD) of 54.8 and 64.3%. Forage fragments were separated by particle size and evaluated histochemically for tissue type and fragmentation pattern. Fragmentation pattern was similar to that previously observed due to ingestive mastication. There was longitudinal separation of vascular bundles (VB) and severing at VB ends. Microscopically, similar size fragments from RUS were indistinguishable from those of RLS. The major difference between RUS and RLS was the distribution of different size particles. Larger particles were associated with the RUS in cattle consuming immature and mature CB. More large particles were associated with mature compared with the immature CB in the RUS and RLS. The distribution of different size particles in the F was similar for both maturities, suggesting that similar particle size reduction was required regardless of maturity. Smaller particles in the rumen and F appeared to contain more lignin (determined histochemically) and were composed of indigestible fragments of cuticle and lignified vascular tissue. Cattle grazing mature CB had higher ruminal fills (2.40 vs 2.02 kg dry matter/100 kg body weight), reduced rates of passage and lower voluntary intake (2.50 vs 3.14 kg DM/100 kg body weight). Lower intake of mature CB may have resulted from a reduced rate of particle size reduction. Similarities in fragmentation patterns due to ingestive and ruminative mastication were interpreted to indicate that mastication was responsible for most of the particle size reduction of CB and that mastication facilitated digestion of potentially digestible tissues.

In trial 1, Coastal bermudagrass pellets were fed for 63 d to 21 growing rams of Barbados Blackbelly (B), Dorset (D) and Blackbelly × Dorset (B×D) breeding to determine the effect of breed on voluntary intake (VI) and total gain. Initially, each breed group consisted of seven rams with mean weights of 18.6, 31.0 and 26.4 kg, respectively. At the end of the intake and growth phase, digestibility, rate of passage and gastrointestinal tract (GIT) fill were determined. Chromium-mordanted fiber was used as a particle phase marker. Intakes during the digestibility-marker phase averaged 75% of the VI determined in the intake and growth phase. Breeds did not differ in VI, total gain or digestibilities of dry matter (DM) and neutral detergent fiber (NDF). Apparent crude protein (CP) digestibility was approximately 3 percentage units higher (P<.10) for B×D than for B or D rams. Rate of passage was faster (P<.05) for B×D rams than for the other breed groups, while D rams were intermediate. Total mean retention time (MRT) was shorter (P<.05) for B×D rams compared with the other breed groups. Ruminal retention time (RRT) was shorter (P<.05) and GIT fill was less (P<.10) for D and B×D than for B rams. In trial 2, chopped orchardgrass-alfalfa hay was fed for 26 d to 19 of the same rams used in trial 1. Breed, number of rams and initial weights (kg) were: B, 5, 25.6; D, 7, 39.4; BXD, 7, 35.0. The B rams had higher (P<.05) VI than D rams. Apparent CP digestibility was highest for B×D rams (P<.05). Breeds did not differ in apparent DM or NDF digestibility. Rate of passage tended to be faster and RRT shorter for B×D rams. The B×D rams had shorter (P<.05) MRT than D rams. Under the conditions of this study, B and B×D rams were competitive with D rams with respect to VI, gain and forage digestibility. Differences in digestive tract kinetics suggest that the B and B×D breed groups could be a valuable genetic resource for sheep production in areas where environment and feed quality limit the use of British and European breeds.

Six heifers (1/4 Brahman × 1/4 Jersey × 1/2 Angus, 290 kg average weight) with ruminal, duodenal and ileal cannulae given ad libitum access to corn silage with or without 100 mg monensin•head−1•d−1were used to determine interrelationships among behavior, ruminal motility and intake. Voluntary intake was positively related (P<.001) to eating time, duration of the main morning meal and daily mastication time. Intake was negatively related to unitary times [min•g dry matter−1•(kg body weight•75)−1] of eating (P<.05), rumination (P<.001), mastication (P<.001), unitary number of rumination boli (P<.001), and latency time between termination of morning meal and onset of rumination (P<.05). Both daily and unitary eating and ruminating time were positively related (P<.001) to daily and unitary mastication time and unitary number of strong craniodorsal ruminal contractions. Positive relationships (P<.01) were found between mean duration of daily meals, main evening meal and unitary eating time, and between mean duration of rumination periods and unitary ruminating time. Daily and unitary number of rumination boli and mean duration of one bolus were positively related (P<.01) to unitary ruminating time. Interrelationships between intake, eating and ruminating activities and associated cranio-dorsal ruminal motility were all influenced (P<.01) by individual heifer, which indicates potential in identifying individuals with better than average mastication behavior and ruminal motility. Results are interpreted to suggest that fast eaters and fast-efficient-ruminators appear to have highest intake capabilities.

Eating and ruminating behavior and associated ruminal motility of six heifers (1/4 Brahman × 1/4 Jersey × 1/2 Angus, 290 kg average weight) given ad libitum access to corn silage with or without 100 mg monensin·head·−1·d−1were examined according to a two-period crossover design. There was no effect (P>.05) of monensin on level of intake, daily and unitary eating, ruminating and masticating times [min·g dry matter−1·(kg body weight.75)−1], duration or number of these activity periods, duration of main meals or latency time for onset of rumination following cessation of main eating activities. With the monensin treatment, daily numbers of normal boli and total boli were decreased (P<.05) and mean duration of one rumination bolus cycle was longer (P<.05). Analysis of covariance indicated relationships between intake of corn silage and duration of the main morning meal, duration or number of rumination boli and total ruminal contractions were affected (P<.01) by monensin. Frequency and unitary number of strong cranio-dorsal ruminal contractions were similar for both treatments. During eating, number of contractions per minute was about twice (2.55/min) that during idling and rumination activity (1.43/min and 1.22/min, respectively). The unitary daily number of contractions was negatively (P<.05) related to level of intake. Total daily ruminal contractions were slightly reduced (−3.96%, P>.05) by monensin. Results are interpreted to suggest that monensin indirectly affects rumination through a lowered motility and thereby affects turnover, gut fill and intake.

Effects of diluting the energy content of a corn-soybean meal diet with either alfalfa meal or corn cobs on nutrient digestibility and rate of passage of feed residues and particle markers were measured in crossbred (Yorkshire × Landrace × Chester White × Large White) barrows with a mean body weight of about 80 kg. The excretion pattern in the feces of Cr-mordanted diet and of rare earths initially bound individually to the mixed diet or to the corn or soybean meal suggested a model having a single age-dependent compartment with time delay. The compartmental turnover rate parameter (λ1) estimated by this model did not differ for the rare earths individually used to mark the corn-soybean meal diet (Yb), the corn (La) or the soybean meal (Sm). In contrast, λ1for Cr was smaller (P<.001) than that of the mean for the three rare earths. The residence time due to displacement flow (τ) did not differ among markers. These results were interpreted to indicate that the high specific gravity of Cr-mordanted feed slowed flow due to mixing but not due to displacement. Correlations between λ1and τwere <.71. These results suggested that the flow of rare earths initially bound to feed ingredients provides a reasonable estimate of the flow of their undigested residues through the gastrointestinal tracts of nonruminant animals. Inclusion of the fibrous feeds reduced digestibility of dry matter, cell contents, crude protein and acid detergent lignin and increased digestibility of cell walls, cellulose and acid detergent fiber. Digestibilities of cellulose and acid detergent fiber were greater with alfalfa than with corn cobs as the fiber source. Differences in digestibility of crude protein and acid detergent fiber existed due to litter in one replicate of the experiment. Variation in digestibility was not significantly related to variation in λ1or τwithin or among treatments and litters. This suggests that variations in λ1and τ were not important causes of the observed variation in digestibility.

Diet selection and mastication by cattle (Bos taurusL.) can be examined using extrusa from esophageal fistulas. Experiments were conducted to investigate the use of an esophageal plug and the influence of a 16‐h fast, prior to collection, on the particle size distribution and quality of masticated forage. Extrusa was freeze‐dried and sieved into seven size‐classes, and the weight of masticate in each class was determined. The proportions of large (>1.7 mm), medium (0.5–1.7 mm), and small (<0.5 mm) particles and the mean and median particle sizes were analyzed. Neutral detergent fiber (NDF) and in vitro dry matter disappearance (IVDMD) were also determined. In both experiments, significant differences between single pastures of Coastal bermudagrass [Cynodon dactylonL. (Pers.)] with 3500 kg ha−1and flaccidgrass (Pennisetum flaccidumGriseb.) with 2900 kg ha−1available forage were found in particle size, NDF, and IVDMD. Averaged over experiments, the proportions of large, medium, and small particles found in bermudagrass were 20, 54, and 26%, respectively, and 35, 47, and 18%, respectively, for flaccidgrass. The NDF of bermudagrass and flaccidgrass averaged 550 and 505 g kg−1, respectively, and the IVDMD averaged 640 and 700 g kg−1, respectively. Neither the esophageal plug nor the fasting had an influence on the particle size distribution, NDF, or IVDMD.

Understanding the dynamics or kinetics of digestion and passage may suggest ways to manipulate forages to improve the diet, and consequently the production, of domesticated ruminants. The methodology of fitting exponential curves was tested using simulated cell‐wall‐disappearance curves with both an exponential and a gamma‐distributed exponential driving function and normal‐random variation superimposed. Nonlinear regression gave more precise estimates of the true rates of digestion and lag times than linearization. The gamma distributed exponential gave lower residual sum of squares but estimates of digestible and indigestible fractions as well as lag times and disappearance rates were not improved over the simpler exponential function. Estimates were less precise when the gamma‐distributed exponential was the driving function for the disappearance curve and the gamma function was skewed. The use of the exponential decay constant (k) in studies of cell‐wall composition is confounded with extent of disappearance as a result of plant anatomy. A surface area fermentability index (SAFI) is proposed as (kCd)/(1000 − Ci) where Cdand Ciare the digestible and indigestible fractions, respectively, of the cell wall. The SAFIwas used to adjust literature values for kas well as to partially explain observed negative correlations between extent and rate of cell‐wall disappearance (k) in bromegrass (Bromus inermisLeyss.) that had been selected for high and low in vitro dry matter disappearance. Study of in vivo kinetics is complicated by variable rate of passage, particle size, and extent and rate of particle size reduction. Models of in vivo kinetics offer an opportunity for synthesis of understanding of the animal system. However, the many interactions make interpretation and incorporation of in vitro data into models of in vivo ruminant digestion difficult.

Coastal bermudagrass (Cynodon dactylon [L.] Pers.) hay was fed in the long form to four ruminally cannulated steers to evaluate the effects of intake level on the physical reduction of feed particles during ingestive mastication. The experimental design was a 4 × 4 latin square with 18-d periods. Treatments based on previous intakes were set at 50, 70, 90 and 110% of feed consumed per animal and fed at 12-h intervals. Boli of ingested, masticated hay were collected at the cardia and separated by wet-sieving into fragments retained on sieves of .0027 to 4.0 mm (mesh aperture). In addition, particles retained on the 4.0-mm and 2.0-mm sieves were scanned by image analysis. Ingestive mastication resulted in a linear decrease (P< .05) in the proportion of boli particles retained on the top (4.0-mm) sieve and linear increases on the 2.0- (P< .02), 1.0- (P< .06) and .25-mm (P< .06) sieves with increasing level of intake. Sixteen to 21% of the ingested hay particles passed through a 1.0-mm sieve. Mean particle size, as determined by sieving data, decreased linearly (P< .05) as feeding level increased. Image analysis showed no differences in length (L) or width (W) of particles retained on the 4.0- and 2.0-mm sieves as feeding level increased, but L/W decreased linearly (P< .05) on both sieves. The frequency of jaw movements following feeding was very similar across feeding levels and averaged 71.3 chews/min. Results indicate that ingestive mastication is an efficient process and suggest that, during eating, feed is chewed to a similar size for bolus formation and ease of deglutition.

Effects of feeding level on chewing behavior and physical reduction of particles during ingestive mastication and rumination were examined in four ruminally cannulated steers fed Coastal bermudagrass (Cynodon dactylon[L.] Pers.) hay in the long form. The experimental design was a 4 × 4 latin square with 18-d periods. Treatments based on previous intakes were set at 50, 70, 90 and 110% of feed consumed per animal. Steers were fed at 12-h intervals. Chewing behavior was monitored over 4 d with a computerized system. Boli of ingested, masticated hay (MH), collected at the cardia, and serial samples of digesta taken from the ruminal upper strata (RUS) were wet-sieved. Dimensions of MH particles retained on 4.0- and 2.0-mm sieves were determined by image analysis. Increasing feeding level resulted in a linear increase in the number of boli ruminated (P< .01) and in linear increases in eating (P< .01), ruminating (P< .05) and total chewing time (P< .01). Linear increases in number of eating (P< .01), ruminating (P< .01) and total chews (P< .01) also were observed. Feeding level had no effect on bolus duration, number of chews/bolus, number of boli/min rumination time and number of chews/min rumination time. The extent of particle breakdown during ingestive mastication was a determinant of the number of boli ruminated daily and their associated number of chews. Number of boli ruminated daily also was related to the RUS mean particle size. Eating time did not provide a reliable indication of how well feed was chewed. Results suggest that under certain feeding and dietary conditions, eating time could inflate the chewing index (total chewing time/kg DM intake) of feeds.

Effects of time of day and dietary monensin on the distribution of size of digesta particles in different digestive tract sites and their intersite relationships were examined in six heifers (290 kg BW) with ruminal, duodenal and ileal cannulas given ad libitum access to corn silage, with or without 100 mg monensin-head−1d−1, in a two-period crossover design. Ingestive masticate and digesta of corn silage were collected via esophageal, ruminal or intestinal cannulas. The distribution of particulate matter retained on sieves with apertures larger than 20 µm was determined by wet-sieving. The cumulative distribution of particulate matter on a series of sieves was regressed on retaining sieve aperture to estimate the sieve aperture that would retain 50% weight of the particulate matter (median retaining aperture, MRA). The MRA of masticate was 6,494 µm. The MRA of digesta particles decreased (P< .05) from ventral rumen (1,847 µm) to dorsal rumen (1,797 µm) to duodenum (346 µm), but increased to the rectum (359 µm). The MRA was lower (P= .044) for the monensin treatment only in feces. The MRA of particulate matter in the dorsal and ventral rumen, duodenum and rectum all changed (P< .05) over 24 h. An inverse pattern between the MRA of ruminal and duodenal digesta occurred, presumably the result of a nycterohemeral pattern of eating and ruminating activity. Across sampling times, an inverse relationship existed between MRA of ventral rumen and duodenal digesta. This relationship suggests that a ruminal digesta raft composed of larger particles (immediately following major meals) is more effective than a raft of smaller particles (prior to such meals) in preventing flux of large particles to the duodenum.

Two forms of corn, whole (WC) and ground (GC), were fed with hay (WH; 900) or 4 h after hay (AH; 1300) feeding to evaluate their effects on growth, starch utilization and digesta passage characteristics in growing steers. Twenty-four Angus steers with ad libitum access to fescue hay were individually limit-fed ground or whole corn (2.27 kg/d) in a 2 × 2 factorial arrangement of treatments: GC- WH, GC-AH, WC-WH and WC-AH. Average daily gain was improved with feeding GC (P< .08), especially when fed at the same time as hay. Fecal starch (%) was less (P< .001) for GC (7.07) than for WC (15.68). No difference was observed in rate of liquid or solid (hay) passage. Mean retention time (MRT) of whole corn (stained with neutral red and crystal violet) was decreased (P< .07), and rate of corn passage tended to be faster for steers fed WC-AH than for those fed WC-WH. Although time of corn feeding had no effect on performance, steers fed WC-AH had more whole corn particles recovered from the feces and the MRT for whole corn particles was reduced, indicating that time of supplementation may affect passage characteristics. Altering time of supplement feeding may have applications in other feeding situations, such as with protein supplementation where ruminal escape would be advantageous.

Coastal bermudagrass hay was labeled with Cr by the Cr-mordant procedure and with 177Lu applied to the same fiber. Neutral detergent fiber prepared from the same Coastal bermudagrass hay was labeled with Yb, 169Yb, Tb and 160Tb by soaking overnight following by thorough washing and drying. Wood chips were similarly labeled with Sm or La, and Solka Floe was labeled with 147Nd and 141Ce. The carriers, labels and times of administration to cattle were: bermudagrass fiber with both Cr and 177Lu, bermudagrass fiber with 169Yb and Solka Floe labeled with 147Nd at 0 h; bermudagrass fiber with Yb, Solka Floe with 141Ce and wood chips with Sm at 24 h; wood chips with La at 48 h; and bermudagrass fiber labeled with 160Tb at the beginning and labeled with Tb at the end of a meal. Fecal collection followed and passage characteristics were determined with a two-compartment, age-dependent model. Markers labeling the different fiber sources had different (P< .01) passage rates (Solka Floc > Coastal bermudagrass > wood chips), but there was no difference within fiber source for rare earth passage. There also was no difference between the passage characteristics of Cr-mordant and 177Lu. However, passage rate of particles administered at the beginning of the meal (160Tb) was 42% higher than for particles at the end of the meal (Tb). These results indicate that the flow characteristics of rare earths applied by the soak and rinse procedure were the same as those of the more strongly bound Cr-mordanted fiber and that the kinetics of passage can be affected by the time of dosing relative to the consumption of the meal.