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Effects of advancing forage maturity and drought-induced summer dormancy on site and extent of digestion and microbial protein synthesis in beef steers grazing native blue grama rangeland were evaluated in four sampling periods. Five steers (avg initial wt 227 kg) fitted with ruminal, duodenal and ileal cannulae and three steers cannulated at the esophagus freely grazed a 12-ha study pasture. Sampling periods lasted 11 and started June 2, which was during the early growing season (EGS); June 22, during early summer dormancy (ESD); July 21, during late summer dormancy (LSD); and August 25, 1985, during the late growing season (LGS). Dietary N content was lower (P<.05) in ESD and LSD than in EGS and LGS. Neutral detergent fiber (NDF) content was lower (P<.05) in EGS than in other sampling periods. Ruminal organic matter (OM) digestion was lower (P<.05) in ESD than in EGS, probably because of increased dietary NDF and lower N content. Ruminal OM digestion was greater (P<.05) in LSD and LGS than in ESD because of increased fiber digestion. Neutral detergent fiber and acid detergent fiber (ADF) digestion occurring in the rumen was greater (P<.05) in LSD and LGS than in EGS and ESD. Organic matter digestion in the small intestine and OM, NDF and ADF digestion in the hindgut were similar for all sampling periods. Over 90% of the fiber digestion occurred ruminally. Nitrogen intake was lower (P<.05) in ESD and LSD than in EGS and LGS; while N flow to the duodenum increased (P<.05) from EGS to ESD, decreased from ESD to LSD and increased from LSD to LGS. Apparent ruminal N digestion was negative in ESD and LSD and near zero for EGS and LGS. Efficiency of microbial protein synthesis was lower (P<.05) in LSD than in other trials. Results indicate that decreased dietary N content resulting from drought-induced summer dormancy does not markedly alter site and extent of OM and fiber digestion; however, efficiency of microbial protein synthesis may be depressed in the later stages of summer dormancy.

Four sampling periods on blue grama rangeland in northeastern New Mexico evaluated effects of advancing forage maturity and drought-induced dormancy on dietary nutrient and botanical composition, intake, digesta kinetics and ruminal fermentation in grazing beef steers. Six ruminally cannulated and three esophageally cannulated steers freely grazed a 12-ha pasture during the study. Sampling periods lasted 11 and started June 2, during the early growing season (EGS); June 22, during early summer dormancy (ESD); July 21, during late summer dormancy (LSD); and August 25, 1985, during the late growing season (LGS). Forage availability was not limiting in any sampling period. Steers consumed a greater (P<.05) percentage in forbs and lower percentage of grasses in EGS and ESD than in LSD and LGS. Dietary in vitro organic matter digestibility was lower (P<.05) in ESD than in EGS, LSD and LGS. Dietary N content was higher (P<.05) in EGS and LGS than in ESD and LSD. Neutral detergent fiber content was lower (P<.05) in EGS than in other sampling periods, while dietary lignin contents were similar for all sampling periods. Voluntary organic matter intake was similar for all sampling periods; however, estimated gastrointestinal tract fill was greater (P<.05) in ESD and LSD than in EGS and LGS. Particulate passage rate was slower (P<.05) and total mean retention time longer (P<.05) in LSD than in other sampling periods. Rate and lag time of neutral detergent fiber digestion were not different among sampling periods. Ruminal pH was greater (P<.05) at 3 and 6 h after sunrise in ESD than in other sampling periods. Ruminal ammonia concentrations were lower (P<.05) in ESD and LSD than in EGS and LGS at 3 and 6 h after sunrise. Total volatile fatty acid concentrations were lower (P<.05) in ESD than in EGS and LSD at 3 h after sunrise and lower (P<.10) than EGS and LGS at 9 h after sunrise. Molar proportions of acetate were greater (P<.05) at 3 h after sunrise in ESD and LSD than in EGS and LGS. Changes in digesta kinetics and ruminal fermentation patterns observed in this study appeared to be related to both forage maturity and dietary botanical composition.

Two trials were conducted to evaluate effects of dietary potassium (K) concentration and lasalocid (L) in lamb finishing diets. Treatments were arranged in a 2 × 2 factorial and consisted of a 65% concentrate diet with 1.1% K (C), .9% K + 21.3 mg L/kg (CL), 2.1% K (P) and 2.5% K + 20 mg L/kg (PL). In trial 1, 64 fine-wool lambs (initial wt 35.3 kg) were randomly assigned to the four treatments (two pens of four wethers and four ewes/treatment) for a 56-d feeding trial. Average daily gain was not affected by K, but was increased (P<.10) by L (.23 vs .26 kg/d). Lasalocid tended (P<.20) to improve feed:gain ratio (5.6 vs 6.3). A K × L interaction (P<.06) was noted for feed intake, with values of 1.37, 1.45, 1.49 and 1.51 kg•head-1•d-1for the C, CL, P and PL groups, respectively. In trial 2, 16 wether lambs (four/treatment fed at 2%of body weight) were used in a site and extent of digestion and nitrogen (N) balance trial, using the same four diets as in trial 1. Site of digestion was estimated from abomasal samples taken at slaughter, with indigestible acid detergent fiber as a marker. Total tract organic matter digestibility was increased (P<.05) with added K (80.1 vs 76.4%) and L (80 vs 76.4%). Ruminal starch digestibility was not affected by added K, but was decreased (P<.10) by added L (71.8 vs 88.2%). A K × L interaction (P<.05) was observed for ruminal neutral detergent fiber (NDF) digestibility. Neutral detergent fiber digestibility in the C and CL diet was similar (48.0 vs 51.5%), while NDF digestion in the PL diet was greater (P<.10) than for the P diet (59.9 vs 37.3%). Nitrogen digestibility and N balance were similar among treatments. Ruminal ammonia concentration in samples taken at slaughter was not affected by treatments. Interactions (P<.05) of K × L occurred for molar proportions of ruminal acetate and propionate and for serum urea nitrogen (BUN). The CL diet had lower acetate and higher propionate (P<.05) than the C diet, but acetate and propionate levels were similar in the P and PL diets. The CL diet resulted in lower (P<.10) BUN than the C diet, but the PL diet resulted in higher (P<.10) BUN than the P diet. Potassium levels up to 2.5% of dietary dry matter do not have major effects on feedlot performance of lambs, but may alter digestion and fermentation patterns.

Sixteen wether lambs (37 ± 3.7 kg) cannulated at the rumen and cecum were used in a 2 × 2 factorial to study the influence of cottonseed meal supplementation and cecal antibiotic infusion on feed intake and digestive physiology. Lambs were housed in metabolism crates and adapted to a basal diet (50% prairie hay, 50% oat straw mixture; 5.8% CP) for 15 d. Half the lambs were fed 189 g · head−1· d−1of cottonseed meal. On d 16, four lambs in each diet group were allotted randomly to receive either daily cecal infusion of 2 g neomycin sulfate or an equal volume (50 ml) of .15 M saline. On d 24, Co-EDTA and Cr-EDTA were infused into the rumen and cecum, respectively. Nylon bags containing basal forage were incubated ruminally on d 25 and 26. Beginning on d 28, feces and urine were collected for 7 d. On d 29, Yb-labeled hay was dosed ruminally. In vitro cecal gas production and fecal nucleic acid concentration were decreased (P< .01) by antibiotic infusion. Protein supplementation increased (P< .01) fecal nucleic acids. Forage intake and in situ DM and NDF disappearance were not influenced by treatment; however, in vivo DM and NDF digestibilities and N retention were increased (P< .05) by protein supplementation, but not by antibiotic infusion. Ruminal fluid dilution rate was decreased (P< .05) by both protein supplementation and antibiotic infusion. Cecal fluid kinetics and passage of Yb-labeled hay were not altered by treatment. Cecal antibiotic infusion had little influence on feed intake and digestibility, but cottonseed meal supplementation increased NDF digestibility and N retention.

Six beef steers (British × Brahman) cannulated at the rumen, duodenum and ileum (avg wt 334 kg) and three mature steers (British × British) cannulated at the esophagus were used in a replicated 3 × 3 latin square design and fed no supplement (C), .5 kg soybean meal (SBM) or .5 kg steam-flaked sorghum grain (SFS)·head−1·d−1(DM basis) while grazing blue grama rangeland. Periods of the latin square included a minimum of 14 d for adaptation and 11 d for esophageal masticate collection and digesta sampling. In September, October and November, respectively, forage collected by esophageally cannulated steers averaged 74.5, 88.8 and 71.0% grasses; 2.06, 1.53 and 1.77% N and 68.3, 82.6 and 77.1% NDF (OM basis). Forage OM, ADF, NDF and N intakes were not affected (P> .10) by treatment, but total N intake was greater (P< .05) for SBM vs C and SFS treatments. No differences (P> .10) were detected among treatments in OM, NDF, ADF and N digestibilities in the rumen, small intestine or hindgut, but total tract OM digestibility was greater (P< .10) for SBM and SFS than for C, and total tract N digestibility was greater (P< .10) for SBM than for C or SFS. Duodenal ammonia N flow was greater (P< .05) when SBM was fed that when SFS and C were fed, but microbial N and non-ammonia, non-microbial N flows and microbial efficiency were not altered by treatment. Likewise, ileal N flow was not affected (P> .10) by treatment. Particulate passage rate, gastrointestinal mean retention time, forage in vitro OM disappearance and in situ rate of forage NDF digestion also were not affected (P> .10) by treatments. Ruminal fluid volume was greater (P< .05) for SFS vs SBM and C treatments, but no differences were noted in fluid dilution rate. Ruminal fluid ammonia concentration was greater (P< .05) when SBM was fed than when SFS and C were fed (13.5, 9.9 and 8.7 mg/dl, respectively), whereas pH and total VFA concentrations were not different (P> .10). Proportion of acetate in ruminal fluid was less (P< .10) for SBM and SFS than for C. Small amounts of supplemental SBM and SFS had little effect on forage intake, ruminal fermentation and site of digestion but both increased total tract OM digestion in steers grazing blue grama rangeland.

Sixteen wether lambs (37 ± 3.7 kg) cannulated at the rumen and cecum were used in a 2 × 2 factorial experiment. Lambs were adapted for 15 d to a basal forage diet (50% prairie hay:50% oat straw; 5.8% CP) or basal plus 189 g · head−1· d−1of cottonseed meal (CSM; 42.5% CP). Starting on d 16, lambs in each diet group were infused via cecal cannulas with 2 g neomycin sulfate or .15 MNaCl. On d 24, ruminal, cecal and blood samples were taken at −2, 0, 1, 3, 6, 9, 12, 16, 20 and 24 h after CSM supplementation. Supplementation increased (P< .10) serum insulin and decreased (P< .10) somatotropin, but it had no effect on serum free fatty acids. Cecal antibiotic infusion decreased serum free fatty acids (P< .05). Protein supplementation increased (P< .10) serum urea-N, and antibiotic infusion increased (P< .10) serum urea-N in supplemented, but not in unsupplemented, lambs. Supplementation increased (P< .10) cecal NH3 concentrations at 0, 6, 9, 12, 16 and 20 h, whereas antibiotic had no consistent effects. Antibiotic increased (P< .10) ruminal NH3at all times, except 12 and 20 h, in supplemented, but not in unsupplemented, lambs. Supplemented, antibioticinfused, but not supplemented, saline-infused, lambs had lower (P< .10) ruminal acetate (mol/100 mol) than either unsupplemented, saline or antibiotic-infused lambs at 3, 9 and 12 h. Supplementation with CSM increased cecal total VFA (P< .10) at all sampling times. Cecal acetate was higher and propionate lower (P< .10) at most sampling times in unsupplemented, antibiotic-infused lambs compared with lambs in other treatment groups. Cecal antibiotic infusion appeared to enhance N recycling in CSM-supplemented lambs but not in unsupplemented lambs.