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Lactating Hereford and Hereford × Angus cows, 2 to 6 yr old, and their calves were used in two trials to study the effects of energy intake on the postpartum interval (PPI) and the adequacy of the cow's milk production for calf growth. In trial 1, 23 cows were fed either 120 (high energy, HE) or 100% (adequate energy, AE) of National Research Council energy recommendations. Cow weights, calf weights and milk production were determined 1 wk after calving and every 2 wk until 17 wk postcalving. One half of the calves received creep feed from the start of the study, whereas the other half did not receive creep feed until their average daily gain was below .23 kg/d. In trail 2, 19 cows were fed energy levels of 100 (AE) or 85% (low energy, LE). Estimated percentage body fat (EPBF) was estimated twice during the study; other variables were determined as in trial 1. In trial 1, the HE cows gained more weight (P<.01) and produced more milk (P<.01). In trial 2, the AE cows gained more weight (P<.01) and produced calves 17 kg heavier (P<.01) at the end of the study than did the LE cows. Milk production was not influenced by energy treatment (P>.2). It required 7.5 kg of milk and 2.3 kg of creep feed to produce 1 kg of calf gain in trial 1. By wk 9 of lactation, milk production was not adequately supporting calf growth; however, creep feed intake was able to compensate and rapid calf growth was maintained. Increasing milk production extended the PPI whereas increasing energy intake and EPBF shortened the PPI. A multiple regression including the linear and quadratic effects of energy intake and EPBF accounted for 71% (P<.01) of the variation in PPI.

The urea dilution technique was evaluated as an estimator of body composition in 40 nonpregnant cows. The cows ranged from 3 to 16 yr of age and included 29 beef type (28 Angus and Angus crossbreds and one Hereford) and 11 dairy type (Holstein). The percentage urea space was calculated using either live weight (LW) or empty body weight (EBW). Urea space was compared with carcass fat estimated from carcass specific gravity and from chemical composition of the 9-10-11th rib section. Coefficients of determination for estimating percentage fat calculated from carcass specific gravity using urea space (LW) were .35 and .17 for the beef and dairy types, respectively, and .50 and .28 using urea space (EBW). The coefficients of determination for estimation of chemical composition using urea space (LW) were .36 and .09 and using urea space (EBW) were .64 and .41 for the beef and dairy types, respectively. Specific gravity of the 9-10-11th rib section was correlated to chemical composition (r2= .86). Live weight was correlated to chemical composition (r2= .47 and .27 for beef and dairy types, respectively). Including change in plasma urea-nitrogen (PUN) as an additional factor in a multiple regression with live weight improved the estimation of chemical composition (r2= .66 and .62 for beef and dairy, respectively). Including initial PUN as a third factor in the regression equation increased the estimate of chemical composition in the dairy, but not in the beef type. The results indicate that the urea dilution technique can be a useful estimator of body composition between groups of mature cows.

In trial 1, 16 wether Iambs were utilized in a N depletion-repletion experiment and were randomly allotted to four diets: (1) chopped smooth bromegrass-alfalfa hay, 50% of diet as fed; (2) chopped untreated wheat straw, 41.6% (WS); (3) water plus NH3-treated wheat straw, 57.8% (ENS-S), and (4) stacked NH3-treated wheat straw, 43.6% (S-S). Water (50%) and anhydrous NH3(3%) were added to the ENS-S and the product was stored in a silo-press bag. The S-S product was stacked under plastic and 3.8% anhydrous NH3was added. The rest of each diet consisted of steam-rolled barley and supplement. Nitrogen retention for the final 3 wk of repletion averaged 15.5, 7.4, 25.7 and 44.0 g/wk and dry matter digestibilities were 75.4, 66.8, 70.9 and 75.0% for the four diets, respectively. In trial 2 these diets were fed to four ruminally fistulated wethers in a Latin square, split-plot experiment. Ruminal samples were taken on d 10 and 15 of each period. Mean ruminal NH3-N values were 13.1, 8.0, 5.3 and 6.4 mg/dl with the four diets, respectively. Total VFA concentrations were 87.3, 73.5, 75.8 and 73.2 µmol/ml, respectively. Propionate concentration was higher (P<.01) in sheep fed NH3straw diets than in sheep fed WS. Similar diets were fed to calves in trial 3. Hay, WS and ENS-S were fed for 133 d and S-S for 105 d. Average daily gains for the 39 bulls and steers were 1.10, 1.04 and .91 kg with hay, ENS-S and S-S, respectively. Heifer gains were .98, .70, .84 and .65 kg/d with the four diets, respectively. Ammonia treatment of straw resulted in higher N retention, dry matter disappearance and dry matter intake.

Growth rate and individual feed consumption of 24 Jersey × Angus (J × A) and 22 Simmental × Angus (S × A) bulls were used to compare requirements for maintenance and gain during the postweaning growth period. The 82% concentrate diet contained 3.4 Meal digestible energy (DE)/kg. Periods I and II were each 84 d long for J × A and 112 d for S × A. Bulls were fed either 1.2, 1.7 or 2.2 times their estimated feed requirement for maintenance during periods I and II and full fed to slaughter during period III. After period I, the bulls from each feed level were further subdivided for the three feed levels for period II. Bulls averaged 242 d of age and weighed 221 and 240 kg at the start of period I for J × A and S × A, respectively. Averaged over periods I and II average daily gains (ADG) were .14, .66 and 1.09 kg/d for 1.2, 1.7 and 2.2 feed levels, respectively, for J × A and .26, .84 and 1.26 kg/d, respectively, for S × A. No compensatory gains were observed among bulls moved from low intakes in period I to higher intakes in period II. Weight (W) and ADG were related (P<.01) to DE, Mcal/d in period I, while W, ADG and the interaction breed-cross (B) × ADG were related (P<.01) to DE in period II. The B × W interaction effects were very small and nonsignificant in both periods. Multiple regression equations for period I were DE = 2.319 + .028 W + 10.773 ADG for J × A and DE = −1.324 + .039 W + 11.270 ADG for S × A. In period II DE = .132 + .039 W + 9.013 ADG for J × A and DE = −.787 + .033 W + 11.588 ADG S × A. Thus, at average W and ADG, DE requirements for J × A and S × A differed in both periods I and II, but maintenance requirements based on W were the same. The J × A required less feed for gain than S × A in both periods I (Pì.10) and II (ì.01).

Effects of low quality roughage, multiple feeding of supplement and various energy intakes on ruminal characteristics and beef cow performance were examined in related trials. In trial 1, four rumen-fistulated cows were fed four diets in a Latin square design. The diets were: (1) smooth brome-alfalfa hay; (2) wheat straw (WS); (3) WS + pelleted supplement fed once daily and (4) WS + pelleted supplement fed twice daily. Diet affected (P<.05) ruminal ammonia, VFA concentrations and pH. Mean ruminal ammonia was 2.7, .0, 7.6 and 14.5 mg/dl with treatments 1, 2, 3 and 4, respectively. Mean concentration of total VFA was 102, 65, 75 and 83 µmol/ml, respectively. Mean values for ruminal pH were 6.37, 7.15, 6.93 and 6.71, respectively. In trial 2, diets 1, 3 and 4 were fed to 61 dry pregnant beef cows at three energy intakes: (100) 100% TDN requirement; (80 to 100) 80% TDN requirement through d 255 of gestation period, then raised to 100% TDN and (80) 80% TDN requirement. On d 275 of gestation, cows were assigned to either 100% gestation or 100% lactation energy requirements and maintained on these diets until 30 d postcalving. Precalving average daily gain (ADG) was lowest (P<.05) for cows receiving 80% of their TDN requirement. Diet affected precalving ADG, with values of .54, .11 and .29 kg for cows fed hay, straw + supplement once daily and straw + supplement twice daily respectively. In addition to precalving ADG, hay fed cows had a lower (P<.05) weight loss 48 h postcalving, higher (P<.05) milk production 30 d postcalving, higher calf weights at 30 and 60 d and a shorter interval to conception. There were no interactions between diet type and energy intake on precalving and postcalving treatments.

Two trials were initiated to evaluate the proportions of wheat straw that could be used successfully in winter diets of dry, pregnant beef cows. In addition, barley and(or) soybean meal-urea supplements to replace alfalfa hay as a supplement of wheat straw-diets was studied. In trial 1, 92 Angus cows, 3 yr old or older, in the final third of gestation, were randomly assigned to one of the following treatments: (1) 100% alfalfa hay (17.0% crude protein); (2) 67% alfalfa hay + 33% wheat straw (3.4% crude protein); (3) 33% alfalfa hay + 67% wheat straw, and (4) 75% wheat straw + 25% barley-urea (22.6% crude protein). Average daily dry matter intakes/cow during the 100-day wintering trial were 6.8, 6.9, 6.7 and 7.4kg for treatment groups 1, 2, 3 and 4, respectively. Cows wintered on diet 1 lost less weight (P<.05) from the beginning of the trial to 48 h postcalving than cows on treatments 3 or 4. After calving, all cows were fed 11.7 kg alfalfahay/d until they were placed on spring pasture. Cows were bred by artificial insemination at each observed estrus for 90 d postpartum. There were no differences (P>.05) attributable to winter treatment in days to first estrus or postpartum interval to conception. Cows wintered on diet 2 had a higher (P<.05) conception rate than cows on diets 3 and 4. Conception rates were 81.8, 90.5, 59.1 and 64.0% for cows fed diets 1, 2, 3 and 4, respectively. In the second trial 60 Angus cows were wintered on the following treatments: (1) 67% smooth bromegrass-alfalfa hay (9.1% crude protein) + 33% wheat straw; (2) 87% wheat straw +13% soybean meal, barley, urea supplement (53.3% crude protein); (3) 33% bromegrass-alfalfa hay + 67% straw and (4) 75% wheat straw + 25% barley-urea (12.7% crude protein). The first three diets were formulated to supply 80% and diet 4 to supply 100% of the cows' TDN requirements. Average daily dry matter intakes/cow were 6.8, 6.9, 7.4 and 8.8 kg for groups 1, 2, 3 and 4, respectively. Cows wintered on diets 2 and 4 gained more (P<.05) weight precalving than cows fed diet 3. Weight loss to 48 h postcalving was greater (P<.05) for cows wintered on diet 3 than for cows fed diets 1 and 4. Cows fed diet 3 took longer (P<.05) to reach first estrus than cows fed diet 4 (50.4 vs 36.1 d). Conception rates for cows fed the four diets were 54.6, 80.6, 66.5 and 78.8%, respectively, and they were not different (P>.05). There were no differences in calf performance in either trial. Dietary crude protein did affect (P>,01) in vitro dry matter disappearance (DMD). Higher crude protein diets had higher in vitro DMD. TDN intakes calculated from these in vitro DMD were 91, 77, 64 and 75% of suggested requirements for cows fed the four diets in trial 1 and 61, 64, 57 and 86% for those fed the four diets in trial 2. Weight changes up to 48 h postcalving were related (P<.05) to apparent digestible dry matter intakes (DDM). The relationship (R2= .41) is described by the equation, weight change = -97.1 + 20.7 DDM. On the basis of these results, a daily crude protein requirement of at least .65 kg is suggested for 460 to 480 kg cows wintered on high wheat straw diets.

Two experiments were conducted to determine some of the factors that led to hypomagnesemic tetany associated with the feeding of two orchardgrass hays. Sixteen mature Columbia and Suffolk wethers (62 to 72 kg), four of which were fitted with ruminal and abomasal cannulae, were fed one of two tetany-prone orchardgrass hays or a non-tetany-prone bromegrass hay. In Exp. 1, 12 wethers were used in a completely random design metabolism experiment to measure apparent absorption and retention of macrominerals. In Exp. 2, four wethers with ruminal and abomasal cannulae were used in a completely random design experiment to monitor pre-intestinal mineral absorption and ruminal characteristics. This experiment was replicated once, with wethers remaining on their diets for 16 d in each replication. In the metabolism experiment, apparent absorption and retention of Mg as a percentage of intake were lower (P<.01 and P<.05) for one tetany-prone orchardgrass hay compared with the other orchardgrass hay. Pre-intestinal absorption of Mg in the cannulated wethers was greater (P<.01) for the orchardgrass hays than for the bromegrass hay. Pre-intestinal Mg absorption was higher (P<.01), both in terms of grams per day and as a percentage of intake for the orchardgrass hay with the highest Mg content. In the noncannulated wethers, the percentage of water-soluble Mg in the feces was lower (P<.01) for the tetany-prone hays, indicating that a decrease in Mg solubility in the intestine may have influenced Mg apparent absorption. Wethers consuming bromegrass hay showed negative Ca and P balances although Ca levels were above requirements. Plasma Mg, Ca, inorganic P and K were not different among wethers fed the three hays. Ruminal ammonia-N production was higher (P<.01) for the orchardgrass hays compared with bromegrass, and the two orchardgrass hays differed from one another. Volatile fatty acid concentrations were not different among treatments. There was a tendency for ruminal K concentrations to be higher (P<.06) in wethers consuming the tetany-prone hays, but ruminal Na concentrations were not affected by treatment. These data are interpreted to suggest that the solubility in the gastrointestinal tract may be a critical factor in mineral bioavailability for some forages.

The effect of dietary crude protein (CP) level and time of supplementation on ruminal dry matter disappearance of wheat straw (WS) diets was evaluated using beef cows fitted with ruminal and duodenal canulae. The dietary treatments were 8% (8×1) and 10% (10×1) CP with supplemental protein fed once daily and 10% (10×2) CP with supplemental protein fed twice daily. Wheat straw fed once daily was 82% of the diet. Using indigestible acid detergent fiber as the indicator, the percentage dry matter fermented in the rumen was 35.8, 37.0 and 44.5%, respectively, and was affected by diet (8×1 < 10×1, P<.05; 10×1 < 10×2, P<.001). Ruminal N digestibility was −22.5, −12.3 and 16.8% for diets 8×1, 10×1 and 10×2, respectively. Diets 10×1 and 10×2 differed (P<.05). Ruminal ammonia-N (NH3-N) measured every 3 h for 24 h was extremely low with all diets. Daily mean values differed (P<.05) and were .6, 3.4 and 1.8 mg/dl, respectively. A more uniform level of NH3-N was achieved when supplement was fed twice a day. Protein supplement fed in two equal portions 12 h apart (10×2) increased ruminal dry matter fermentation. In another experiment the effect of incremental levels of CP and supplemental energy on digestibility and microbial activity in beef cattle fed WS diets was evaluated in four cows fitted with ruminal and duodenal canulae. Three levels of dietary CP, 8, 10 and 12%, were compared. Two 12% CP diets were fed with supplement fed at 12 (low energy, 12a) or 18% (high energy, 12b) of dry matter intake. Treatments were continued after an initial 15-d period and 30 g urea was added directly to the rumen in two equal portions at 8 and 16 h after feeding to determine whether increased levels of NH3-N would stimulate microbial activity. There were no dietary or urea treatment effects on ruminal dry matter digestibility or outflow of nonammonia-N and microbial protein-N. Total tract apparent dry matter digestibility was improved (P<.01) when dietary CP content was increased from 10 to 12%. Mean dry matter digestibilities were 49.3, 49.7, 53.2 and 54.1% for diets 8, 10, 12a and 12b, respectively. Apparent ruminal-N digestion for diets without additional urea was −52.7, −19.8, −9.7 and −19.4%, respectively. Addition of urea to the rumen tended to improve N digestion (P<.06), with values of −48.1, −18.1, 3.5 and 5.1%, respectively. Ruminal NH3-N increased (P<.001) when dietary CP increased. In spite of low levels of ruminal NH3-N associated with the 8 and 10% CP diets, increasing ruminal NH3-N levels with urea did not improve dry matter digestibility or microbial protein production.

Seventy-five crossbred heifers (mean weight 304 kg) were stratified by weight and source and placed in eight pens. Four duplicate treatments were randomly assigned to pens as follows: (1) intact-control (n = 19), (2) intact + melengestrol acetate (MGA, n = 20), (3) hysterectomy (n = 17) and (4) ovariectomy (OVX, n = 19). All heifers were preconditioned for 2 mo before beginning the finishing study. Surgery was performed through a high lumbar incision approximately 3 wk before the finishing study began. Group 2 heifers were fed .4 mg•head-1• d-1MGA. Blood samples were collected at 28-d intervals throughout the 112-d finishing phase. Ovaries were collected at slaughter and carcass data were obtained 24 h after slaughter. On d 112, progesterone was higher (P<.05) in hysterectomized heifers than in other treatment groups. Serum progesterone concentration was nondetectable (<.35 ng/ml) in OVX and MGA heifers. There were no differences (P>.05) in feed efficiency or carcass characteristics among the four treatments. Hysterectomized and MGA heifers had similar (P>.05) average daily gains, which were higher (P<.15) than control and OVX heifers. These data suggest that the elevated levels of progesterone from the maintained corpora lutea and the subsequent absence of estrous activity in the hysterectomized heifers are advantageous for promoting growth in feedlot heifers.

Eight mature, pregnant cows were fed diets consisting of: (1) 63.4% mixed hay (MH) + 36.6% wheat straw (WS), 7.16% crude protein (CP); (2) 86.57% WS + 13.34% soybean meal supplement, 10.42% CP; (3) 71.34% WS + 28.57% MH, 5.29% CP, or (4) 75% WS + 25% urea supplement, 6.02% CP. Diets 1, 2 and 3 were fed at 80% and diet 4 at 100% of energy requirement in a 4 × 4 Latin square experiment. The diets supplied 121, 177, 90 and 102% of the recommended CP level, respectively. Apparent dry matter digestibilities (DMD) were determined from fecal grab samples using Cr2O3. Ruminal fluid, for determination of pH and NH3-N was obtained at 0 and 4 h postfeeding on d 16 via stomach tube. Apparent DMD, mean ruminal pH and NH3-N were: 49.5, 58.6, 44.4 and 48.1%; 7.17, 6.91, 7.28 and 7.08, and 7.55, 17.57, 4.92 and 6.25 mg/dl, for the four diets, respectively. All measurements differed (P<.05) except those of diets 1 and 4. In a second study, DMD was determined with eight 250 kg yearling steers fed 81.5% WS and 18.5% of supplements, with soybean meal varied to raise dietary CP to 11, 9.5, 8 or 6.5% in a 4 × 4 Latin square experiment. Ruminal samples obtained by stomach tube at 0, 2, 4 and 8 h postfeeding were analysed for pH and NH3-N. Apparent DMD, mean ruminal pH and NH3-N were: 48.5, 47.2, 45.4 and 42.1%; 6.7, 6.8, 6.9 and 7.0, and 6.9, 4.3, 1.9 and .7 mg/dl, respectively. All measurements differed (P<.05) from the 6.5% basal diet values. Dietary CP intakes. higher than required for nonlactating beef cows result in increased DMD and utilization of WS diets.

The efficacy of monensin as a feed additive for feeder calves wintered on dry mature pasture was evaluated. Monensin, hand-fed in a dry grain supplement, was evaluated using 40 yearling Angus steers with two replicates per treatment. Steers fed 200 mg/head/day of monensin gained faster (P<.05) than steers that received no monensin, .64 vs.57 kg/head/day, respectively. Self-feeding of monensin was evaluated using liquid supplements. Forty-eight yearling Angus steers grazing winter pasture were fed a liquid supplement ad libitum. When monensin intake was 100 mg/head/day during the first 56 days of the trial, supplemented calves gained faster (P<.05) than nonsupplemented controls. However, monensin intake dropped to 50 mg/head/day or less during the last 56 days and gains were not different for the 112-day period.

The rate of release of ammonia-N(NH3-N) from corn urea (CU) and molasses urea (MU), two gelatinized starch-urea products (GSUP) and a slow release liquid supplement (SLS) were studied in vitroand in vivoto determine if they gave a slow continual release of NH3over a 12- to 24-hr period. The rate of NH3-N release in a buffered urease solution was the same for the GSUP and CU supplements. The SLS had an initial rate of NH3-N release that was faster than MU; however, only 40% of the total N that was present was released as NH3from the SLS. Similar results were also found in in vitrorumen fluid incubations.The supplements were fed with cottonseed hull diets to 12 (trial 1) and eight (trial 2) fistulated wethers. On sample days either 50% (trial 1) or 75% (trial 2) of the normal daily supplement was placed directly into the rumen at T0.Samples for rumial NH3-N and pH in both trials and plasma urea-N (PUN) in trial 2 were collected at 0, .5, 1, 2, 4 and 8 hr after feeding. NH3-N accumulation from GSUP-2 was lower (P<.01) than NH3-N accumulation from either CU, GSUP-1 or MU. The SLS had the same initial rate of NH3-N accumulation as MU; however, a major portion of the N appeared to be tightly bound, resulting in less total NH3accumulation. Ruminal pH and PUN were lower (P<.01) when the two liquid supplements were fed, suggesting that the lower rumen pH resulted in decreased NH3absorption.

Microbial protein production and nutrient digestibility were measured using 275 kg steers, fitted with permanent rumen and abomasal cannulas, and fed low quality roughage with soybean meal (SBM) or SBM plus urea supplement. The 5 kg daily feed consisted of ground weathered range grass (nitrogen .5%) and nitrogen supplement. Urea replaced 0, 25, 50 or 75% of the supplemental SBM nitrogen. Ground milo was added with urea to make all rations isocaloric and isonitrogenous. Steers were fed hourly with an automatic feeding system to maintain a constant flow of digesta. Nitrogen retention and digestibility of dry matter (DM), organic matter (OM), and nitrogen decreased (P<.01) as urea replaced more SBM in the ration. Microbial protein production was relatively constant among rations regardless of source of nitrogen. However, the amount of total nitrogen and feed protein reaching the abomasum decreased (P<.01) as urea replaced more SBM in the supplement. Microbial protein synthesis per 100 g dry matter digested in the rumen was 9.9, 10.4, 10.9 and 11.6 g for rations containing 0, 25, 50 and 75% of the supplemental nitrogen from urea, respectively. Protem synthesis appeared to be limited by available energy and rumen turnover time rather than by nitrogen availability. Urea apparently was well utilized for the synthesis of microbial protein when fed in combination with low quality roughage at hourly intervals, but was inferior to SBM for supporting digestion of DM and OM and retention of nitrogen.

Four 350 kg steers, fitted with permanent rumen and abomasal cannulas, were fed cottonseed hulls with soybean meal (SBM) or urea supplements to study the influence of level and source of nitrogen on microbial protein production. Protein levels on a dry matter basis were 11.1, 8.5, 10.8 and 12.6% for S-100, U-75, U-100 and U-115, respectively. Total daily feed intake was 5 kg/day. Steers were fed hourly to maintain relatively steady state ruminal conditions. Mean ruminal ammonia-nitrogen concentrations were 8.1, 3.7, 12.4 and 22.2 mg NH3 -N/100 ml rumen fluid for steers fed the four rations, respectively. SBM was superior to any level of urea for total apparent dry matter (DM), organic matter (OM) and cellulose digestion. The DM, OM and cellulose digestibilities were improved by the addition of urea above the lowest level (P<.01); a negative nitrogen balance was noted with this ration, also. Microbial protein synthesis per 100 g OM apparently digested in the rumen was 18.9, 17.9, 19.9 and 19.3 for S-100, U-75, U-100 and U-115, respectively. An estimated 20% of the SBM-nitrogen escaped ruminal degradation. No significant differences were noted among rations for di-gesta passage, dilution rate or rumen turnover time. Microbial protein production per 100 g DM digested in the rumen appeared to plateau at dilution rates of 5.5 to 6% per hour.

Two trials were conducted to determine the influence of roughage level and corn processing method on the efficiency of nitrogen (N) utilization by beef steers fed high concentrate rations. In trial 1, abomasally cannulated steers were fed rations containing whole shelled corn (WSC) with cottonseed hull (CSH) levels of 0, 7, 14 and 21% and in trial 2, rations contained steam flaked (SF) and dry rolled (DR) corn with 0 and 21% CSH levels. Lignin was used as a marker for determination of digesta passage through the abomasum. In trial 1, the grams of microbial protein synthesized per 100 g dry matter fermented in the rumen (MP/DDM) were 7.5, 10.3, 11.8 and 12.7 for rations 0, 7, 14 and 21, respectively. More of the N intake bypassed the rumen undegraded (BPN) on rations 14 and 21 resulting in more (P<.05) total protein reaching the intestine (AP). In trial 2, MP/DDM for main effects, SF, DR, 0 and 21 were 7.0, 10.5, 6.3 and 11.2, respectively. N was absorbed from the rumen on all rations, but the N loss was less (P<.01) on the 21% CSH rations. BPN and AP were higher (P<.05) on the 21% roughage rations and tended to be higher for DR than SF corn.

Forty-eight Angus and Hereford cows were used in a completely random design with a 2 × 2 factorial arrangement of treatments to evaluate the effects of monensin and two sources of Mg on performance, ruminal characteristics and mineral status of mature beef cows. Cows were fed wheat straw treated with NH3during late gestation and straw treated with NH3plus bromegrass haylage after calving along with a protein supplement that contained either MgO (7.5 g/kg) or MgSO4(11 g/kg). One half of the cows received monensin (1 g/kg) in their protein supplement. During the precalving period, MgO without monensin increased (P<.01) plasma Mg compared with Mg in plasma of cows receiving MgSO4or MgO with monensin. Magnesium oxide also increased cow weight gains during late gestation (P<.05) compared with weight gains of cows supplemented with MgSO4, but magnesium source had no effect on weight gain postcalving. Plasma glucose was increased by monensin (P<.05) and by MgSO4(P<.01) during early lactation. Monensin increased the proportion of propionate before (P<.05) and after (P<.10) calving in the ruminal fluid of cows. Monensin also increased (P<.05) cow weight gain precalving and tended to decrease (P<.10) milk fat in lactating cows. Calf weight gains were not affected by treatment of dam. Magnesium oxide appeared to have higher Mg bioavailability than MgSO4for cows precalving, but Mg bioavailability was not different postcalving. Monensin increased cow weight gains precalving and improved feed efficiency postcalving.

The osmotic pressure of rumen fluid was investigated as a possible factor influencing the occurrence and numbers of rumen holotrich protozoa in sheep. Fasting osmotic pressures, measured just prior to the daily feeding, ranged from 224 to 270, mOsmoles/kg, while holotrich numbers varied between 0 and 32.4 × 103/cm3of rumen contents. These two variables were not correlated (r = 0.17). Correlations between holotrich numbers just prior to feeding and osmotic pressure at varying times after feeding were inconsistent between trials, ranging from −.72 to 0.90. Restricting feed intake tended to lower both osmolality and holotrich numbers. Inoculation with rumen contents containing both Isotrichaand Dasytricha, while on restricted feed intake, allowed establishment of both genera in all animals. When the sheep were returned to the normal feed level, both osmolality and holotrich numbers increased markedly. Fasting osmotic pressure increased significantly during water restriction; however, except for a temporary initial increase in rumen fluid osmolality after feeding, results of the water restriction experiment were quite similar to those for feed restriction. Withholding water for 27 hr. increased the average fasting osmolality from 246 to 298 mOsmoles/kg and the osmolality 1½ hr. after feeding from 393 to 416 mOsmoles/kg. Total holotrich numbers were determined 24, 96 and 168 hr. after the start of the experiment, and remained quite constant throughout. Within both the normal ranges of osmotic pressure and extremes caused by manipulation of feed and water intake, this parameter does not appear to be associated with the occurrence or numbers of holotrich protozoa.

Cheviot wethers were defaunated by using dioctyl sodium sulfosuccinate and were constantly infused with urea to provide 2.87% of the daily N intake. Defaunation resulted in higher rumen dry matter and lower rumen pH. The molar per cent propionate was higher in defaunated sheep, whereas the molar per cent butyrate and acetate was lower. Apparent nitrogen digestibility, nitrogen utilization, and nitrogen balance were higher in defaunated sheep when compared with faunated animals. Urea infusion resulted in lower apparent nitrogen digestibility, nitrogen utilization, and nitrogen balance in faunated sheep, but did not affect nitrogen metabolism in defaunated sheep. Rumen ammonia-N levels in defaunated sheep were lower than those observed for faunated animals, and urea infusion into faunated sheep increased rumen ammonia-N levels to a greater extent than did the urea infusion into defaunated animals. Significant correlations were demonstrated between rumen ammonia-N levels and C2/C3, C3/C4and C2/C4volatile acid ratios. From this it was concluded that, as rumen ammonia-N levels increased, there was a shift from propionate to higher proportions of butyrate and acetate.