Performance data on nearly 16,000 head of cattle that were used in trials to document effects of monensin on feedlot cattle were summarized. Cattle fed monensin-containing diets gained 1.6% faster, consumed 6.4% less feed and required 7.5% less feed/100 kg gain than cattle fed control diets. Monensin resulted in the greatest improvement in feed/gain at 2.9 Mcal metabolizable energy (ME)/kg diet dry matter (DM). Within the range of monensin concentrations used in the trials that were summarized (31.8 +/- 7.5 mg/kg DM), high monensin concentrations did not improve feed/gain over that obtained with lower concentrations. Carcass characteristics were not significantly influenced by monensin. Responses of cattle to monensin and implants were additive. Energy metabolism data suggested that monensin improved digestibility of DM, reduced fasting heat production and increased dietary net energy maintenance (NEm) values more than it increased net energy gain (NEg) values. Data showing the response of cattle to monensin when fed various dietary protein concentrations or sources of supplemental N suggested that monensin had a protein sparing effect. Monensin has also been shown to reduce lactic acid production, aid in the control of coccidia and bloat and to be toxic to face and horn fly larva in feces of monensin-fed cattle. In pasture trials, monensin improved daily gains. When fed to beef cows, monensin reduced amounts of feed required to maintain cow weight.

Objectives of this study were to determine the influence of trenbolone acetate (TBA) and estradiol (E2) in a combined implant on feedlot performance, carcass characteristics, and carcass composition in finishing steers. Sixty-four large-framed (394.1 kg) crossbred steers were randomly assigned to one of four pens. Subsequently, pens were randomly assigned to one of two treatments, implanted (120 mg of TBA and 24 mg of E2) and nonimplanted. Eight steers/treatment were slaughtered for initial carcass composition. Remaining steers were assigned to one of three serial slaughter dates (d 40, 115, or 143). Implantation increased circulating trenbolone (TBOH) and E2 concentrations throughout the trial. Implantation increased ADG 18% (P < .001) during d 0 to 40, 21% (P < .001) from d 0 to 115, and 16% for the entire 143 d. Implant status had no effect (P > .05) on dry matter intake. Feed efficiency was improved 13% during d 0 to 40 (P < .01) and from d 41 to 115 (P = .07). Longissimus muscle area was larger (P < .05) in implanted steers than in nonimplanted steers on d 115. Carcasses from implanted steers had a smaller (P < .05) percentage of kidney, pelvic, and heart (KPH) fat on d 143 than those from nonimplanted steers. Carcasses from implanted steers possessed more carcass protein (P < .05) on d 40. Implanted steers had an 82% increase (P < .05) in daily carcass protein accretion during the first 40 d. Implantation increased (P < .01) carcass water but did not affect carcass fat accumulation throughout the feeding period. The combined TBA+E2 implant improved feedlot performance and stimulated carcass protein accretion in feedlot steers.

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Objectives of this study were to analyze alterations in circulating IGF-I and insulin-like growth factor binding protein (IGFBP) concentrations due to administration of a combined trenbolone acetate (TBA) and estradiol (E2) implant. This study was part of a larger serial slaughter study in which 64 large-framed (394.1 kg) crossbred steers were randomly assigned to one of four pens. Pens were assigned to one of two treatments: implanted (120 mg of TBA and 24 mg of E2) and nonimplanted. After d 2, 24 steers/treatment remained on the study. These steers were assigned to one of three serial slaughter dates (d 40, 115, and 143). Blood samples were obtained on d 0, 2, 21, 40, 115, and 143 from remaining steers. Serum was harvested and analyzed for IGF-I, IGFBP, and mitogenic activity. Glycyl-glycine (GG) extraction of serum was performed to reduce IGFBP interference in the IGF-I RIA. Implantation with TBA+E2 interference in the IGF-I RIA. Implantation with TBA+E2 increased (P < .001) circulating IGF-I concentrations during the period from d 0 to d 40. On d 21 and 40, steers implanted with TBA+E2 had 16 and 22%, respectively, greater (P < .001) circulating IGF-I concentrations than nonimplanted steers. For steers in the study for at least 115 d, TBA+E2 increased (P < .05) IGF-I concentrations 9, 13, and 19% on d 21, 40, and 115, respectively, compared with nonimplanted steers. Implantation with TBA+E2 resulted in greater (P < .05) serum concentration of a 49/39-kDa IGFBP (IGFBP-3) on d 21 and 40 after implantation. Sera from steers implanted with TBA+E2 stimulated proliferation of cultured muscle satellite cells to a greater extent (P < .05) than did sera from nonimplanted steers on d 21, 40, 115, and 143 after implantation. In summary, TBA+E2 increased serum concentrations of both IGF-I and IGFBP-3. Additionally, implantation increased mitogenic activity of sera from implanted as compared to nonimplanted steers. These alterations may be partially responsible for the positive effects of TBA+E2 implants on feedlot performance and rate of protein accretion in steers.

Twenty Angus-Hereford and 20 Angus-Holstein cows were individually fed 12.9 or 18.0 Mcal metabolizable energy (ME)/head daily from November 28, 1979 through February 21, 1980. Energy retentions for the winter feeding period were calculated by determining body composition at the initiation and at the termination of the feeding period. After the experimental period, all cows were managed the same through weaning. Maintenance energy requirements (Kcal/d) estimated from linear regressions of energy retentions on ME intakes per unit body weight (BW).75 were 127.6 and 140.3 kcal/BW.75 for Angus-Hereford and Angus-Holstein cows, respectively. Estimates of maintenance energy requirements for thin and fat cows within each breed type indicated that fatter cows of Angus-Hereford breeding had 6.1% lower energy requirements than thinner cows. Opposite trends occurred with Angus-Holstein cows, where fat cows had 2.7% higher maintenance requirements. Estimated maintenance energy requirements were higher (P less than .01) for protein than fat tissue. Maintenance energy requirements of fat was -1.55 kcal ME/kg for Angus-Hereford cows, indicating that for cows of the same lean body mass, cows with more fat have a lower daily energy requirement during winter. Angus-Holstein cows had an estimated maintenance energy requirement of fat of 51.11 kcal ME/kg. Because cattle of Holstein breeding have less subcutaneous fat than cattle of the beef breeds, and less subcutaneous fat would provide less insulation, the estimated maintenance requirements of fat in Angus-Holstein cows may be an estimate of the true maintenance requirement of fat. Estimates of the partial efficiency of ME use for tissue gain and the ME sparing effect of body tissue loss were 78.8% and .70, respectively, for Angus-Hereford cows and 53.8% and .46, respectively, for Angus-Holstein cows. Regression of retained energy on cow BW.75, body fat and body protein calculated for Angus-Hereford and Angus-Holstein cows from within energy level indicated that BW.75 accounted for less variation in retained energy than weight of empty body fat or protein. Multiple regressions that contained all three variables accounted for 75% and 32% of the variation for Angus-Hereford and Angus-Holstein cows, respectively. Subsequent performance of the cow and calf was not affected by winter energy levels fed, body composition of cows before calving or body energy changes of cows during the winter.