The Effects of Energy Level and Monensin on Reproductive Performance and Lactation in Beef Females

154 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.A two-year, two-experiment study was conducted to evaluate the effects of energy intake and monensin on reproductive performance and lactation in beef females.In experiment I, a two-cubed factorial arrangement of treatments was utilized to determine the effect of breed, creep feeding and monensin on subsequent reproductive performance and lactation of 32 heifers. Half of each breed (Angus and Hereford) group had access to a 75% TDN creep feed while nursing their dams. Approximately forty days prior to breeding through 120 days of lactation, the heifers were fed a suboptimal energy ration with half of each breed and creep group receiving 200 mg monensin daily. Estrus was synchronized with Synchro-Mate B for the first AI service. Twenty-four hour milk production, % butterfat and % solids-not-fat were determined at 60 and 120 days postpartum. Weaning weights (adjusted to 205 days and for age of dam) were significantly heavier (p < .02) for the creep-fed heifers compared to those not receiving creep feed (219 kg vs 202 kg). A monensin x creep interaction appeared to significantly affect the peak LH value detected from 24 to 36 hours after implant removal. Progesterone levels were not altered by monensin. Monensin-supplemented females gained significantly more weight from the initiation of the monensin treatment to immediately post-calving and gave birth to significantly (p < .02) heavier calves as compared to those not receiving monensin. The energy stressed, monensin-supplemented first-calf heifers exhibited a shorter postpartum interval (p < .02) to first estrus than did those not receiving monensin (55.7 vs 69.1 days respectively). Creep-fed females had a significantly (p < .01) lower daily milk yield at 120 days postpartum.Calf production or cow weight change was not significantly altered by monensin over the 120-day lactation. Propionic acid levels were significantly increased by monensin while acetic and butyric acid levels were concurrently reduced throughout the experiment. No detrimental effects were observed due to long-term monensin supplementation.Experiment II consisted of a two year drylot study involving 80 energy-stressed beef cows (Angus and Herefords) suckling calves. Experimental treatments (breed, monensin and year) were arranged in a two-cubed factorial which evaluated the effect of breed and monensin on reproductive performance and lactation. The cows received 85% of the NRC TDN requirement for the first 56 days of the 140 day trial. On days 56 and 140, milk yield estimates were obtained (weigh-suckle-weigh) along with collection of rumen fluid (for VFA determinations) and blood serum (to monitor nutritional status). Following these collections on day 56, energy levels were increased by allowing ad lib consumption of the forage. The calves, which were separated from the cows during feeding, were given access to a 72% TDN creep ration after day 56. The cows were synchronized with Synchro-Mate B and artificially inseminated 30 days into the trial with 50% of each treatment group bled from 24 to 36 hours after implant removal with samples analyzed for LH. Monensin did not alter progesterone levels determined on serum samples collected at day 7 and 14 after implant removal. Monensin supplementation did not yield a significant difference in cow weight change or calf gains through the first 56 days of restricted energy or throughout the entire 140 day period. Neither milk yield estimates nor blood metabolites were affected by monensin supplementation when sampled on days 56 and 140. Neither calf gains nor conception rates were affected during the entire trial. Propionic acid levels were significantly (p < .01) increased while levels of acetic and butyric acids were concurrently decreased.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD