Influence of prepubertal dietary protein and age at first calving on the performance, reproduction and lactation of replacement beef heifers

The objective of this research was to evaluate the effects of prebreeding dietary protein level and age at first calving on the performance, reproduction and lactation of replacement beef heifers. Dietary protein levels ranging from 11.7% to 22.6% CP were tested in six experiments using several nutritional management systems. Heifers were managed to calve first at 18 or 24 months of age. The effects of prebreeding protein and age at first calving were tested in several prebreeding production settings including normal weaning and creep feeding, early weaning and creep feeding, and early weaning and confinement feeding. Replacement beef heifers utilized in the experiments were Angus, Simmental or a reciprocal crossbred of Angus and Simmental. Increasing prebreeding dietary protein improved (P ≤ 0.05) performance of early-weaned heifer calves grazing pasture but did not influence (P > 0.05) performance of early-weaned heifers maintained in confinement. Increasing creep feed protein levels increased, decreased, and did not influence the performance of suckling heifer calves. Body weights of replacement heifers did not differ (P > 0.05) by 60 days after first calving, regardless of prebreeding protein level or age at first calving. A greater (P ≤ 0.05) percentage of early-weaned replacement heifers were pubertal at eight months of age compared to normal-weaned heifers. No difference (P = 0.30) in percentage of heifers estrus cycling prior to the breeding season was observed between 18 and 24-month calving heifers. Pregnancy, calving and weaning rates tended (P < 0.10) lower for early calving replacements. Decreasing the age at first calving also decreased (P < 0.001) first lactation milk production. Effects of increasing dietary protein were tested in six experiments, one experiment indicated an increase (P ≤ 0.10) another experiment indicated a decrease (P ≤ 0.10) while four experiments indicated no effect (P ≥ 0.10) of increasing prebreeding dietary protein on first lactation milk production. Increasing the level of prebreeding dietary protein fed to replacement beef heifers does not influence performance or reproduction and cannot prevent milk suppression due to prebreeding ADG greater than one kg/d. Calving heifers at 18 months depressed milk production and may reduce reproductive efficiency.