Effects of weaning period length on growth and health of preconditioned, spring-born beef calves originating from the Great Plains. I. Conventional weaning ages1

Spring-born Angus × Hereford calves (n = 433; average initial weight = 229 ± 37 kg) were assigned randomly to treatments that corresponded to a length of time (d) between separation from their dam and transport to a feedlot: 60, 45, 30, 15, or 0 d. Weaning date varied by treatment; transport occurred on a common date and at a common age (220 ± 22 d) for all treatments. Calves were vaccinated against common diseases 14 d before maternal separation and again on the day of maternal separation. Calves were transported < 20 km to a ranch-of-origin weaning facility following separation from dams and penned according to treatment. Calves were fed a complete diet (16.9% CP, 1.21 Mcal of NEg/kg). On a common date, all calves were transported 4 h to a commercial auction market and held for 14 h. They were subsequently transported 1 h to a feedlot. Unweaned calves weighed less (P = 0.02) at transport than weaned calves. Morbidity during the ranch-of-origin weaning period increased quadratically (P = 0.01) as length of the weaning period increased. Transport shrink and receiving morbidity were greater (P = 0.01) in unweaned calves than in weaned calves. Calf BW at the end of receiving was greater (P = 0.01) for weaned than unweaned calves and increased linearly (P = 0.01) as weaning-period length increased. Calf ADG during receiving and finishing were not affected (P ≥ 0.30) by treatment. Preslaughter BW and HCW tended (P = 0.07) to be greater in weaned calves than in unweaned calves but were not different (P ≥ 0.11) among calves weaned for 15, 30, 45, or 60 d before transport. Days on feed decreased linearly (P = 0.01) as length of the pretransport weaning period increased. Conversely, KPH, 12th-rib fat thickness, and USDA YG increased linearly (P ≤ 0.05) as pretransport weaning period length increased. Cow BCS change from 60 d before to 60 d after transport of calves increased linearly (P = 0.01) as weaning-period length increased. Cows suckling calves weaned immediately before transport had lesser (P = 0.03) pregnancy rates than cows suckling calves weaned before transport; moreover, pregnancy rate tended to increase linearly (P = 0.09) as the length of preconditioning increased. Under the conditions of our experiment, pretransport weaning periods ≥ 15 d improved the health and performance of calves during feedlot receiving.