315 Awardee Talk: Late Gestational Nutrient Restriction Alters Postnatal Nutrient Delivery and Calf pre-Weaning Growth in the Primiparous Beef Female
Authors: Redifer Colby A , Meyer Allison M 
Published: October 2023

Abstract
Nutrient requirements increase dramatically in the late gestation beef female to allow for proper growth and function of the uteroplacenta, fetus, and mammary gland. In fall- and spring-calving systems, late gestation grazing beef cattle often have limited forage nutrient availability. Late gestational nutrient restriction research generally associates decreased prenatal nutrient delivery with programming fetal growth and development, resulting in poor postnatal outcomes. However, the effects gestational nutrient partitioning may have on programming the mammary gland to decrease postnatal nutrient delivery is far less understood. In a fall-calving experiment, primiparous females were individually fed either 100% or 70% of energy and protein requirements from d 160 of gestation until parturition. Nutrient restricted females lost body weight and condition and had less nutrients in maternal circulation during late pregnancy. Calf birth weight was not affected, but nutrient restricted dams had less vigorous calves and 40% less colostrum yield. Colostrum of nutrient restricted females was more concentrated with IgG, and calf 48-h serum IgG indicated that offspring of both treatments had successful passive transfer of immunity. Colostrum of nutrient restricted females had less total lactose, but similar total protein and triglycerides compared with control females. Even when females were fed to meet their energy and protein requirements during lactation, late gestational nutrient restriction reduced milk yield by 15% and decreased milk protein concentration during the first 150 d of lactation. Reduced milk production was partially explained by a 19% reduction in blood flow supplying the mammary gland during this time. Less total lactose, protein, and triglycerides provided by the milk resulted in calf body weight diverging by 42 d of age, and calves born to nutrient restricted dams were 13% smaller at weaning. Interestingly, calf metabolic status pre-weaning was not as severely altered as expected considering milk nutrient and calf growth differences. By d 21 of lactation, metabolic status of previously nutrient restricted dams had returned to concentrations similar to control dams. During lactation, previously nutrient restricted females gained maternal weight faster and increased body condition, but had still not fully recovered to that of control females by weaning. Females were managed together and followed through their second parity to examine effects during the subsequent parity. Previously nutrient restricted dams still weighed less, were in thinner body condition, and had less mammary blood flow during their second lactation. Lactational performance and second parity calf growth were similar between treatments. While late gestational nutrient restriction may not always reduce fetal growth, our results illustrate that impaired postnatal nutrient delivery mechanisms in the cow and calf deserve continued consideration. Additionally, persisting effects in the second parity on maternal size, fleshing ability, and mammary blood flow demonstrate the possibility of long-term programming effects on the dam.