Fall-calving primiparous beef females [body weight (BW): 451 ± 28 (SD) kg; body condition score (BCS): 5.4 ± 0.7] were individually-fed 100% (control; CON; n = 13) or 70% (nutrient restricted; NR; n = 13) of estimated metabolizable energy and metabolizable protein requirements from day 160 of gestation to calving. Post-calving, all dams were individually-fed tall fescue hay supplemented to meet estimated nutrient requirements for maintenance, growth, and lactation until day 149 of lactation. Four-hour milk yields were collected on days 21, 42, 63, 84, 105, and 147 of lactation, and milk nutrient composition was determined. Doppler ultrasonography of both pudendoepigastric arterial trunks was conducted every 21 d from days 24 to 108 of lactation. Total mammary blood flow was calculated, and hemodynamics from both sides were averaged. Data were analyzed as repeated measures with nutritional plane, day of lactation, their interaction, calving date, and calf sex (if P < 0.25) as fixed effects. We previously reported that post-calving, NR dams weighed 64 kg less and were 2.0 BCS lower than CON, but calf birth weight was not affected. Milk weight and volume were 15% less (P = 0.04) for NR dams than CON. Milk protein concentration was lower (P = 0.008) for NR dams than CON, but triglyceride and lactose concentrations were not affected (P ≥ 0.20) by nutritional plane. Milk urea N concentration of NR dams tended to be greater (P = 0.07) on day 42 but was lower (P = 0.01) on day 147 of lactation than CON. Total milk protein, triglyceride, and lactose yields were less (P ≤ 0.05) for NR dams than CON. Total milk urea N yield was less (P ≤ 0.03) for NR dams than CON on days 21, 63, and 147 of lactation. Maternal heart rate was greater (P = 0.008), but pudendoepigastric arterial trunk peak systolic velocity, resistance index, and cross-sectional area were less (P ≤ 0.04) and pulsatility index tended to be less (P = 0.06) for NR dams than CON. Mammary blood flow was 19% less (P = 0.004) for NR dams than CON, but mammary blood flow relative to milk weight or dam BW was not affected (P ≥ 0.14) by nutritional plane. Most milk yield, milk nutrient composition, and mammary blood flow variables were affected (P ≤ 0.04) by day of lactation. In summary, first-parity beef females that were nutrient restricted during late gestation and then fed to meet estimated nutrient requirements during lactation had decreased milk nutrient yield and a similar reduction in mammary blood flow.
Keywords: Doppler ultrasonography; beef heifers; developmental programming; lactation; mammary gland; nutrient partitioning.
Mammary development in preparation for lactation is largely complete at the time of calving, and final prepartum mammary growth and differentiation are occurring for the first time in heifers. Nutrient requirements increase substantially during late gestation, resulting in competition for nutrient use among maternal growth, fetal growth, and mammary growth in primiparous beef females. Undernutrition during late gestation can occur due to poor forage nutrient availability or drought, potentially impacting mammary gland development and subsequent milk production. We report that first-parity beef females that were nutrient restricted during late gestation and then fed to meet estimated nutrient requirements during lactation had 15% lower milk yield, reduced milk protein concentration, and less total milk protein, triglycerides, lactose, and urea N available for their calves than controls. Additionally, previously nutrient restricted dams had 19% less total mammary blood flow, and the major arteries supplying the mammary gland were smaller.
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society of Animal Science.