Effects of nutrient restriction and subsequent realimentation in pregnant beef cows: Maternal endocrine profile, umbilical hemodynamics, and mammary gland development and hemodynamics

Effects of two different rates of body weight gain during the first trimester of pregnancy or supplementing vitamins and minerals throughout pregnancy on primiparous beef cow milk production and composition

We evaluated the effects of nutrition during pregnancy in beef heifers on colostrum and milk production and composition. For Experiment 1, crossbred Angus heifers were randomly allocated to a low (0.28 kg/d, [LG], n = 23) or a moderate rate of body weight gain (0.79 kg/d, [MG], n = 22) for 84 d after breeding, followed by management on a common diet until parturition. Colostrum samples were collected before first suckling and milk samples were collected by manual stripping of the teats 5 to 6 hours after calf removal on d 62 ± 10 and 103 ± 10 postpartum. At d 103, sampling techniques were compared by collecting a second sample after oxytocin administration and 90 s lag time. Colostrum somatic cell count was greater (P = 0.05) in LG (6,949 ± 797 × 103 cells/mL) than MG (4,776 ± 797 × 103 cells/mL) cows. In milk, percent protein was greater (P ≤ 0.01) in MG (3.03 ± 0.05%) than LG (2.87 ± 0.05%) cows. At d 103, oxytocin administration and extended lag time after teat stimulation increased milk fat content (P < 0.01) compared with immediate milk sample collection. For Experiments 2 and 3, crossbred Angus heifers were randomly assigned to receive either 113 g•heifer-1•d-1 of a vitamin and mineral supplement (VTM,) or no supplement (CON) from breeding until parturition. For Exp. 2, CON (n = 12) and VTM (n = 17) heifers were managed on a single pasture after parturition. On d 56 of lactation, 24-h milk production and composition were determined via a modified weigh-suckle-weigh technique using a portable milker. Milk yield and components (P ≤ 0.91) were similar between treatments. For Exp. 3, twice daily milk yield was recorded for 6 CON and 6 VTM heifers for 78 d following parturition. Milk samples were collected on d 32, 58, and 78 of lactation for component analysis. No differences were observed among treatments in milk yield or composition (P ≥ 0.09). Milk production was affected by day, increasing until d 10 and remaining similar (P ≥ 0.27) thereafter; however, protein was greater (P < 0.01) on d 58 compared with d 32 and d 78, and urea was reduced (P < 0.01) on d 78 compared with d 32 and 58. These experiments indicate that nutrition during early pregnancy has a sustained impact on milk protein but no impacts of vitamin/mineral nutrition during pregnancy were observed ion the subsequent lactation.

Nutrient restriction during late gestation reduces milk yield and mammary blood flow in lactating primiparous beef females

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.