103. Late Gestational Nutrient Restriction and Fetal Genetic Growth Potential Interact to Affect Nutrient Partitioning Among the Dam, Fetus, and Colostrum.

Abstract Late gestational nutrient restriction of beef females causes body store mobilization and decreases colostrum production, independent of whether fetal growth is reduced. Despite this, effects of fetal genetic growth potential on nutrient partitioning to the gravid uterus and mammary gland in nutritionally-stressed beef cows are unknown. Thirty-six SimAngus-based fall-calving cows (5-9 yr of age) were artificially-inseminated to either a low or high birth weight EPD SimAngus sire (2nd [low] vs. 99th [high] percentile). At d 160 of gestation, cows (BW: 625 ± 39 kg; BCS: 5.4 ± 0.6) were allocated to receive either 100% (control) or 70% (nutrient restriction, NR) of estimated metabolizable energy and metabolizable protein requirements for maintenance and pregnancy to allow for a 2 × 2 factorial of fetal growth potential × late gestational nutritional plane (n = 9 per quadrant). Cows were limit-fed low-quality chopped hay and supplemented to meet targeted nutritional planes individually in Calan gates. At birth, calf BW and size were measured and total colostrum from one rear quarter was collected pre-suckling. Data were analyzed with nutritional plane, fetal growth potential, and their interaction in the model, including calf sex and treatment initiation date as covariates. Cow BW, BCS, and backfat thickness were unaffected by fetal growth potential at nutritional plane initiation (P ≥ 0.29). From d 160 of gestation to post-calving, NR cows lost BW compared with control maintaining (-68 vs 1 ± 3 kg; P < 0.001), and cows gestating high calves lost more BW than low (P = 0.01). Cow BCS change was affected by nutritional plane × fetal growth potential (P = 0.001), where both NR groups lost BCS compared with control groups maintaining (P < 0.001). Within NR, BCS loss was greater for high than low (-2.1 vs. -1.4 ± 0.1; P < 0.001). Backfat thickness decreased for NR versus maintaining in control (P < 0.001). Gestation length was 3.1 d longer for NR than control (P = 0.05) and tended to be 2.6 d longer for high than low (P = 0.10). Calf birth weight was unaffected by nutritional plane (P > 0.67) but was 17.5% greater in high than low (P = 0.009). High calves had greater length, heart girth, abdominal girth, and shoulder height than low (P ≤ 0.04). Colostrum yield was affected by nutritional plane × fetal growth potential (P = 0.05). Within low, NR produced 39.7% less colostrum than control (P = 0.04). Within NR, high tended to produce 48.8% greater colostrum than low (P < 0.10). Overall, high fetal genetic growth potential increased nutrient partitioning to the gravid uterus, even during undernutrition. High fetal growth potential appears to be able to signal for greater maternal energy store mobilization and increased colostrum production.