17. Effects of Inclusion of Soybean Oil in Beef Heifer Diets on Reproductive Function and Calf Growth Performance

Abstract The objective of this study was to determine the effects of including soybean oil in beef heifer development diets on uterine artery hemodynamics and calf morphometrics. Angus crossbred heifers (n = 80; BW = 248 ± 21 kg) were sorted randomly into 1 of 8 pastures (n = 10 heifers/pasture) and pastures were assigned randomly to 1 of 2 treatment groups (n = 4 pastures/treatment), being: 1) control group fed an isonitrogenous and isocaloric grain supplement with no soy product (CON); and 2) treated group fed grain supplemented with soybean oil at 2% of estimated total diet dry matter intake (SBO). Heifers grazed 2.4 ha mixed grass pastures and when forage was limited heifers were offered bermudagrass hay for ad libitum intake. Supplements were offered beginning approximately 30 days after weaning and continued through the breeding season. At day 35 of gestation, a rectal ultrasound was performed by a trained technician for determination of pregnancy by artificial insemination (AI). Images were taken using color-Doppler ultrasonography to determine blood perfusion and area of the corpus luteum (CL). The hemodynamic measurements of the AI bred heifers (n = 16 CON; n = 16 SBO) were obtained on days 222, 223, and 224 of gestation to measure ipsilateral artery blood flow, contralateral artery blood flow, and total blood flow. Body weights and morphometrics were recorded on all calves at birth and weaning (n = 70). Statistical analyses were performed using the PROC MIXED procedure of SAS 9.4® for luteal perfusion and area, uterine artery blood flow parameters, and morphometric measurements of calves at birth and weaning. The CL area at day 35 of gestation was greater for CON supplemented heifers compared to SBO supplemented heifers (P = 0.0077). However, CL blood perfusion was not different between supplemental treatment groups (P = 0.9992). No differences were found for uterine blood flow for the ipsilateral uterine artery (P = 0.4539), contralateral uterine artery (P = 0.2876), or the total uterine blood flow (P = 0.7447). Calves born to SBO supplemented dams during development and early pregnancy tended to have greater crown rump length at birth compared to calves born to CON supplemented dams (P = 0.0733). Also, calves born to SBO supplemented dams tended to have a greater cannon circumference than their contemporaries (P = 0.0986). All other calf morphometric measurements at birth did not prove to be significant between treatments (P ≥ 0.1195), and there were no differences at weaning (P ≥ 0.1610). In summary, including soybean co-products in developing heifer diets did not have negative effects on reproductive functionality and had some influence on morphometric measurements at birth.