PS5-13. The Effects of the Inclusion of Soybean Oil in Beef Heifer Diets on Reproductive Performance and Economic Viability.

Abstract For this two-year study, nulliparous Angus crossbred heifers (n = 160; BW = 251.33 ± 2.23 kg) were sorted randomly into 1 of 8 pastures (n = 10 heifers/pasture each year). Pastures were assigned randomly to 1 of 2 treatment groups (n = 4 pastures/treatment each year): 1) an isonitrogenous and isocaloric grain supplement with no soy product (CON); and 2) a grain supplement with soybean oil at 2% of estimated total diet dry matter intake (SBO). Supplements were offered beginning approximately 30 days after weaning and continued through the breeding season. Body weights and body condition scores were recorded monthly. On day 140, a rectal ultrasound was performed to determine reproductive tract scores (RTS), and pelvic area measurements were recorded. Heifers were declared peripubertal if RTS was 2 or 3, and cyclic if RTS was 4 or 5. Heifers were bred by artificial insemination (AI) using sex sorted semen from a single sire at approximately 14 months of age. Bulls were introduced 10 days following AI and remained with heifers for 56 days. Confirmed pregnancy was determined on day 204 and 278 by a rectal ultrasound. All costs were recorded during the study for economic viability. Statistical analyses were performed using SAS 9.4®, with PROC MIXED used for body weight and body condition scores. Body weights and body condition scores were analyzed with the repeated measure of day and for treatment, day, and treatment × day interactions. The GLIMMIX procedure was used for RTS, cyclicity using RTS, AI pregnancy rate, overall pregnancy rates, and maintenance of AI pregnancy at second pregnancy check. Per study design, there were no differences in body weight (P = 0.69). Reproductive tract scores, percentage of cyclic heifers using RTS, and pelvic area were not different between treatments (P ≥ 0.22). The percentage of heifers bred by AI was not different between treatments (P = 0.53) but was numerically greater for heifers supplemented with SBO compared to CON (53.2% vs. 48.1%, respectively). Overall pregnancy rates and heifers that maintained AI pregnancy at second pregnancy check were both numerically greater for the SBO supplemented heifers compared to CON (93.6% vs. 87.4%, respectively), and (50.6% vs. 46.9%, respectively), but were not statistically different (P ≥ 0.17). Finally, while supplement costs and total inputs were greater for the SBO treatment, potential revenue captured was still greater compared to the CON treatment. Though there were not any differences between CON and SBO in this study, soybean co-products can be fed to developing heifers without sacrificing reproductive efficiency or profitability for cattle producers.