PSI-13 The impact of divergent nutritional planes on systemic immune status in mature bulls

Bull fertility is a major component of reproductive efficiency. One of the many factors influencing reproductive efficiency in bulls is nutrition. Improper nutrition influences physiological features, such as body weight (BW) and body condition scores (BCS), but also increases stress, which can alter the molecular composition of ejaculate. Following a breeding soundness exam and a 21-d dietary adjustment period, mature Angus bulls (n = 6) were randomly assigned to one of two nutritional planes for a 119-d period: 1) restriction (RES; targeted a 1.36 kg/d decrease in BW) or 2) maintenance (MNT; targeted to maintain BW). Body weight and BCS were evaluated every 14 d and feed amounts were adjusted to meet BW targets. On d 73 and 93, scrotal circumference and blood samples were collected for analysis. Serum samples were evaluated for an immunity score using the Nano Discovery D2Dx Immunity Test, that assesses samples for the presence of immunoglobulin (Ig) G, IgM, cytokines, and complement. Statistical analyses were conducted using R Studio to determine the effects of day, nutritional plane, and the interaction of day Î nutritional plane on BW, BCS, scrotal circumference, and systemic immunity score. At the onset of treatments, all bulls had a greater (P = 0.008) BW (845.5 ± 17.0 kg) and BCS (6.3 ± 0.15) than on d 73 (BW = 757.2 ± 17.0 kg; BCS = 5.5 ± 0.15, respectively) or 93 (BW = 749.6 ± 17.0 kg; BCS = 5.3 ± 0.15, respectively). According to the experimental model design, the MNT bulls had a greater (P = 0.001) BCS (6.1 ± 0.12) compared with the RES bulls (5.3 ± 0.12) throughout treatment. Body weights were also influenced by treatment with MNT bulls (845.7 ± 13.9 kg) having a greater BW (P > 0.001) compared with RES bulls (714.6 ± 13.9). Immunity scores were greater in the RES bulls (0.03 ± 0.003) compared with MNT bulls (0.003 ± 0.003). However, there was no effect of day or the interaction effect (P > 0.05) on the systemic immunity score. There was also no effect of day, nutritional plane, or interaction on scrotal circumference (P > 0.23). In conclusion, we observed nutritional planes to result in divergent BW and BCS and were influential on systemic immunity scores, which may lead to different seminal plasma compositions and reproductive outcomes. Further analyses are needed to elucidate the effects of paternal nutritional stress on the female uterine response after mating and reproductive efficiency. The authors would like to thank USDA-NIFA-AFRI (#2023-67015-39453) and USDA-NIFA Multi-State Project No. 1019048 for providing funding and support for this research. USDA is an equal opportunity employer and provider.