Vitamin a and D Pre-Exposure to Prime Reproductive Success in Breeding Beef Cows

Abstract Vitamins A and D are important for immunity, reproduction, and overall health. The need for supplementation is often overlooked in beef cattle production because it is assumed that beef cows receive adequate amounts of these micronutrients from their environment. However, there is an increasing incidence of vitamin A and D deficiencies in beef cattle due to challenges of environment, forage access and quality. It is well known that nutrition has a major effect on reproduction, but direct effects of vitamins A and D on beef cow reproduction and fertility are understudied. We previously reported increased rates of fertilization after co-culture of bovine embryos with vitamin A and D in vitro. The objective of this study is to determine how vitamin A and D supplementation in mid- to late gestation affects calving ease and birth weights of calves at parturition, as well as cow fertility at the following breeding. Angus and Angus crossbred females (Bos taurus) of first (n = 20) or second (n = 10) parity were selected based on pregnancy status and gestational age and then randomly assigned to treatment groups. Starting in October 2021, pregnant cows were injected monthly with either a vehicle (control) or a commercial supplement made up of 300,000 IU vitamin A and 45,000 IU vitamin D (Vitamin AD, Vet One; n = 15/ treatment group). Blood samples were collected before each injection for analysis of vitamin and hormone concentrations. All cows were monitored from January-April 2022 for signs of parturition to determine calving date and time, calving ease, birth weight, and to provide assistance if necessary. Serum samples were collected from the calves within 12 hours of parturition for analysis of vitamin D concentrations. There was no effect of vitamin A and D treatment on calving ease (Generalized Linear Model analyses; P = 0.58; 1.44 ± 0.2 treated, 1.29 ± 0.19 control group) or on birth weight (Generalized Linear Model; P = 0.52, 32.6 ± 1.24 kg treated, 33.8 ± 1.38 kg control group). Estrus synchronization, additional blood sampling, vitamin/vehicle injections, and artificial insemination (AI) was performed in late April 2022. Conception rates were determined by transrectal ultrasonography 30 to 40 days post-AI. Interestingly, 7 of 15 controls were determined to be pregnant by AI, while this improved to 10 of14 in AD-injected females (one female could not undergo AI due to late calving). Cows not pregnant by AI were pastured with a bull of proven fertility, and rectal palpation was used in the fall to determine overall pregnancy rates and any pregnancy loss. These data suggest a modest increase in serum vitamin A and D will improve pregnancy rates after AI. Additional studies should reveal if these treatments improve markers of calf health. Supported by The Oregon Beef Council (C. Bishop).