Effects of vitamin A supplementation on liver retinol concentrations of beef cows and their calves managed in confinement

Objective: Our objective was to evaluate how the amount of vitamin A supplementation provided from mid- gestation through early lactation affected liver retinol con- centrations of a cow and, subsequently, of her calf in a confinement system. Materials and Methods: Multiparous Angus cross beef cows (n = 54; BCS = 6.0) in mid-gestation (5 mo) with an initial BW of 578 kg (SD = +/- 74) were stratified by BCS and time spent in the confinement system and were assigned to pen (n = 9). Pens were assigned (n = 3) to each of the following treatments: current NASEM (2016) recommendation (31,000 IU/d; 1X) or 3 times (93,000 IU/d; 3X) or 5 times (155,000 IU/d; 5X) the cur- rent NASEM recommendation for supplemental vitamin A. Cows were limit fed a diet consisting of wheat straw, corn silage, and wet distillers grains. Liver biopsies were collected from cows 24 d before treatment initiation and at d 40 and 81 of supplementation, and both cows and calves were sampled 32 d after calving (d 165 of supplementa- tion, SD = +/- 22). Results and Discussion: No differences in initial cow liver retinol concentrations (mean 186 mu g/g of DM) were observed between treatments. A significant treatment x day interaction was observed for cow liver retinol. Liver retinol concentrations of 1X remained below adequate ref- erence ranges (300-700 mu g/g of DM) throughout the study (<= 189 mu g/g of DM), whereas 3X and 5X were elevated into the adequate range by d 81 (334 and 412 mu g/g of DM, respectively). Calf liver retinol concentration also differed among treatments, as calves of cows in 1X had lesser liver concentrations than 3X and 5X calves, which did not dif- fer. Liver retinol concentrations considered adequate for calves at 32 d of age (100-350 mu g/g of DM) were not observed in 1X calves (51 mu g/g of DM) but were observed in calves from 3X and 5X cows (119 and 165 mu g/g of DM, respectively). Implications: Providing the amount of supplemental vitamin A recommended by NASEM to cows in a long-term drylot did not result in cow or calf liver retinol con- centrations within the adequate reference ranges. Supple- menting cows with 93,000 IU/d of vitamin A for 165 d brought liver retinol concentrations of cows and their calves up within adequate reference ranges. Cows being fed diets consisting mainly of brown forages and concen- trates long term may need more supplemental vitamin A than currently recommended to ensure calves receive enough vitamin A from colostrum.