Provision and utilisation of vitamin D pre-calving have substantial implications during the transition and early lactation periods. The negative effects of hypocalcaemia with immune function, risk of disease, lactation performance and reproduction have been well-documented. As vitamin D is a key contributor to calcium (Ca), attempts have been made to prevent hypocalcaemia by supplementing vitamin D, since the amount and source of vitamin D can influence Ca balance during the transition phase. Vitamin D primarily can be supplemented by either cholecalciferol (CHOL) which is the endogenous format in the skin, or calcidiol (CAL) which is the converted format in the liver. CHOL is routinely supplemented in the diets of high-producing cows, but there are indications that CAL may be more effective. The authors cited aimed at testing this hypothesis, and therefore their objectives were to test the effects of CAL versus CHOL at levels of 1 and 3mg per day in late gestation on production outcomes in Holstein cows.
One hundred and seventy-seven pregnant Holstein cows were enrolled in the study. The cows were blocked by parity and previous lactation milk yield and assigned randomly to receive 1 or 3mg per day either CAL or CHOL in a 2 x 2 factorial arrangement (CAL1, CAL3, CHOL1 and CHOL3). These were provided to individual cows as a top-dress to the pre-calving diet from 250 days in gestation until calving. The pre-calving diet had a dietary cation-anion difference of -128mEq per kg of dry matter. Production and disease were evaluated for the first 42 days in milk, and reproduction was evaluated until 300 days in milk.
The incidence of after calving diseases did not differ among treatments. Feeding CAL compared to CHOL increased yields of colostrum and colostrum fat, protein and total solids, resulting in an increased amount of net energy for lactation secreted as colostrum. An interaction between source and amount was observed for milk yield: CAL3 increased milk yield compared with CHOL3 (CHOL3 = 34.1 vs CAL3 = 38.7 ± 1.4kg per day), but milk yield did not differ between CAL1 and CHOL1 (CHOL1 = 36.9 vs CAL1 = 36.4 ± 1.4kg per day). The concentrations of serum calcidiol on day of calving and average serum Ca from days 2 to 11 post-calving were positively associated with milk yield in the first 42 days in milk. Interactions between source and amount of vitamin D were also observed for pregnancy after first AI: the percentage of cows receiving CHOL1 and CAL3 that became pregnant was smaller than that of cows receiving CHOL3 and CAL1. However, pregnancy per AI and pregnancy by 300 days in milk did not differ among treatments. Overall, CAL3 increased milk yield compared with CHOL3, whereas in cows fed 1mg per day the source of vitamin D generally had no effect.
The effect of CAL3 may be explained in part by serum CAL concentrations and post-calving serum Ca, which were associated with milk yield. The results overall therefore indicate milk production benefits for CAL3 supplementation pre-calving, but not for CHOL3.