EFFECT OF DIETARY CAROTENE ON FERTILITY OF HIGH-YIELDING DAIRY-COWS

Multiparous dairy cows were allotted at the beginning of the dry period to 2 groups, each .apprx. 75 cows. In addition to a basal diet containing a low concentration of carotene and a normal concentration of retinyl ester, the carotene group was given a daily supplement of 500 mg carotene during the dry period and 750 mg carotene during lactation, and the retinol group was given an equivalent amount of retinyl ester. Plasma carotene concentrations were higher in the carotene group (.apprx. 4.2 mg/l vs. .apprx. 0.35 mg/l), but plasma retinol did not differ. Peripheral plasma progesterone concentration or the interval between calving and 1st insemination were not affected by carotene supplementation. Conception rate was improved by carotene supplementation in the younger cows (2nd and 3rd lactation) during the 1st part of the experiment (cows calving in Sept.-Dec.) when conception rates at 1st insemination were 0.70 vs. 0.33 (P < 0.05); for all inseminations, conception rates were 0.71 vs. 0.38 (P < 0.01). During the 2nd part of the experiment (cows calving in Jan.-April), no significant differences were found between the 2 groups. Plasma tocopherol concentration in cows given carotene was significantly higher during the dry period and the 1st wk of lactation, but these differences disappeared in the later stages of lactation. Plasma cholesterol concentration in the carotene group was highly correlated with plasma carotene concentration (r = 0.52-0.59, P < 0.001), even higher correlation coefficients were found between plasma cholesterol and plasma tocopherol in the carotene and the retinol groups (r = 0.61-0.80, P < 0.001). Plasma concentrations of triglycerides, glucose, urea and aspartate aminotransferase were not affected by treatment. A high plasma carotene concentration was not a prerequisite for conception, but under certain conditions which need defining, the carotene concentration appeared to be a limiting factor.