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[Concentrations of thyroxine, beta carotene, vitamin A and pregnancy in heifers after administration of beta carotene].

Serum concentrations of thyroxine (T4), beta-carotene (beta K), vitamin A and conception after the first insemination on the zero, sixth and 21st day were studied in 63 heifers fed synthetic beta K supplement in groups I to IV at stanchion housing and low-carotene diet. The following amount of beta K was supplemented per head/day: I--0 mg (control, n = 17), II--100 mg (n = 16), III--200 mg (n = 14) and IV--400 mg (n = 16). The parameters were evaluated separately according to groups and conception. By comparing the T4 concentration between experimental groups and control, separately in non-gravid and gravid animals, a significant increase (P less than 0.05) was determined only on the zero day in the fourth group. At the same time, in the fourth group, in comparison with the others, insignificantly lower T4 values were recorded in non-gravid animals. beta K values on the zero day in gravid animals increased in a direct proportion to the higher supplement (P less than 0.05; P less than 0.01; P less than 0.001); in non-gravid animals the increase was slower (P less than 0.05; P less than 0.05; P less than 0.01), but differences in relation to gravidity were not significant. An increase of beta K had no significant effect on serum concentrations of vitamin A. Differences between groups as far as conception was concerned were insignificant. It follows from the results that increasing rates of beta K, with the exception of serum concentrations, had no significant effect on other parameters. Higher concentrations of T4 and a slower increase of beta K in non-gravid animals, however, point at mutual co-operation relation and its positive effect on conception in animals.