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The effect of sex on levels of metabolic hormones and biochemical standards in blood serum was investigated in the first days of life in calves. Forty-five calves were randomly included in two trials (trial I - 8 heifers and 12 bullocks; trial II - 14 heifers and 8 bullocks). The first trial was conducted in October and the second in April. After delivery, the calves were moved to a prophylactorium area where they stayed for two weeks. The health of all calves was good.

Calf nutrition consisted of colostrum at an amount of 31, arid of 61 of genuine milk. Feeding took place twice a day. Blood samples were taken by punctures of the vena jugularis in four hours after morning feeding on days 1, 2, 3, 5, 7 and 14 of age. The blood serum for insulin and thyroid hormone determination was deep-frozen and stored at a temperature of -20-degrees-C untill the analyses were performed. The values of biochemical standards were determined m the day of sampling in all cases.

Procedures of determination of the investigated standards in blood serum insulin levels (RIA-INS-kit, MJ96 model from CPDR, Poland), thyroid hormone concentrations (RIA test T4 and T3 from URVJT at Kosice), glucose, urea levels and total proteins (Bio-Lachema-test, Brno) and amino-acid nitrogen (together with ninhydrin by photometry). Before the insulin readings were subjected to mathematico-statistical processing of the results, these values were converted to logarithms (lnx) on the basis of reference data (P o r t e r, B a s s e t t , 1979). The results achieved were processed by analysis of variance (ANOVA) and the differences were determined by Sheffe’s test.

No significant effect of sex on insulin concentration in the blood serum of calves was observed in the first two weeks of calf age in either of the trials (Tabs. I and II). In trial I, the insulin concentrations tended to be high in heifers. Similarly, them were no significant differences in either of the trials between the males and females in the values of biochemical standards nor in the values of the T3/T4 ration. Insulin and urea levels were increasing in the males and females with growing age in trials I and II.

In trial I, the thyroxine (T3) level was in general higher in the first two weeks of age in heifers in comparison with bullocks (P < 0.01). The differences was significant on day 1 and 5 of calf age (P < 0.01) (Tab. I). In trial II, the thyroxine level was insignificantly higher during the period of investigation, while the difference was significant on day 2 (P < 0.05) (Tab. II).

As for triiodothyronine (T4) in trial I, its level was significantly higher in heifers than in bullocks over the first two weeks of age, similarly like in thyroxine concentrations. The significantly higher level was demonstrated on day 2 (P < 0.05). In trial II, the differences between the values were not significant.

In trials I and II, the thyroid hormone levels were sharply decreasing in the first five to seven days of calf age, both in the males and females. The higher level of thyroid hormones in heifers if compared with bullocks is probably related to different degrees of ontogenetic development at birth.

When individually feeding the males and females the same amount of milk, the different levels of thyroid hormones between the heifers and bullocks do not probably result from the plane of nutrition in our trials. This is documented by the readings of biochemical standards in blood serum, which were not different in the males and females.

The level of thyroid hormones in the neonatal period influences the viability of born calves. The mortality rate of bullocks is much higher than that of heifers. It is probable that the higher level of thyroid hormones in the blood of heifers in the first days of life is a reason for their lower mortality and for their better adaptation to the extrauterine life if compared with the bullocks.

In conclusion it is apparent that the higher thyroid levels in heifers than in bullocks may probably be related to the different degree of ontogenetic development at birth.