Yearly production trends and genetic and environmental influences in a genetically closed Brahman herd. I. Calf weights and mortality

Abstract

Variation and its causes of 1 454 weights at birth (PN), 1 310 at weaning (P205) and 1 116 to 18 months (P18) were studied in a Brahman herd registered under selection and genetically closed, by means of analysis of variance per square minimum. The farm, located in Venezuela with a tropical climate, was planted with Panicum maximum. The selection of females was carried out mainly for reproductive efficiency (pregnancy condition) and maternal ability (P205 of calves) and that of males was carried out according to P18 and fertility. Approximately 50% of the cows were mated with sires tested by progeny tests and 50% with sires preselected by phenotypic value (first) or by estimated genetic value (after) of P18. The basic model of analysis of variance by least squares included father (P; 45 for PN and P205, 41 for P18), sex (S; males, females), age of the mother (E, 3, 4,… 14 years) , year (A, 1966, 67,… 76) and month (M, 1, 2,… 7) of birth and A × M. were important (P <01) S, M and A × M in the three weights, P in PN and P18, E in P205 and A in P205 and P18. Cows of 5 and 6 years weaned the heaviest calves and those of 13 and 14+ years the less weight. Calves born early weighed at birth, weaning and 18 months more than those born late. The average weight of the cows at weaning was 399.5±1.5 kg and the regression of calf weight on cow weight was .11 kg. Mortality until weaning and up to 18 months was 6.6 and 8.7% of live births. Of the deaths, 61% occurred during the first month and 15% in the first 3 days.