Sources of variation in the fertility of a herd of zebu, British, and zebu × British cattle in Northern Australia

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Abstract
Results of matings made in the Belmont breeding programme from 1954 to 1968 have been analysed. The analysis has been made within three groups. (1) The foundation cows in the years 1954–9; (2) theF1generation which were mated within line in the years 1957–62; (3) theF2andF3generations which were mated within Africander cross (AX), Brahman cross (BX) and Shorthorn-Hereford (SH) lines in the years 1960–8. Fertility was based on the number of calves born, whether alive or dead, to the number of cows mated.In the foundation cows, Hereford cows had a fertility 9% higher than the Shorthorns. The difference between the bull breeds was not significant although the mean for the Brahman bulls was 16% below the mean of the other three breeds. There was a large variation in the fertility of the Brahman bulls. The fertility of the Shorthorn cows was depressed (by 8%) in the lactating cows as compared with dry cows, while there was an opposite effect in the Herefords. These latter effects showed year to year variation.In theF1generation the differences between the breeds were not significant although both the AX (76·4%) and the BX (81·2%) were more fertile than the SH (70·1%). Estimates of heterosis in theF1generation were 42% for the AX, 43% for the BX and 12% for the SH. Lactating cows were 7% more fertile than non-lactating cows. There were significant differences between the BX bulls used but not between bulls of the other two breeds. The effect of sires within breed on fertility of daughters was significant only within the SH, and the heritabilities of fortuity were estimated from the variance components for sires within breed to be 9%, 14% and 22% for AX, BX and SH respectively.In theF2andF3data the breeds were significantly different in fertility with averages of 77%, 61% and 67% for AX, BX and SH respectively. Thus by comparison with the fertilities of theF1cows no loss of heterosis for fertility occurred in the AX, a very marked loss in the BX and only a slight loss in the SH. A significant interaction between age of cow and lactational status showed that in the mature cows, wet cows had a higher fertility than dry, while the converse applied in the 3-year-old cows. The interaction of lactational status with breed consisted of the wet zebu cross cattle having a relatively low fertility while the wet British cattle had a relatively high fertility. There were significant differences between bulls within each of the three breeds. The effect of sires was significant in the BX and SH. Heritabilities estimated from between sires within breeds variance components were – 12%, 22% and 25% for the AX, BX and SH respectively.