Factors affecting reproductive performance of range-fed beef cows at Athi River Ranch; Kenya
A study of breed and environmental factors, both internal and external, affecting calving interval of range-fed beef cattle was conducted at Athi River ranch in Kenya. Breeding females comprised the Boran, the small East African Shorthorn Zebu (EASZ) and their Fl progeny from Hereford bulls. Fertility was highly seasonal (P ~ 0.005) and the shortest mean calving intervals were recorded from animals that conceived between November and January followed by the period from May to July. When mating was confined between May and July, fertility became significantly influenced (P < 0.005) by the calving month preceeding conception, with heifers being the most affected. Under such conditions animals that calved down during March-April had a higher fertility during the subsequent breeding period. Increased rainfall during the month prior to conception was associated with a highly significant (P < 0.01) linear improvement in fertility depending on conception period. significant (P.£. 0.005). The target joining weight was around 318 kg. Year effects were highly Above the target weight, up to about 410 kg, fertility became a function of the absolute bodY weight such that heavier cows at joining were more fertile (P ~ 0.01) than lighter ones. There was evidence to suggest that the depressing effect of suckling could dominate influence of body weight unless joining weight was well above 318 kg. Below the target weight, relatively higher fertility was associated with females that gained weight or suffered less body weight loss during the month prior to conception. Previously dry cows tended to gain weight prior to conception to the "detriment of fertility (P<O.OOS). Monitoring of proportionate body weight changes during the month prior to conception was a better method of assessing fertility than weight changes conception. Influence of dry season supplementation was significant far-removed from but was modified by a number of factors such as age, year and previous parity. There was no significant difference in fertility of cows that were supplemented before calving with energy or energy plus urea. Generally, cows that were supplemented after calving had a higher fertility. Fertility improved as the dam's age increased from three years onwards but declined after about nine to ten years. Young females were more vulnerable to adverse post-partum nutritional conditions but they benefitted by calving down one month earlier than the older cows. Effects of weaning period, weaning month,previous parity and weaning year were marginally significant (P<O.OS). The mean calving interval of the EASZ was shorter (P~O.OS) than that of the Borans by only 17 days. However, there was evidence that when mating occurred between November and January or when supplementation was provided, the Boran and Boran crosses had a higher fertility compared to the small zebus and their crosses.