Phenotypic and genetic parameters of reproductive traits for Ayrshire cattle on large-scale farms in Kenya Livestock Research for Rural Development (18) 10 2006 Guidelines to authors LRRD News Citation of this paper Phenotypic and genetic parameters of reproductive traits for Ayrshire cattle on large-scale farms in Kenya J O Amimo*, R O Mosi , J W Wakhungu, T K Muasya and B O Inyangala Department of Animal Production, University of Nairobi, P.O Box 29053 00625 Nairobi Kenya *Corresponding author (address PO Box 7256-00200 Nairobi Kenya, 0722 507 957) amimodr@yahoo.com Abstract Data consisting of 2757 records from ten Kenyan Ayrshire herds made between 1980 and 2005 were used to examine environmental factors affecting age at first calving (AFC) and calving intervals (CI) and consequently estimate genetic and phenotypic parameters and trends. The overall means and standard errors for AFC and CI were 39.4 ± 7.2 months and 487.5 ± 151.6 days respectively. The respective heritability estimates were 0.091 ± 0.05 and 0.044 ± 0.032, while the repeatability estimate for CI was 0.096 ± 0.001. The genetic trends for CI and AFC were -0.6d/yr and -0.01mo/yr respectively and were both significant (P<0.001), indicating a decrease in mean breeding value over the study period. Phenotypic trends were -0.31 mo/yr and -0.35 d/yr for AFC and CI respectively though non-significant (P>0.05). The low heritability for CI and AFC indicated that temporary environmental influences were much greater than genetic influences or permanent environmental influences on these traits. Key words: Age at first calving, Ayrshire, calving interval, genetic parameters and trends Introduction Most of the current breeding programs globally use indices that give more weight to yield and type traits than reproductive traits (Lucy 2001). This practice has resulted in substantial genetic progress for yield, but is likely to cause reduced fertility given the reported positive genetic correlation between fertility and yield (Hansen et al 1983; Raheja et al 1989; Oltenacu et al 1991). Several studies demonstrated an inverse relationship between reproductive efficiency and milk yield especially in the tropics (Lucy 2001, Ojango and Pollot 2001, Roth 2004). Management can enhance reproductive efficiency, even though reproduction has some genetic variation (Lucy 2001; Biffani et al 2003). Low fertility is of economic importance in dairy enterprises especially in tropics, because it results in higher levels of involuntary replacement, slippage in calving pattern, veterinary intervention and reduced annual milk production (Esslemont and Peeler 1993). Improving fertility also increases effectiveness of treatments or vaccinations and also reduces breeding costs because of isolation, treatment, culling and replacement of problem cows (Campos et al 1994, Damatawewa and Berger 1998). Evidence in literature (Njubi 1990; Rege 1991; Lucy 2001; Biffani et al 2003) shows that heritabilities of reproductive traits in dairy cattle are generally low, making selection for those traits difficult. However there is sufficient variation to permit genetic improvement of reproductive performance. Thus incorporation of measures of cow (daughter) reproduction into sire selection decisions has been recommended (Clay et al 2000; Weigel and Rekaya 2000). http://www.utafoundation.org/lrrd1810/amim18147.htm (1 of 7)15/11/2006 09:33:58