EFFECTS OF GENOTYPE AND FEEDING SYSTEM ON THE REPRODUCTIVE-PERFORMANCE OF DAIRY-CATTLE

Potential factors influencing reproductive performance were examined i n a high producing Holstein Friesian dairy herd, There were two geneti c groups: a selection line (S) consisting of heifers and cows with the highest genetic merit for kg fat plus protein in the UK, and a contro l line (C) consisting of heifers and cows of average genetic merit for kg fat plus protein. Within each group, heifers were randomly assigne d to either a high concentrate (HC: Annual average intake of about 2.5 ton per cow) or low concentrate (LC: Annual average intake of about 1 .0 ton per cow) complete diet based on grass silage, brewers grains an d concentrates. The mean milk yields during the first 26 weeks of lact ation for cows in the S-HC, C-HC, S-LC and C-LC groups were 6008, 5254 , 5041 and 4573, respectively. Data on the performance of 114 heifer l actations and 287 cow lactations was available for analysis. There wer e no significant differences in the mean intercalving interval, number of days to first observed estrus and number of services per pregnancy for selection versus control line cattle. Also, no difference in repr oductive performance was observed between cows on the high concentrate and low concentrate diets, but heifers fed the high concentrate diet had significantly longer intercalving intervals than heifers on the lo w concentrate diet. Average condition score of cows during the first 2 6 weeks of lactation was negatively related to number of services per pregnancy and intercalving interval. The degree of body condition scor e loss in heifers and cows between calving and week 10 of lactation wa s negatively related to the number of days to first observed estrus. I n heifers there was a significant positive relationship between milk y ield during the first 26 weeks of lactation and number of days to firs t observed post partum estrus, and in cows milk yield was positively r elated to number of services per pregnancy and calving interval.