Study of 305- Day Milk Yield, for First Three Lactations, of Jersey Cows Under Conditions of Small Scale Family Farms I. Effects of Calving Age and Season on Total Variance of Milk Yield.

Data of 935 first lactations, 607 second lactations and 432 third lactations obtained by 1476 Jersey cows that are managed in small scale family farms, under conditions of low input production system were analyzed in order to study effects of factors: age and month of calving on variance of 305-day milk yield. Analyze of variance carried out according to a GML with fixed factors showed statistical effects of these factors – age on calving (CA): first lactation (P<0.05), second lactation (P<0.05), month of calving (CS): first lactation (P<0,001), second lactation (P<0.01), third lactation (P<0.05) and herd (H): first, second and third lactations (P<0.001). Average growth of 305-day milk yield, in second lactation was 198 kg more than first lactation, and 271 kg more in third lactation compared to second lactation. To make possible the comparison of milk yield obtained by cows in different physiological conditions, it is necessary that 305-day milk yield be adjusted in order to reduce effects of calving age and month.