RESEARCH ON THE MILK PRODUCTION DIFFERENCES DETERMINED BY THE CALVING MONTH OF THE DAIRY COWS

The paper aimed to assess the effect of the calving month on milk production and to correct the biases determined by this environmental factor in order to increase the precision in the breeding value estimation and assure a better evaluation of candidate cows as mothers of sires and bulls used in the artificial insemination to improve genetic gain in the dairy herds. A specific mathematical model, xij=m . ai . eij, where xij ? the milk record of the daughter ?j” in the month ?i” (i=1,2,....12), m ? a constant factor, ai ?multiplicative effect of the month ?i”, eij ? the residual multiplicative effect, was used to process the data regarding milk yield and fat yield by month of calving collected from 4,100 Friesian dairy cows under milk testing. In order to eliminate the biases âi , adequate multiplicative adjusting factors were calculated as 1/âi.. The conclusion was that the calvings carried out in the fall-winter months recorded a milk yield by 3.5 % higher in January, by 5.4 % in September, by 6.3 % in December and by 10.5 % in November. The cows with early spring calving have been distadvantaged by 0.4 % in February, by 1.8 % in August up to 7.9 % in May. Therefore, correction factors are imposed to be used in order to increase precision in milk production estimation and finnaly to assure an accurate breeding value for dairy cows and bulls.