A STUDY ON THE HERITABILITY OF PERFORMANC E VARIABLES OF 1ST LACTATION MILK-YIELD IN DAIRY-COWS

Aim of the present study was the estimation of genetic parameters of r ecords of single test-days and various parts of the lactation. The res ults can be summarized as follows: 1. Apart from the effect of herd, y ear, season, and age of calving also the influence of the insemination , i. e. new pregnancy, is an important factor for the last trimester o f the lactation. This is also evident at the beginning of the lactatio n although no biological relationship exists here. The effect of the n ew pregnancy masks genetic differences at the beginning of the lactati on and thus leads to low estimates of heritability. 2. The effect of t he interval between calving and first test-day yield is strong for tes t-day one and also for test-day seven to ten. When test-day records ar e standardized to records for equal 30-day parts of the lactation this effect is accounted for in the later parts of the lactation. 3. Herit ability estimates are high for second to fifth test-day and for the 30 -day parts of lactation between 31 and 210 days. Fat and protein conte nts show high estimates for test-days six to eight and in the 30-day p arts from 91 to 270 days. 4. Estimates of the correlation between the individual traits in parts of the lactation and the corresponding trai ts in the complete lactation are low at the beginning of the lactation and highest in mid-lactation. 5. Milk yield in increasing cumulative parts of the lactation reveals increasingly negative correlations with fat and protein contents.