Some non-genetic sources of variation In milk production of registered Friesland cows.

The effect of year, season and month of calving, herd, lactation number, and their first-order interactions with each other, as well as age at calving and calving interval, on milk, butterfat and FCM yield and butterfat percentage was investigated, using least·square analyses of variance. The data consisted of lactation records, completed from 1965 to 1971, obtained from performance tested registered Friesland cows in three regions; Western Cape, Eastern Cape and Natal. The analyses were performed on the first lactations, the first three lactations and the second up to and including the fifth lactations of cows in six herds in each region. The coefficients of determination (R²) were smaller for the Eastern Cape region than for the other regions and smaller for butterfat percentage than for the other traits. The values of R², obtained from analyses of variance, ranged from 0,365-0,676 for the yield traits and from 0,185-0,444 for the percentage butterfat. Herd, year and herd x year interaction were highly significant sources of variation in production in most cases. Other important sources of variation were season and month of calving, lactation number, and year x season, year x month, herd x lactation number and herd x season interactions, although all were not significant in all cases. The specific effect of season seemed less important in the Western Cape than in the other regions, but significant year x season and herd x season interactions were present. Year x month interaction was only significant for the first lactations of cows in Natal, and herd x season interaction was significant for the first three lactations of cows in the Western Cape region. Herd x lactation number was significant for the first three lactations, but not for the second up to and including the fifth lactations. Age at calving was a less important source of variation in butterfat percentage than in the other traits for the first lactations and the first three lactations. The regression on age at calving and preceding calving interval was highly significant for all traits of the second up to and including the fifth lactations. The average age at first calving in days was 875,1 ± 121,4; 988,2 ± 172,6 and 927,4 ± 153,2 for cows in the Western Cape, Eastern Cape and Natal respectively. The average preceding calving interval in days, based on the  second up to and including the fifth lactations, was 434,2 ± 96,3; 450,9 ± 119,0 and 423,0 ± 94,2 for cows in the Western Cape, Eastern Cape and Natal respectively.