Genetic and non-genetic factors affecting lactation curve components of a Sudanese Butana dairy herd

Weekly milk yield of 1578 lactations of 249 Butana dairy cows during the period 1949–1999, were analyzed to study effects of year of calving, season of calving and cows’ parity order on lactation curve components. Estimates of heritability and repeatability of these traits were also studied. Lactation curve components were estimated using the formula Y= anbe(-cn). The data were classified according to years of calving into 1949–1959, 1960-1969, 1970- 1979, 1980-1989 and 1990–1999 groups. The data were also classified according to season of calving into dry summer (March–June), wet summer (July-October) and winter (November-February) groups. The data were also classified according to cow's parity order into ten parity groups. General linear model was conducted to examine the significance of the tested effects. Week of peak yield and persistency of lactation curve peak significantly increased with years' advancement and the opposite trend was observed for peak yield and constants a and c. Peak yield was significantly (P<0.05) higher for winter group than dry summer and wet summer groups. Constants b and c significantly increased with parity order increase indicating flatter shape of lactation curve at early parities. Heritability and repeatability estimates of lactation curve components were low and ranged between 0.01–0.06 and 0.02–0.22, respectively. The study concluded that all traits those determined shape of lactation curve of Butana cows were not inheritable and were affected by environmental factors such as management systems change during years, climatic change during seasons and cow's age.