LACTATION CURVES OF HOLSTEIN-FRIESIAN AND HOLSTEIN-FRIESIAN X GIR COWS

Linear y = a' + b'x and gamma type y = ea-cx xb models for the lactation curve (y = daily production on day x of lactation) were adjusted to daily production records of 870 lactations. Within- lactations pooled R2 = 0·71 and R2 = 0·74 were respectively obtained for the linear and gamma type models, the latter being preferred for further analyses on account of its general validity under varied circumstances. The parameters a, b, c of individual gamma type lactation curves were analysed by least squares within a subset of 634 lactations from Holstein-Friesian (HPB), 1/2 HPB: 1/2 Gir (F1) and 3/4 HPB: 1/4 Gir (B1) cows. Crossbreds had both higher initial production and higher persistency of lactation than purebred cows. F1's had higher initial production than B1's, both groups having similar persistency of lactation. Cows calving during the rainy season had higher initial production than cows calving in the dry season, which, however, had more persistent lactations. Parity affected only the initial production a parameter, which was lower for first calvers, while the other parity classes had similar a values. © 1979, British Society of Animal Science. All rights reserved.