The objective of this study was to evaluate the effects of genetic group and environmental factors, including farm of origin, season of calving and lactation number, on the individual lactation curves of 1,010 Holstein-Gyr, Guzerat-Holstein, Holstein-Nellore, and Holstein-Zebu F1 cows on the experimental farms of the Agricultural Research Company in Minas Gerais (Empresa de Pesquisa Agropecuária de Minas Gerais, EPAMIG). Least squares analysis and the incomplete gamma function were used to investigate differences in early milk yield, peak milk yield, time from onset to the lactation peak, persistency, total milk yield and lactation duration. The highest early yields were observed for Holstein-Gyr and Holstein-Zebu, and the lowest were observed for Holstein-Nellore cows. Early yield increased with lactation number, and the lactations that began in the late rainy and early dry seasons had the highest early yield. The time between lactation onset and the lactation peak was longer for primiparous Holstein-Gyr and Holstein-Guzerat cows. The lowest yields at the lactation peak were observed for the first lactation number. The highest values of persistency were observed for the first lactation number in Holstein-Guzerat cows. Total production from Holstein-Gyr and Holstein-Zebu cows were higher by approximately 14% and 29% compared with Holstein-Guzerat and Holstein-Nellore cows, respectively. The highest total yields were observed in lactations beginning in the dry season. Total yield increased with lactation number. The shortest lactation periods were observed in lactations beginning late in the rainy season. The incomplete gamma function was used to fit the individual curves from the genetic groups evaluated. The total yield differences were linked to early yields and yields at the peak of lactation.
bovine; F1; incomplete gamma function
