Fatores genéticos e ambientes que influenciam as características das curvas de lactação de quatro grupos genéticos de mestiças Holandês-zebu

Abstract

Average and individual lactation curves of 1,340 F1 Holstein-Gyr (HG), Holstein-Guzerat (HGU), Holstein-Nelore (HN) and Holstein-Zebu (HZ) cows were fitted using incomplete gamma function and data on 14 days-milk-production controls. Cows were raised in two Experimental Farms of Minas Gerais Agricultural Research Company (EPAMIG). Animals were kept in pastures during the rainy season and they were supplemented with maize silage, sugar cane (Saccharum spp.) and urea during the dry season and concentrates at the moment milking depending on production. The parameters of the function (‘a’, ‘b’ e ‘c’) were estimated and them peak yield (PY), time from the beginning of lactation to the peak (TP), persistence (PERS) and total milk production (P) were estimated. In addition, lactation length (LL) was analyzed. Average curves were individually fitted for each parity and each genetic group. Fixed effects of genetic group (GG) and non-genetic factors (farm, season of calving, SC, and parity) on the function parameters and on the four lactation traits (PY, TP, PERS, P, and LL) were evaluated using least squares methodology. Twenty three percent of the individual curves were atypical. Adjusted determination coefficients (R2A) were greater than 90% for most average curves and 99% of the typical individual curves presented R2A greater than that value, indicating the good fit quality of the data to the incomplete gamma function. For all GG and for most parities average lactation curves showed an initial milk production (IP) followed by an ascending phase, a peak and a descending phase. However, the curve for the fifth or above parities of HN cows presented a linear and declining pattern from the beginning of the lactation. For all GG, differences between the average curve of the first and all others parities were observed. When modeling average curves, variations between IP and PY were not expressive, showing a discrete peak. Average TP was of 23.3 days, varying from 15.8 days for HN’s to 32.6 days for HZ’s. From individual curves, estimates of parameter ‘a’, an indicator of initial production (IP), was influenced by GG, SC and parity. The highest IP were observed for HG and HZ animals and the lowest for the HN group. In addition, IP increased with parity and cows calving at the end of rainy season and at the beginning of dry season presented higher IP. The effects of farm, SC and the interaction GG x parity on TP were important. TP was lower for lactations beginning at the end of rainy season, and for the HG’s and the HGU’s higher TP were observed for first parity cows. The effects of farm and the interaction GG x parity on PY were important. PERS was influenced by farm and the interaction GG x parity, higher PERS were observed for the first parity HG cows and, for the other GG, PERS were similar for all parities. The P was influenced by the effects of farm, GG, SC and parity. Total production observed for HG and HZ groups were from 14 to 29% higher than the P estimated for HGU and HN. In addition, higher P was observed for lactations starting in the dry period, and in general, P increased with parity. The LL was influenced by SC and the interaction GG x parity. Smaller LL was observed for lactations starting at the end of the rainy season. Differences between LL in some parities were observed for HG and HN. The interactions GG x OL were mostly in magnitude.