The objective of this work was to evaluate the effects of year of calving (YC), season of calving (SC), genetic group (GG), calf sex (SEX), reason for drying-off (RD), cow age at calving (AGE, in months), lactation length (LL, in days) and sire (SIRE) on milk yield (MY) in a Holstein breed herd. A total of 684 observations, from 267 cows raised in a physical models of dairy cattle production system maintained at Embrapa - Centro de Pesquisa de Pecuária do Sudeste, São Carlos, SP, was included in the analysis. Data on milk yield were collected from October 1984 to December 1993 and analyzed by the least-squares method through a mathematical model including the fixed effects of YC, SC, GG, SEX and RD, linear and quadratic effects of AGE and LL as covariates and the random effect of SIRE. There were effects of YC, SC, RD, AGE (linear and quadratic), LL (linear and quadratic) and SIRE on MY. GG and SEX did not influence MY. In the studied period, there was a linear increase of MY in 347.6±22.02 kg/lactation.year. The cows freshening in the summer (January-March) were less productive (MY = 4,325±82 kg) than those freshening in the other seasons of the year (4,795±49 kg). The cows with lactations terminated because of pre-calving presented higher MY (4,903±78 kg) than those with lactations terminated because of low production/other reasons (4,452±61 kg). The MY was influenced by linear (18.86±2.39 kg/mo.) and quadratic (-0.2934±0.0456 kg/mo²) effects of AGE. There were linear (15.83±0.41 kg/day) and quadratic (-0.0057±0.0023 kg/day²) effects of lactation length on MY.
dairy cattle; environmental factors; genetic effects; Holstein cattle; reason for drying-off
