Factors affecting the occurrence of atypical lactations for Holstein-Friesian cows

Lactation curve traits were determined for Holstein-Friesian cows in Tunisia. A total of 6495 lactation records on 4340 cows were used in the analysis. Factors a, b, and c representing the beginning of lactation, the increasing phase before peak yield, and the decreasing phase after peak yield, respectively, were estimated from fitting the incomplete gamma function to lactation records by non-linear regression. The effects of fixed environmental factors (calving year, calving season), management factors (production sector, times of milking per day, DIM at first test-day), and parity on the probability of occurrence of atypical curves were studied by logistic regression. Approximately 25% of the lactation curves were defined as atypical: the curve failed to show an increase in the first phase (b<O), or the curve showed steadily increasing production (c<0). Seven factors were significantly associated (P<0.01) with occurrence of atypical lactations. The odds of observing an atypical curve were reduced by approximately one-half for investor or farmer herds compared to the odds of observing a typical curve. The probability of occurrence of atypical curves compared to that of typical curves also increased by 36% and 45% in the spring and summer seasons, and by 20% in second lactation, respectively. The probability of occurrence of atypical curves also changed by 6% with calving year and increased by 4% for each 1 day delay in the first test-day date. (C) 2003 Elsevier B.V. All rights reserved.