Application of non-linear mathematical models to describe effect of twinning on the lactation curve features in Holstein cows.

The objective of this study was to evaluate effect of twinning on the lactation curve characteristics for milk yield (MY), fat (FP) and protein (PP) percentages, fat to protein ratio (FPR) and somatic cell score (SCS) in Holstein cows. The data set consisted of 5,917,677 test day production of 643,625 first lactation cows from 3146 herds in Iran. Calvings were classified into single or twin births. Six non-linear models (Brody, Wood, Sikka, Nelder, Dijkstra and Rook) were fitted to monthly productive records of single or twin calvers. The Rook model provided the best fit of the lactation curve for MY and FP in single and twin calvers and single calvers for SCS and twin calvers for FPR. The Dijkstra model provided the best fit of the lactation curve for PP in single and twin calvers and single calvers for FP. Also, the Wood model provided the best fit of lactation curve for SCS in twin calvers. Twin calvers had greater predicted 200-day and 305-day cumulative milk yield than single calvers. Time to the peak milk yield was observed later for twin calvers (92 days in milk vs. 80 days in milk) with greater peak milk yield (32.16 kg vs. 31.70 kg) compared with single calvers. The results indicated the efficiency of different models for modelling productive performance of single or twin calvers. The application of the association between production and twinning in dairy cows is crucial because present strategies used in the dairy industry are intended to maximize production performance.