title: Non-linear modelling to describe lactation curve in Gir crossbred cows.
abstract: BACKGROUND
  The modelling of lactation curve provides guidelines in formulating farm managerial practices in dairy cows. The aim of the present study was to determine the suitable non-linear model which most accurately fitted to lactation curves of five lactations in 134 Gir crossbred cows reared in Research-Cum-Development Project (RCDP) on Cattle farm, MPKV (Maharashtra). Four models viz. gamma-type function, quadratic model, mixed log function and Wilmink model were fitted to each lactation separately and then compared on the basis of goodness of fit measures viz. adjusted R2, root mean square error (RMSE), Akaike's Informaion Criteria (AIC) and Bayesian Information Criteria (BIC).
  
  
  RESULTS
  In general, highest milk yield was observed in fourth lactation whereas it was lowest in first lactation. Among the models investigated, mixed log function and gamma-type function provided best fit of the lactation curve of first and remaining lactations, respectively. Quadratic model gave least fit to lactation curve in almost all lactations. Peak yield was observed as highest and lowest in fourth and first lactation, respectively. Further, first lactation showed highest persistency but relatively higher time to achieve peak yield than other lactations.
  
  
  CONCLUSIONS
  Lactation curve modelling using gamma-type function may be helpful to setting the management strategies at farm level, however, modelling must be optimized regularly before implementing them to enhance productivity in Gir crossbred cows.
authors.affiliation: Network Project on Sheep (Deccani) Improvement, Mahatma Phule Krishi Vidyapeeth, Rahuri, Maharashtra 413722 India.
authors.forename: Yogesh C
authors.initials: YC
authors.lastname: Bangar
authors.affiliation: Division of Livestock Economics, Statistics and IT, Indian Veterinary Research Institute, Bareilly, Uttar Pradesh 243122 India.
authors.forename: Med Ram
authors.initials: MR
authors.lastname: Verma
keywords: Gamma-type function; Gir crossbred; Lactation curve; Non-linear modelling