عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The effects of days in milk (DIM) on the curves of milk yield and milk fat percentage in Holstein cows of Iran were studied. A total of 964,606 and 885,346 test day records were used for milk yield and milk fat percentage, respectively. The data were grouped in four classes including: 1) 240 < DIM < 301, 2) 300 < DIM < 401, 3) 400 < DIM < 501 and 4) 500 < DIM < 601 days. To fit the curves, nine non-linear models for every one of the classes were applied. The Narushin & Takma 1 function which previously has been used for fitting the production and growth characteristics of chickens was used in this study. It showed a relatively high goodness of fit in fitting milk yield and milk fat percentage curves compared to other functions. Wood, Dijkstra and Rook functions showed higher Adjusted R2 whereas the third order Legendre polynomial function showed the lowest value in all the classes. The cows had longer lactation period, showed higher persistency and yielded higher daily milk at peak and post peak stages of lactation period. The forth class of cows yielded considerably higher milk and fat (based on 305 day production) compared to other groups. On the contrary, the rate of reduction of milk yield was significantly high for cows located in the lowest lactation length class and the slope of declining stage of milk yield curve was sharper.