COMPARISON OF DIFFERENT PROCEDURES FOR LACTATION LENGTH ADJUSTMENT OF MILK YIELD IN SAHIWAL CATTLE
2004
I. R. Bajwa, M. S. Khan, M. A. Khan1 and K. Z. Gondal2
The purpose of the present study was to compare three procedures for lactation length adjustment of milk yield. For this purpose, weekly milk yield records (n = 2039) of Sahiwal cows were used. Multiplicative adjustments using simple linear regression of milk yield on lactation length was the first procedure, in the second procedure last recorded milk yield was used to predict milk yield of unrecorded lactation, while the third procedure was similar to the second procedure except that predictions included average daily milk yield of the recorded lactation as well. The bias (the difference between actual and predicted milk yield) was lowest in the third procedure. The standard deviation of bias was 235 kg for milk yield adjusted by using last test day milk yield and average daily milk yield of the known lactation as compared to milk yield adjusted by linear regression i.e. 496 kg for lactation length of 56 days. The standard deviation of bias decreased to 23 kg of milk for the milk yield adjusted by using last test day milk yield and average daily milk yield of the known lactation as compared to 66 kg for the milk yield adjusted by linear regression towards the end of the lactation length. The correlation coefficient between actual and predicted milk yield was 0.881 for the milk yield predicted by using last test day yield and average daily milk yield of the known lactation length as compared to 0.10 for milk yield predicted by linear regression for the lactation length of 56 days. The correlation coefficient also increased with the increase in lactation length. Lactation length adjustment of milk yield should be done using last test day information along with average yield of the recorded lactation.
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