Abstract
A rotal of 21,441 milking records of biweekly test-days were collected from six dairy farms of Almarai company, Saudi Arabia to determine the effect of days open on lactation curve and milk production during the period of 1991 to 1996. These records included cows calved in two seasons: winter, for cows calved from October to March and summer, for cows calved from April to September. Season of calving did not have a significant effect on the last biweekly points of the curve, and this is due to the effect of the evaporative cooling system. Days open had a marked effect on milk production. The difference in milk yield between cows with days open <60 days and days open >150 days was 1,021 liter. Moreover, the difference in milk yield at early lactation decreased from 1,021 to 829 liter as the days open increased from 75 to 125, due to the decrease in the effect of conception on milk production with advancing lactation. These data also showed that the middle parr of the curve (105-255) was the least affected part by the variation in days open because the pregnancy effect become more obvious after five months of conception. These data showed that the dairy cattle produce more than 70% of the milk yield during the first 250 days of the lactation curve.