The effect of age, breed, days open at conception, stage of lactation and pregnancy upon daily body weight and milk weight in lactating dairy cattle were studied. The cattle used in the study were those of the University of British Columbia Research Farm at Oyster River, B.C. 76 Holstein and 60 Holstein X Ayrshire crossbred milking cows at different stages of lactation and both either open or pregnant, were used in this study. The study was carried out during the months of May and June of 1975. The procedure involved weighing the animals daily after the afternoon milking and recording the daily milk yield along with the weight of each cow. The cows were classified into three groups, namely All, Open and Pregnant, the first group being a combination of the last two. For All and Open cows, the effect of age, breed, age by breed and number of days in milk on body weight and milk weight were investigated. For the Pregnant cows, the effect of age, breed, number of days open at conception, age by breed, age by number of days open at conception, breed by number of days open at conception and number of days pregnant on body weight and milk weight were investigated. Using the number of days open at conception as a dependent variable, the effect of age, breed and age by breed were investigated. The results showed that among All, Open and Pregnant cow groups, the older cows were significantly (P <0.05) heavier and were producing significantly (P <0.05) more milk than the younger cows. Age accounted for 0.9%, 1.5% and 0.8% of the body weight variation and for 0.1%, 0.5% and 1.3% of the milk weight variation in All, Open and Pregnant cow groups respectively. Among the All, Open and Pregnant groups, the older cows weighed more by 143.22, 133.12 and 125.07 lbs and produced 3.72, 6.98 and 12.14 lbs more milk per day respectively than the younger cows. Among All, Open and Pregnant groups, a significant breed effect (P <0.05) on body weight and milk weight was observed. In the three groups, breed accounted for 2.3%, 3.8% and 0.1% of the body weight variation and for 0.1%, 0.3% and 0.6% of the milk weight variation respectively. Among All and the Open cow groups the Hoi steins were heavier by 223.00 and 214.04 lbs and were respectively producing 5.20 and 5.26 lbs more milk daily than the crossbreds. Among the Pregnant cow group the Hoi steins were lighter by 49.33 lbs and produced 7.74 lbs less milk daily than the crossbreds. The number of days open at conception was a significant source of variation (P <0.05) affecting body weight but not milk weight in the Pregnant cow group. It accounted for 1.3% of the body weight variation. Those cows who stayed open longer than 123 days were heavier by 159.21 lbs than those who became pregnant earlier or on day 123 after calving. Age by breed for the All cow group was significant (P <0.05), accounting for 0.1% of the body weight variation. This interaction was not significant for body weight of the other two groups and for milk weight of all the three groups. Age by number of days open at conception was significant (P <0.05) for the Pregnant cow group milk weight and accounted for 1.2% of the variation. This interaction was not significant for body weight. Breed by number of days open at conception was not significant for the Pregnant cow group body weight and milk weight. Number of days in milk for All and Open cows had a significant (P <0.05) effect on both body weight and milk weight. It accounted for 10.2% and 2.9% of the variation in body weight and 35.6% and 18.4% of the variation in milk weight in All and Open cow groups respectively. In the Pregnant cow group, the number of days pregnant resulted in significant (P <0.05) effect on both body weight and milk weight. It accounted for 30.8% and 61.4% of the body weight and milk weight variation respectively. When number of days open at conception was used as a dependent variable, age, breed, and age by breed showed a significant (P <0.05) effect in the pregnant cow group. Age, breed, and age by breed accounted for 0.3%, 0.4% and 0.9% of the variation respectively. / Land and Food Systems, Faculty of / Graduate