Abstract
Data on Sahiwal cows kept at the Livestock Production Research Institute, Bahadurnagar, Okara during the period 1963-87 were utilized for this study. The 305-day milk yield based on 2030 lactation records averaged 1670.45 ± 12.71 kg with a C.V. of 34.51%. The postpartum oestrus interval and service period averaged 130.40 ± 2.88 days and 146.40 ± 2.03 days, respectively and number of services per conception were 2.10 ± 0,04. The correlation and regression coefficients between level of milk production and postpartum oestrus interval, service period and services per conception were 0.017 and 0.003 ± 0.005, 0.044 and 0.008 ± 0.004 and 0.000 ± 0.000, respectively. The correlation and regression coefficients between level of milk production and service period were non-significant while the correlation and regres-sion coefficients between level of milk production and number of services per con-ception were significant (P< 0.01).
INTRODUCTION
Reproductive efficiency is one of the most important factors determining the suc-cess of a dairy farm. Some of the measures of the reproductive efficiency are known to be wholly environmental while others are determined by the heredity and environment both, The environmental factors include their nutritional status, management, disease control especially of the reproductive tract, quality of semen used and time of insemina-tion (Maurer and Chenault, 1983). Similarly, the biological factors such as age of the cow and level of production could he important in influencing the fertility parameters. The parameters of reproductive performance:in-clude, age at maturity, age at first calving, postpartum oestrus interval, service period and calving interval, etc. For better return, it is essential to improve the fertility parame-ters so that the productive life as well as calf crop could be increased, This study was planned to determine the effect of level of production on some fertility parameters
such as postpartum oestrus interval, service period and number of services per concep-tion in Sahiwal cows.
MATERIALS AND METHODS
The pedigree and performance records of a purebred Sahiwal herd maintained at the Livestock Production Research Institute, Bahadurnagar, district Okara, Pakistan during the period 1963-87 were utilized for this study. Only the normal and complete records of the cows were included in this study. incomplete !aerations of less than 180 days and the cows having less than 6 com-plete lactation records were excluded from the study. The lactation was considered ab-normal if it followed a stillbirth, abortion or a premature birth. The data on the following traits were tabulated from the history sheets of the cows for statistical analysis: i. Number of services per conception ii. Postpartum oestrus interval iii. Service period at each conception iv. 305-day milk yield in each lactation
