Effect of Days Open on Lactation Production
Accurate evaluation of dairy production records is essential if dairymen are to establish and maintain profitable dairy herds and efficient culling programs. Improper record evaluation may result in saving breeding stock that otherwise might be eliminated. Many factors may influence the validity of individual production records. Adjustment factors, currently in use by the United States Department of Agriculture (USDA) for sire and cow evaluations, are generally accepted as removing the majority of these influences. Consequently, these factors are invaluable to practical dairymen. The adjustment factors, suggested by McDaniel in 1967, take into account the geographical location, season of calving, breed, age ac calving, and milking frequency for both milk and fat, independently of one another. These factors are regarded as being adequate in removing variation arising between herdmates. Reproductive efficiency is of economic importance to dairymen. The necessity for regular calving is vital in an efficient dairy operation. in order to maintain longevity and maximize lifetime net returns. Calving interval and days open prior to conception are related measures: of reproductive efficiency which may assist in production record evaluation. Days open prior to conception actually determine the calving interval, assuming a constant gestation length; consequently, examination of days open would appear more applicable in evaluation of reproductive influences on production·. It must be realized that certain environmental factors, such as careful and skillful observation of animals, type of housing system, and nutritional practices, may enter into relationships between production and reproductive efficiency and therefore, these factors must be accounted for in the analysis of data. This study was undertaken to determine the relationships which exist between days open prior to conception and lactation production in South Dakota Holstein herds.