Farm environmental factors and cow physiological traits affecting the gender ratio of newborn dairy calves.
Farm environmental factors in the KwaZulu-Natal Midlands were investigated to determine whether any of them had an influence on the calf gender ratio in dairy herds in this area. Heat detection score (P<0.001), number of inseminators (P<0.001), geographical location (P<0.001), bull .presence on the farm (P<0.001), and timing of insemination (P<0.001), had a highly significant effect on the calf gender ratio. The probability of female calves increased when heat detection included visual observation and two aids, three inseminators, geographical location around Boston, Kamberg, Umzimkulu or Underberg, when a bull was present on the farm and when timing of insemination was according to an assessment of oestrous behaviour of each cow (where insemination was immediate if a cow was thought to have been on standing heat for a period before being observed, or delayed until the next milking if this was not the case). These factors could be manipulated, where possible, in an attempt to skew the gender ratio of newborn dairy calves in favour of females, as this would lead to economic gain in a dairy enterprise due to the comparative worth of heifer calves compared to bull calves. An additional experiment was conducted to examine the relationship between rectal and vaginal temperature with oestrus and ovulation in the cow. Rectal temperature was found to be the best predictor of both oestrus (P<0.001) and ovulation (P<0.001), when measured within 24 hours of the start of oestrus. Rectal temperature should, therefore, only be used to predict the onset of oestrus if the approximate time of oestrus was known from heat expectancy records. Rectal temperature could also be used to determine when to inseminate relative to estimated time of ovulation to increase the probability of male or female calves, if used in conjunction with the observation of oestrous behaviour.