Farm environmental factors and cow physiological traits affecting the gender ratio of newborn dairy calves.
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Date
2002
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Abstract
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.
Description
Thesis (M.Sc.Agric.)-University of Natal, Pietermaritzburg, 2002.
Keywords
Dairy cattle--Reproduction., Dairy cattle--Physiology., Cows., Calves., Dairy farming., Theses--Animal and poultry science.