A 60-day waiting period between parturition and the first rebreeding insemination has long been the practice recommended. Only recently has this practice been challenged. Recent work reveals that waiting 60 days is not necessary. Considerable time can be saved by beginning rebreeding at the first heat after 40 days postpartum. Reducing the voluntary waiting period by 20 days will shorten the calving internal by about 15 days. About five days are lost because of missed heats and a slightly reduced conception rate. Estimates of services required per conception at various days open are in Table 1.
Table 1. Relationship of Days Open to Services Per Conception
|Days Open||Services per Conception*|
*It is estimated that an extra 0.2 ampules or straws of semen will be needed per cow in a herd that has a voluntary waiting period of 40 days as contrasted to 60 days.
By beginning rebreeding at 40 days, a dairyman can usually reduce expenses enough through reducing days open to offset the slight cost of the extra semen used. With rebreeding beginning at 40 days, the 15 days saved might be valued at $15.00. This savings will be achieved at a cost of $2.00 (0.2 service x $10.00 per straw). The concerns about greater embryonic loss, abortion, metritis, retained placentas, or weak calves at birth have not been observed in several extensive studies with early rebreeding.
With earlier rebreeding, about 45 percent of the cows bred will become pregnant to the first insemination. Since a 40 to 60 day dry period is required for cows to produce at their potential the next lactation, those conceiving prior to 60 days postpartum will have lactations shorter than 305 days. This should be a minor concern with most cows, since they will produce more during their lifetime in the herd by being dried off and allowed a 40 day dry period before their next lactation.
Proper timing of insemination in the heat period affects the conception rate. Careful monitoring indicates that the average heat period length is 18 + 5 hours. The first six hours of this period is the time of poorest conception. The next 12 hours, which extends to the end of heat, is a period in which the probability of conception doubles. The period extending to six hours after heat is an intermediate period in conception probability.
This figure indicates that a substantial increase in the conception rate (up to 15 percent) could result if insemination timing is precisely controlled. The reference point for proper timing is the entry into heat and this usually is difficult to determine with intermittent observation or with marking devises. A cow either observed or found marked at 6:00 a.m. may have come into heat anytime after the last observation the previous evening. When a cow is found in heat, shorter time periods since the previous observation will help in estimating when she came "into heat." This will in turn help to estimate the most fertile time to breed.
Dr. Olds of Kentucky has developed a "breeding wheel" based upon the best time to breed. Rules of thumb from this "wheel" follow:
Fertility will be lowered if the semen is deposited at the entrance of the cervix compared to deposition in the cervix. Little difference in fertility exists when semen in placed in the cervix, body of the uterus, or split and deposited in both uterine horns. Deep penetration of inseminating intruments offers risk of disruption of an existing pregnancy. Ohio data indicates that cows will show heat in about 5 percent of the pregnancies, and that two-thirds of those heats occur within the first 60 days of pregnancy. Because of the possibility of disrupting an established pregnancy, it is important that cows serviced for two or more times be inseminated so that the instrument penetrates only to within and not through the cervix.
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This page was last updated on November 16, 2002