Vet Column 11-29-10 |

Vet Column 11-29-10

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

Although natural service can be used in conjunction with estrous, or heat, synchronization of cattle, most producers use artificial insemination in their synchronization breeding plans. For beef and dairy heifers, this provides a distinct advantage for using progeny proven sires, especially for calving ease. Research over the years has provided several products and protocols for consideration.

One option has been the use of long-term progestin-based products. These progesterone like products are used to keep heifers from displaying heat, and then when treatment with these products stop, a high percentage of the animals display heat. An example is the use of melengestrol acetate (MGA) in the feed at the rate of 0.5 mg per head per day for a period of 14 to 19 days depending upon the protocol. Once the MGA feeding is withdrawn, a standard dose of prostaglandin is administered 19 days later. As a result, it takes a minimum of 35-37 days of planning prior to an expected date for breeding.

Another product that can be used as a long-term progestin-based product is the controlled internal drug release (CIDR) product. CIDRs are placed in the heifer for 14 days followed by use of a standard dose of prostaglandin 16 days later. This results in a planning period of 32-33 days prior to an expected breeding date.

Scientists from the University of Missouri recently published data comparing these two protocols. Emphasis was placed on both the display of heat as well as the timing of the associated ovulation, or release of the egg from the ovary for fertilization. Getting synchronized heifers to display heat is one thing. Getting them to synchronize ovulation can be another. The important difference is that the more closely ovulation can be synchronized, the more likely that inseminating all animals by establishing a fixed time might become possible. This could greatly minimize the labor of heat checking and the resulting insemination time over a period of days as they come into heat.

For this study, one treatment group involved feeding MGA at the rate of 0.5 mg per head per day for 14 days followed by a 25 mg intramuscular injection of Lutalyse 19 days later. A second treatment group involved placing CIDRs treatment animals for 14 days followed by a 25 mg intramuscular injection of Lutalyse 16 days later. Heat Watch estrus-detection transmitters were used in both groups to aid in heat detection. Two different experiments were designed and 495 beef heifers were used in this study. Heifers in both experiments were inseminated 12 hours after the onset of estrus.

Results of these studies demonstrated that the heat response after use of Lutalyse did not differ between the MGA or CIDR use. The average time from use of Lutalyse also did not differ. The range of time over which heat was displayed as a group was also not different, although it tended to be tighter in pre-synchronized cycling heifers with MGA. In groups with more pre-pubertal heifers at the time of synchrony, response and tighter synchrony favored the use of CIDRs.

Most importantly, based upon heat response, synchrony of heat, and the resulting fertility after treatment, MGA in the feed or intra-vaginal CIDR placement protocols to heat synchronize heifers can be used equitably.

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