By Bethany Johnston and Aaron Berger
Have you wanted to have more calves born earlier in your calving season, but didn't want to deal with the increase in labor, cost and facilities to use estrus synchronization and artificial insemination?
One protocol (see figure below) can increase the number of cows coming into estrus early in the breeding season, with one time through the chute, one injection and breeding using only natural service.
This protocol calls for bulls to be turned out with the cows on Day 0. On Day 5, cows are given a shot of prostaglandin that synchronizes most of the cows to be in heat or estrus from Day 6 through Day 10.
The injection of prostaglandin causes any cows with a corpus luteum present on one of their ovaries to regress, ceasing progesterone production. This triggers the cows to come into heat or estrus.
If the cow conceives during Day 1 to 5, she will not abort when given the prostaglandin injection on Day 5 because the developing corpus luteum at the site of ovulation on the ovary has not yet reached maturity and will not respond to prostaglandin.
Research conducted at the Fort Keogh Research Center near Miles City, Mont., used this protocol over a three-year period, achieving pregnancy rates of more than 85% in a 32-day breeding season.
Research from the University of Nebraska showed 75% of cows calved in the first 21 days of the calving season using this estrus synchronization protocol compared with only 63% of cows from nonsynchronized natural service breeding.
Using natural service with this method of estrus synchronization requires adequate bull power. A bull to cow ratio of 1:15 with yearling bulls, or 1:25 with mature bulls, should be sufficient.
Because early breeding and synchronized estrus is occurring over a 10-day period, fertile and active bulls with adequate libido should be able to handle the number of cows that will be coming into heat. Breeding bulls should undergo a breeding soundness exam before the breeding season.
Producers considering this synchronization tool should remember that several factors affect pregnancy rate, including cow body condition score, plane of nutrition, cattle health and bull fertility.
Estrus synchronization can shorten the calving season. The BeefWatch article "Use of Natural Service Sires with Synchronized Estrus" highlights other advantages of estrus synchronization and natural service.
Note that a relatively new prostaglandin product in a high-concentration formula allows for a 2-millileter dose to be injected subcutaneously (under the skin). Other prostaglandin products have an intramuscular injection label requirement, which requires a longer needle for deep muscle penetration.
IM injections have a greater risk to develop lesions in the muscle that affect meat quality. The 2018 Nebraska Beef Report article "Comparison of Two Alternate Prostaglandin Products in Yearling Beef Heifers" showed beef heifers performed similarly to either the IM injection of prostaglandin or the subcutaneous injection of high-concentration prostaglandin.
Johnston is a former Nebraska Extension educator, and Berger is a Nebraska Extension beef educator.