Observe Estrus in Cow Herd to Monitor Bull Fertility During Breeding Season

Bull condition and fertility is paramount in successful breeding, which is why Bob Weaber, Cow-calf specialist at Kansas State University, is urging farmers to keep an eye on the bull through the breeding season and detect estrus in cows to monitor bull fertility.
calendar icon 9 July 2013
clock icon 4 minute read
Uni Kansas State

All too often when producers and specialists talk about ‘fertility’ we gravitate to a discussion of reproductive failure in cows and subsequent culling for management strategies. It is, however, important to recognize the importance of bull fertility as well, especially in commercial operations that rely heavily on natural service sires. With the increased cost of bulls, it is tempting to stretch the bull to cow ratio. This strategy may decrease the breeding cost per pregnancy but it may also put your herd’s reproductive rate at greater risk.

"Detect estrus at least once and preferably twice per week during the breeding season"
Bob Weaber,
Cow Calf Specialist

The risks of male reproductive failure tend to increase following nutritional or environmentalinsults to the bull. Environmental conditions such as drought or extended winter and cold temperatures can not only adversely affect body condition score and fertility in cows, they can take their toll on bull fertility as well. The wide range of precipitation and temperatures experienced recently should cause us to pause and consider strategies to assure asuccessful breeding season.

It is always a good idea to have a breeding soundness exam performed on bulls prior to turnout to identify infertile and sub-fertile bulls. However, a bull with satisfactory breeding potential before the season starts may not stay that way. In multi-sire breeding pastures, injuries are fairly common causing either injury to feet and legs or the reproductive tract, disrupting the libido and serving capacity of the bull. Fertility issues can and do present themselves in otherwise healthy and fit appearing bulls. Some of the fertility problems are associated with a range of venereal diseases including Trichomoniasis (a protozoan disease) which causes early embryonic losses in cows bred by infected bulls. (see sidebar map for affected counties and January 2012 Beef Tips for more information on Trich). Other causes are less specific and hard to diagnose but result in delayed rebreeding and or low pregnancy rates.

The value of a tight calving distribution is often measured in the weight advantage of older calves at weaning. A tight breeding season is a good sign of the success of nutrition and management decisions. If you still have a large percentage of cows calvingin the last 30 days of a 90 day breeding season an investigation into the cause is warranted.

Producers are advised to periodically monitor estrous behavior (heat) in their cowherd to see if the service sires are settling cows. At the beginning of the breeding season, most of the cows should be at least 30 to 45 days postpartum and beginning to show estrous behavior. If all cows in a group have resumed normal estrous cycles then about 5 percent of the cows should be in estrus each day. Over the first 42 days of the breeding season nearly all thecows should exhibit estrus and be bred at least once. A percentage of these cows will not conceive and will return to estrus. If we assume that 80 percent of the cows bred during the first 42 days of the breeding season conceive, then 20 percent of these should return to estrus. During the middle and latter parts of the breeding season, approximately one percentor fewer of the cows should cycle each day. If after 45 days of the breeding season you still have three to five percent of the cows cycling each day action is needed; either pull bulls for a breeding soundness exam to confirm fertility and/or replace these bulls with bulls known to have recently passed a breeding soundness exam.

It is common for bulls to lose body condition during the early part of the breeding season when working hard to cover a high percentage of the cows in the herd. If body condition continues to decline measurably after the middle of the breeding season and bulls are still very active in monitoring and breeding cows, you should check for fertility problems in the bulls. Bulls in low body condition score should be replaced in the breeding pasture by bulls in adequate condition. You should record body condition scores on bulls at turnout and then periodically during the breeding season for use in management decisions. 

Detect estrus at least once and preferably twice per week during the breeding season. Estrus should be observed at a distance from the cows early in the morning or near dusk when it cools off just as you would for an artificial insemination program. While finding a bad bull during the breeding season is a bad thing, learning of a problem at pregnancy check time in the fall is even worse. Remember, getting cows bred is profit mission number one! Pregnant cows require fertile bulls. It pays to keep tabs on the breeding performance of your bulls during the breeding season.

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