Heifer calving date influences weaning weights, longevity

Many producers will be in the middle or perhaps near the end of the 2015 calving season, hopefully with many healthy calves on the ground.

I have written in the past about the importance of front-loading the calving season.

Dr. Bob Larson from Kansas State University coined the phrase, and I’ve adopted it in many of my presentations.

Larson is referring to the goal of having 65 percent of a herd’s cows calve within the first 21 day period of the calving season.

The percentage is a direct reflection of successful breeding during the first 21 days of the previous breeding season. Cows and heifers that were cycling earlier were more likely to be bred early in the breeding season and are now calving early.

Body condition at calving and into the breeding season is one of the most important factors that determine how long it takes cows to begin cycling after breeding. Numerous studies have demonstrated the importance of body condition and its impact on the interval between calving and breeding.

Heifers naturally have a longer interval between calving and breeding after their first calf. On average, they will take 80 to100 days to begin cycling, compared to 50 to 60 days for older cows.

Front loading the calving and breeding seasons for heifers is even more important.

The calves that are born earlier in the calving season are also at an advantage. They will be older and heavier at weaning and if they are heifers, they will be more likely to reach puberty at an earlier date and start cycling sooner.

Researchers are starting to discover that heifers that have their calves early in the calving season may even be more valuable in the long term.

A recent paper in the Journal of Animal Science collected longevity data on more than 2,000 heifers from producers in South Dakota and more than 16,000 heifers at the U.S. Meat Animal Research Center in Clay Center, Nebraska.

U.S. Department of Agriculture researchers followed these heifers over their subsequent life span until they left the herd. They divided them into three groups, depending on when they calved as a first calf heifer.

They then compared the subsequent productivity of the heifers that calved for the first time in the first 21 days of the calving period to heifers that calved in the second or third 21 day period.

Remarkably, the heifers that calved for their first time in the first 21 day period had significantly longer life spans in the herd: 8.2 years compared to 7.6 and 7.2 years for the two subsequent calving groups.

Longevity of the females in the herd is an important factor that can affect a ranch’s profitability.

This study clearly showed that heifers that are bred earlier and calve earlier tend to stay in the herd longer, which demonstrates the importance of front-loading the calving season for heifers.

These heifers also weaned heavier calves. That isn’t surprising given that they calved earlier, but the researchers also showed that heifers that calved in the first 21 days tended to continue to stay in that first 21 day calving group in subsequent years.

As a result, they had significantly heavier weaning weights in their offspring for all of their first six pregnancies. It is a huge advantage to be able to create a heavier calf every year for six years simply by ensuring that the heifer calved in the first 21 days of the calving period for its first calving.

The report authors suggest that herd managers might consider breeding extra heifers than they need for their herd replacements.

This would allow producers the opportunity to select the replacement heifers that they will eventually keep by having their veterinarian estimate the fetal age at pregnancy checking time.

The report also suggested the strategy of focusing on the youngest heifers before breeding because they are at the greatest risk of not cycling early and either being open or becoming pregnant later in the season.

These heifers might be targeted for estrus synchronization programs, which in turn might help induce ovulation and improve the chances that they would conceive earlier in the breeding season.

The authors also said selecting replacement heifers based on the date they become pregnant is better than selecting them solely based on age.

It would appear that Larson’s motto of front loading the breeding season is even more important when it is applied to the first-calf heifers.

John Campbell is head of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.

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