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Studies Show Selecting Performance Traits Hurts Reproduction

Studies Show Selecting Performance Traits Hurts Reproduction
Studies on cows and bulls suggest traditional performance traits are antagonistic to reproductivity.

Evidence keeps growing that decades of selection for production traits have damaged reproductivity of the nation's beef animals.

Two studies, one on cows and the other on bulls, bear out this hypothesis.

Researchers in Ireland used records from 156,056 beef cows to estimate genetic parameters for reproductive performance in beef herds. They also calculated genetic correlations between reproductive traits and performance traits and published their study this year.

Taller = poorer: Two recent studies have added to the knowledge showing traditional production traits hurt cow and bull reproduction.

All of the cows were of Bos taurus breeding and the majority of the cows were crossbreds. Researchers measured these reproductive traits:
• Age at first calving
• Whether or not cows calved in the first 42 days of the calving seasons

• Calving interval between consecutive calving events
• Survival to the next lactation

Researchers measured these performance traits:
• Calving dystocia
• Linear-type traits which describe the skeletal, muscular and functional characteristics of an animal
• Live weight and price
• Carcass traits
• Producer-subjectively scored traits of weanling quality and docility

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The researchers did not report anything unusual in their heritability estimates for these traits and said they confirmed what other research has suggested. However, they did note these four correlations.

1. Animals genetically predisposed to calving for the first time at an older age had inferior genetic merit for calving interval, meaning longer intervals.

2. Genetic merit for calving interval was an important contributor to the genetic differences in survival, meaning cows with shorter calving intervals stay in the herd longer.

3. Genetically taller, longer and wider animals were of inferior genetic merit for reproductive performance.

4. Greater genetic merit for increased muscularity was genetically correlated with reduced number of animals calving in the first 42 days of the calving season and longer calving intervals.

In a study published in 2013 researchers measured the relationship between feed efficiency and fertility in young bulls.

Canadian researchers at the University of Guelph measured feed efficiency as residual feed intake. RFI is defined as the difference between an animal's actual feed intake and its expected intake based on body weight and growth rate.

Positive RFI animals eat more than expected in relation to their weight and gain, so they are less efficient. A negative RFI value is better and indicates a more efficient animal. Animals selected purely on negative RFI animals, however, are also known to be larger-framed.

In conjunction with RFI, the researchers measured bull fertility traits such as sperm motility, viability and scrotal circumference.

In the Canadian study, 110 crossbred beef bulls were selected for semen collection from a total of 328 bulls based on their desirability for use in the University of Guelph breeding herd over a six-year period from 2002 to 2007.

All of the bulls went through a 112-day performance test and measured for individual feed intake. The average age of the bulls at the start of the performance test was 274 days, with average initial weight of 796 pounds and final weight of 1,246 pounds. At the end of the test, researchers measured scrotal circumference and collected semen.

These researchers reported that the 10 bulls with the greatest feed efficiency, meaning the lowest RFI ranking, showed decreased sperm motility, viability and scrotal circumference when compared with the 10 least feed-efficient bulls, or those with the highest RFI.

The researchers say these data suggest low-RFI bulls are less fertile than high-RFI bulls and that selecting for improved feed efficiency by itself would be counterproductive to reproductive soundness.

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