7 May 2009

3 minute read

What they found, said Jennifer Bormann, a researcher with K-State Research and Extension, was that although RFI has been shown to be moderately heritable, heifers that were sired by bulls with low RFI merit performed no differently than heifers sired by bulls with high RFI merit.

RFI is the difference between what an animal eats, and what it is predicted to eat, based on its size and growth. Animals with negative or low RFI eat less than is expected for their level of production, and are more efficient than animals with a high RFI. A low RFI is more desirable because that animal can more efficiently and inexpensively turn feed into beef.

“RFI has become the measurement of choice in the beef industry, but there is not a lot of research about how cows (dams) fit into the picture or if the heifers would fare differently on a more concentrated diet than the one we fed,” she said. She presented the study’s findings at K-State’s Cattlemen’s Day in March.

“Previous research has shown that RFI is moderately heritable,” the researcher said. So she, along with colleagues Dan Moser, of K-State, and Twig Marston at the University of Nebraska-Lincoln, studied how heifers sired by bulls with high and low RFI ratings would fare when it came to their own RFI ratings.

To begin the study, bulls with low or high genetic merit for RFI were selected from the Australian Angus Association sire summary and mated to Angus cross commercial cows at K-State’s Cow-Calf Unit in 2005 and 2006.

“There were no significant differences between heifers sired by low or high RFI EBV (estimated breeding values) system bulls in RFI, feed intake, feed conversion ratio or gain,” Bormann said.

Heifers in the study were developed at a relatively low rate of gain, she noted. Genetic differences in RFI calculated in growing bulls may not have been expressed on the lower plane of nutrition of the developing heifers.

Heifers sired by low RFI bulls in the study also had more rump fat and tended to have more ribeye area and intramuscular fat.

“Our results indicated that selection for RFI should not negatively impact carcass quality,” she said.

There is no doubt, Bormann said, that a producer would prefer to breed for animals that are as efficient as possible at converting feed to high-quality beef. But there is much still to be learned about selecting for RFI in beef breeding programs. Several other universities are also studying various aspects of the practice. K-State will continue studying the issue to try to determine the best practices when selecting for RFI.

“The bottom line,” said Bormann, “is that as selection for RFI becomes more adopted by the industry, the relationships between diet, growth rate, body composition, and RFI in heifers need to be better understood.”

April 2009