Target body weight can provide guidance in nutritional management of heifers
Target body weight is an important aspect of post-weaning nutritional management for heifer development. Conversations around target body weight started as early as the 1920s and 1930s, according to University of Wyoming Extension Beef Specialist Shelby Rosasco.
“Research during the 1960s, 70s, 80s and even 90s led to guidelines stating replacement heifers should reach 60 to 65 percent of their mature body weight by the start of the breeding season,” Rosasco explains.
“This is a safe number targeted at getting a high percentage of heifers pubertal prior to the start of the breeding season. This is important if we’re having issues with the percentage of heifers attaining reproductive maturity,” she continues.
Reaching a target body weight also helps in maximizing pregnancy rates, she adds.
However, in arid Western environments, reaching 65 percent of mature body weight can provide an economic challenge, especially when feed prices are high.
“Newer research started looking at whether we can develop heifers to a lighter target body weight while not impairing reproductive performance,” Rosasco says. “Studies show heifers can be developed to 50 to 55 percent of their body weight while out grazing, utilizing winter or native range and corn residue to reduce development costs and still maintain reproductive performance.”
Other research has shown similar results, which effectively gives ranchers options for developing heifers, she states.
In 2013, Mulliniks did research in New Mexico comparing heifers developed on native range to heifers in a drylot. On the range, heifers received a 50 percent rumen undegradable protein (RUP) supplement or a 36 percent RUP supplement.
“Reproductive performance was really similar among all three of the treatments,” Rosasco notes. “The 36 percent and 50 percent RUP supplemented heifers were certainly not limited by being developed on native range.”
Heifers were followed through the fourth breeding season to look at retention rates.
“We had significant impacts of the dietary treatments on longevity,” she notes. “The 50 percent RUP supplement had an increased proportion of heifers remaining in the herd through breeding year four, compared to both the 36 percent and the drylot heifers.”
She continues, “I think there’s a really important concept in that heifers developed on the 50 percent RUP diet on native range may have been more adapted to their reproduction environment. They had an opportunity to adapt to and learn how to perform in their production environment. I think it allowed us to put some selection pressure on those heifers to be able to handle the nutrition stresses they were going to see while pregnant.”
She notes developing heifers on range may allow ranchers to put selection pressure on heifers for their future production environments.
“We are allowed to pick out heifers which are able to handle nutritional stresses and perform for us from a reproductive standpoint,” she explains.
“This research gives us opportunities to consider what fits best in our production system to get heifers to meet the bulls we have with them,” Rosasco concludes.
Rosasco presented in a monthly webinar for the Beef Reproduction Task Force on Feb. 15, 2022. Watch for a follow-up article in the Wyoming Livestock Roundup on why timing of nutrition plays an equally important role. View the full webinar at beefrepro.org.
Saige Zespy is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to firstname.lastname@example.org.