For producers with years of experience, picking replacement heifers may seem to come naturally. It's not all about the looks, though, says Ryon Walker in a recent report from the Noble Research Institute out of Ardmore, Oklahoma.
Walker, livestock consultant at Noble, stresses the importance of the calendar and the scale when deciding which females to keep in the herd. He adds heifer development is where good herd management really shines.
"Reproductive traits are very lowly heritable," he says. "Ten percent is controlled by genetics. The other 90% is controlled by management and environment."
At 13 to 15 months of age a well-developed heifer reaches puberty, weighs 65% of her mature body weight and has a 5 to 6 body condition score (BCS). Most importantly, she conceives within the first 21 days of the breeding season. While there are other considerations like pelvic measurement (a minimum of 150 square centimeters) and reproductive tract scoring, Walker says success pretty much comes down to meeting those basic criteria.
Puberty in heifers, Walker explains, is a factor of body weight, age and breed. Bos indicus cattle, for example, tend to reach puberty at 16 to 18 months of age. Bos taurus cattle reach that point between 12 and 14 months of age. This is where crossbreeding and the value of heterosis can play a role in building a herd where heifer puberty can be genetically managed to some degree.
Another genetic management tool that effects age of puberty in heifers, is scrotal circumference on breeding bulls. Walker says basing bull selection partially on scrotal circumference (SC) can change the dynamics of a herd, as each 1 centimeter increase in SC equals that bull's daughters reaching puberty four days earlier.
"Research out of Canada has shown scrotal circumference has increased 0.5 to 3.5 centimeters in seven Bos taurus breeds of bulls from 1972 to 2011. This has had a positive impact on lowering puberty age," he says.
While genetics has its role, heifer body weight, a direct result of nutrition, is the biggest influencer of age at puberty. To develop a heifer to 60 to 65% of her mature body weight, it's important to accurately know the average weight of females in the herd. If an operation doesn't have scales, Walker suggests using an average weight on cull cows as an indicator. Many producers, he notes, tend to guess too low on body weights.
Some research has pointed to low-weight development as a cost-saving tactic in heifer development, with evidence that conception rates to natural service are not negatively impacted over a defined breeding season while feeding less. Walker says there are management considerations with this approach.
"Heifers should reach 85% of their mature body weight at calving to ensure their pelvic area has developed to a size big enough to deliver a calf," he says. Otherwise the stresses of calving and lactation could have long-term negative impacts on the female's fertility.
While Walker says producers can successfully breed heifers at lighter weights, a shorter breeding season will increase the likelihood of those heifers remaining in the herd longer. In fact, preliminary data has shown that lighter heifers (less than 60% of mature weight) conceiving to AI fall out of the herd sooner than heifers weighing more than 60% of their mature weight at breeding.
Research out of Nebraska showed pregnancy rates to natural service were similar over 45 days in lighter versus heavier developed heifers, he added. And those heifers that started out lighter do catch up to that 85% level by calving due to compensatory gain. The key is good management and nutrition.
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