Post-weaning development strategies increase pregnancy success in heifers
“Every operation is different, and what is great about heifer development is we have a lot of different opportunities to have an influence,” says Shelby Rosasco, Extension beef specialist at the University of Wyoming. “I don’t think one opportunity is necessarily better than the other. It’s about what works best in each environment.”
Rosasco adds, “We should make sure we’re monitoring nutrition during the post-weaning period and the post-breeding period to ensure heifers that do get bred early are able to maintain pregnancy and be successful.”
Timing of gain
While making sure heifers are in optimal shape to breed is important, Rosasco notes the timing of weight gain is very important, pointing to a 1997 study.
“In this study, researchers compared heifers developed on an even rate of gain from weaning through the start of the breeding system to heifers on a late-gain system – or stair-step system – when they had a period of lower rate of gain during the first half of development and then stepped to a higher rate of gain during the second half of the development period,” she explains.
The study saw differences in average daily gain and a nice compensatory gain response in heifers from March to April when they were fed a higher rate of gain diet.
The results found heifers are more efficient at utilizing feed during this point in time, which may decrease development costs.
“We also saw, however, the heifers on the late-gain system started at a similar body weight and ended at a similar body weight,” she says. “We ended at the same point, regardless of the development system, and there was no difference in pregnancy rates in these heifers.”
The significant finding, argues Rosasco, is gain can be delayed in heifers, allowing ranchers to save money when feed costs are high without sacrificing reproductive rates.
Further research
Additional research has also been conducted related to timing of weight gain to assess impact on ovarian reserve and reproductive longevity of heifers.
One study looked at comparing stair-step development systems with constant-gain systems to determine the impact on the ovarian reserve.
“When looking at the ovarian reserve by analyzing number of primordial follicles, researchers saw an increase in the number of primordial follicles in the stair-step developed heifers, as compared to their counterparts in the study,” Rosasco explains.
“This was really exciting because it was one of the first experiments in cattle which showed we could potentially impact the ovarian reserve with nutritional management, when those heifers are postnatal,” she adds.
Additionally, research suggested the stair-step system could increase reproductive longevity.
A 2015 study looked at the timing of the stair-step system.
In this study, all heifers followed the same protocol. At eight months of age, a subset of heifers was ovariectomized and the number of primordial follicles was counted. The same process was done at 11 months of age, when the low rate of gain was ending.
“There was no difference in the number of primordial follicles at 11 months of age, but at 13 months of age – at what would be the start of the breeding season – we see the stair-step heifers had an increase in the number of primordial follicles compared to their control counterparts, again suggesting the period of low-high nutrient intake in these heifers potentially impacts the ovarian reserve and positively impacts longevity,” Rosasco summarizes.
Rosasco notes the mechanisms of this system are unknown, but some suggest stair-step development delays activation of primordial follicles, slowing the rate they are used, which allows heifers to have more to utilize over their lifetime.
Grazing systems
Although the above research was conducted in a dry lot situation, Rosasco’s own PhD work focused on the ovarian reserve for heifers on native range compared to dry lot. Additionally, she looked at other reproductive determinants.
“Because we ovariectomized heifers, we couldn’t assess pregnancy rates, so we looked at dominant follicle diameters, which can be an indication of potential fertility of the follicle,” she notes. “We saw a significant group effect where we had an increase in the dominant follicle diameter in our native range heifers compared to our drylot developed heifers.”
Additionally, follicular fluid hormone concentrations were analyzed, including estradiol concentrations. Larger follicle size likely increased estradiol concentration, which suggests a further increase in fertility.
“We may have seen enhancements in the fertility of native range heifers because of their ability to select a higher-quality diet than what we can anticipate,” Rosasco says.
“When we looked at the effect of nutritional programming, we were able to see similar results, where we had an increase in primordial follicle numbers in our stair-step drylot and our stair-step native range developed heifers compared to their constant gain counterparts,” Rosasco summarizes.
“This shows us the U.S. Department of Agriculture’s study could be repeated in a new location, but native range heifers can be supplemented on a stair-step regiment and still see an increase in primordial follicle numbers,” she adds.
The big picture, Rosasco says, is ranchers have more options in developing heifers.
“What was interesting is constant-gain, native range heifers were intermediate,” she says. “Because heifers on native range naturally select their food sources, following a stair-step system as the grass creeps up, the diet on native range naturally follows the stair-step system, which can be utilized to ranchers’ advantage.”
Continued data
Most recently, data released in 2021 compared survival between 300 heifers developed on a stair-step diet and 300 heifers developed on a constant diet.
“What they saw was, by six years of age, there was a greater percentage of stair-stepped heifers remaining in the production herd and still calving, compared to their constant gain counterparts,” Rosasco says. “We are actually seeing an increase in longevity in these stair-step heifers.”
Additionally, she notes further research is being conducted to look at the possible mechanisms to explain the increased reproductive longevity of heifers fed on a stair-step diet in development.
Rosasco presented in a monthly webinar for the Beef Reproduction Task Force on Feb. 15, 2022. View the full webinar at beefrepro.org under Resources and Webinars.
Saige Zespy is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.
