Cow herd management along with heifer selection and development are keys to achieving high pregnancy rates at breeding time. The management practices you choose help ensure that those rates stay strong year in and year out.
“For mature cows, a pregnancy rate of 90–95% over a 90-day breeding season is a reasonable goal,” said Jason Banta, a beef cattle specialist with Texas A&M AgriLife Extension Service. “For yearling replacement heifers, a pregnancy rate of 92–97% over a 90-day breeding season may be possible.”
Body Condition Matters
The most critical factor determining whether your cows achieve a 90th percentile pregnancy rate over a three-month breeding season is their body condition score (BCS) at calving. “Mature cows should have a body condition score of 5 or better,” Banta said. “Two-year-old and 3-year-old cows should have a condition score of 6 or better.”
The BCS reflects the cows’ ability to rebreed quickly because cows in good condition at calving come into estrus postpartum earlier than cows in poorer condition. Delayed onset of estrus after calving delays opportunities for cows to rebreed in a timely manner.
Banta recommended managing yearling heifers’ physical development so they achieve 65% of their mature body weight by the onset of the breeding season. Achieving that level of development as yearlings helps ensure that most heifers are cycling by the start of the breeding season. It also better prepares them to reach the target of 85% of mature weight needed when they calve as 2-year-olds.
While managing heifer development to achieve a yardstick of 65% of a heifer’s mature body weight at the start of breeding season may be optimal for many producers’ management systems, David Lalman, Extension specialist for beef cattle at Oklahoma State University, suggested lowering the heifers’ target body weight to 55–60% of mature body weight.
The lower body weight may reduce the percentage of heifers cycling and becoming pregnant during the early part of the breeding season. But it’s a management strategy that could put more rigorous selection pressure on replacements. “This system lets you select replacement heifers that are a good natural fit for your environment,” Lalman said. (See “Steps to Building Herd Fertility.”)
Nutrition Management
While good body condition at calving is important in helping cows rebreed readily, minimal weight loss after calving also plays a critical role. “Manage nutrition so that you can control weight loss after calving, through the breeding season and into early gestation,” Banta said. “I recommend that cows not lose more than one body condition score from calving time to the start of breeding season.”
In spring-calving herds, adequate nutrition postcalving typically comes from grazing forages. Typically, managing forage availability to ensure sufficient feed is the most cost-effective means of controlling weight loss in females.
“But it also depends on where you’re located and when you’re calving,” Banta said. “If you need to feed hay after calving, test hay for energy and protein, and supplement accordingly.”
Disease prevention can also play a role in sustaining good pregnancy rates. Banta said to make sure leptospirosis, IBR, and BVD are part of a well-designed vaccination program.
When it comes to mineral supplementation, he advised a reasonable approach. “Minerals are not a silver bullet to getting good conception rates,” he said. “You can actually reduce conception rates if you overdo mineral supplementation. Work with a nutritionist, and use a single product. Avoid using multiple supplementation products with added trace minerals at the same time.”
Consider Bulls
The number of bulls you run with females also impacts pregnancy rates. For mature bulls, Banta recommended a ratio of one bull to 30–35 cows, assuming a stocking rate of fewer than 5 acres per cow/calf pair. You might increase the number of bulls on bigger pastures with a stocking rate of more acres per pair. In such settings, cows spread out, and bulls have to cover more ground.
If there is only one bull per herd, Banta suggested that bull may cover as many as 40–45 cows. Having a single bull in one pasture lets the bull focus solely on breeding cows because it is not distracted by fighting with another bull.
Long yearling bulls are best limited to just 10–15 cows per bull, Banta said.
Monitoring herds during the breeding season lets you see whether bulls are breeding cows effectively. Banta recommended keeping track of cows that have been bred and then start watching those cows during the 18–24-day period when they might repeat if they haven’t conceived. Taking the time to monitor cows also lets you replace an ineffective bull.
The criteria to select herd sires can have a lasting impact on the ability of home-raised replacement females to conceive and remain productive year after year. When evaluating potential sires, Lalman recommended paying close attention to your breed association’s expected progeny differences related to fertility such as heifer pregnancy and longevity.
Steps to Building Herd Fertility
Selecting heifers best fitted to your locale and management system can build fertility into your cow herd.
To identify yearling heifers that are the best fit for your system, Oklahoma State University Extension beef cattle specialist Dave Lalman suggested aiming for a target body weight of 55–60% of mature weight at breeding season. Then, expose those heifers to bulls for a 30–45-day breeding period. An alternative is to use an extended breeding season and work with your veterinarian to detect the early-bred females through one of several fetal-aging options available.
“Sort off and keep those heifers becoming pregnant in the first 30 days,” Lalman said. “By cycling early and breeding early, those heifers are showing you they’re a good fit for your environment.”
Open or later-bred heifers can be marketed as stocker cattle in midsummer, or as bred heifers in fall.
Lalman advised managing these early-bred yearlings separately from the cow herd as 2-year-olds so they can maintain the needed body condition to rebreed after calving.
Crossbreeding is another powerful tool that can improve herd fertility in one generation. “Fertility traits have low heritability,” Lalman said. “But these traits are also high in heterosis, the hybrid vigor resulting from crossbreeding.”
He recommended this strategy to improve herd fertility through female selection:
Cull open cows.Only keep replacement heifers that are born early in the calving season and that breed early in the breeding season.Use a cross-breeding system.
“Over time, you can create a really fertile cow herd,” he said.
