A replacement heifer represents the costliest improvement in the genetics of a herd. Some of the more important influencers that are critical to the success of retaining these genetics over time include hiring the right female, reaching a target breeding weight, proper development, and a defined breeding season.
As we know, reproductive traits are very lowly heritable; 10% controlled by genetics and the other 90% controlled by management and environment. This forces us to be intentional managers within the environment we live. Being better managers naturally forces us to use more data to make decisions that can improve fertility responses from good nutritional management.
The goal in developing replacement heifers is reaching puberty by the beginning of the breeding season. Puberty in heifers is controlled by three factors—body weight, age and breed.
With breed, we know that on average Bos taurus cattle reach puberty between 12 to 14 months of age; whereas Bos indicus cattle reach puberty later at approximately 16 to 18 months of age.
Heifer body weight has a very influential impact on age at puberty. Research conducted in the 1980s reported that heifers weighing approximately 60-65% of their mature body weight reached puberty prior to breeding.
Hire the right female
Not every heifer will make a good replacement heifer. The female you want to hire conceives early in the breeding season. However, to get there we need to make some decisions earlier on in the hiring process.
You need a pool of candidates to start with based on your job description. The job description will lay out minimum requirements the heifer has to have in order to go through the development process.
These requirements should include, at minimum, no freemartins, minimum body weight, minimum age, structural soundness and not out of terminal sires. Throughout the development process, checkpoints need to be in place to monitor the heifers’ performance of your heifers.
And finally, after the breeding season, is she pregnant and preferably early on in the breeding season? Then, and only then, should we hire her for the job!
Target breeding weight
To properly develop heifers to 60-65% of mature body weight, you need to know the mature body weight of your cows, otherwise it is just a guess. If you don’t know what your cows weigh, you don’t know what 65% of mature weight is and what those heifers should weigh at breeding.
At minimum, know what the average weight of your cow herd is so you can determine what her expected mature body weight is. If you have the capabilities to determine this individually, then you can avoid breeding heifers that are too light or too big. The implications of developing heifers too light is not reaching puberty. Developing heifers that are too heavy (or too fat) is a reduction in fertility rates and increased development cost.
Proper development starts with knowing how much she weighs, how much she needs to weigh, how many days you have to get her there and what her nutrient requirements are. Then, you can determine what her average daily gain needs to be and monitor progress to make sure she is on track for breeding.
Typical forage-based development diets for heifers consist of 9% crude protein and 60% total digestible nutrients to achieve a steady rate of gain of 1.5 pounds per head per day. Some programs develop heifers at different rates of gain at different stages of development. Overall, results seem to be similar, regardless of the development stages, as long as she reaches her target breeding weight and is on a positive nutrition plan throughout the development period.
Drastic changes in diets prior to the breeding season can cause wrecks sometimes. Some work out of South Dakota showed significant decreases in pregnancy rates in replacement heifers that were turned out on grass pasture from a dry lot in late spring at the beginning of the breeding season. To avoid this, make sure heifers are on the same forage system (diet) at least 30 days prior to the breeding season.
Define your breeding season
Without a defined breeding season, we can’t manage for fertility. Work out of Clay Center, Neb., in the 2000s reported a higher retention rate across nine calving seasons in replacement heifers conceiving in the first 21 days of their first breeding season compared to heifers conceiving after day 42 of the breeding season.
Preliminary data out of Louisiana State University reported similar results where 28% more heifers conceived to timed-AI during their first breeding season and developed to more than 65% of mature body weight remained in the herd through their fifth calving season, compared to heifers bred by natural service within a 75-day breeding season. In that study, heifers that reached more than 65% of mature body weight at breeding were out of smaller cows, and as a result had a lighter target mature body weight.
The perfect development protocol is one that results in heifers reaching the target age, weighs 60 to 65% of mature body weight, has reached puberty, conceives within the first 21 days of the breeding season and does not require taking out a loan to develop her. Nutrition is critical to ensuring heifers reach puberty by the start of the breeding season. Intentional management plays a major role in achieving high fertility rates during the breeding season at a manageable cost.