Sexten: Single Trait Selection for Birthdate

As bull sale season gets into full swing, sale catalogs arrive daily in your mailbox. Sort through any two sale books and you are sure to find a variety of breeding objectives and methods to showcase those priorities to buyers.

Breeder preferences for sale catalog organization will vary based on how they market to customers, some sort by sire groups, others use genetic merit or even breed composition. Regardless of the organization, buyers will find an abundance of individual data on each bull ranging from actual weights to genomic-enhanced EPDs and even a picture or video. While some bull buyers prefer age-advantaged bulls, most search the catalog looking for bulls to complement their breeding objectives for calving ease, performance and carcass merit with little, if any, thought as to the bull’s birthdate.

If buyers consider the bull’s age, they generally are focused on how many cows he may service using the rule of thumb, beyond puberty - one cow for every month of age. Let’s contrast this to the most common recommendation for selecting replacement heifers: “Select replacements only from those born in the first half of the calving season.”

I admit there are likely some long-time readers who will recall reading this advice under my byline. I am the first to recommend making data-based decisions so let's re-consider the data informing the management behind selection for birthdate in replacement females.

First an interesting contrast between bulls and heifers. Why do we evaluate bulls’ fertility relative to successful semen check, while fertility in heifers is typically evaluated according to when their mother calved relative to the rest of the herd? Reproductive tract scoring of replacements gets little press compared to relative birth date. If birthdate is an indication of fertility shouldn’t we put the same if not greater selection pressure on the bulls? The lack of including an animal’s relative birthdate in selection indexes would suggest the environment may have a greater influence on heifer and cow fertility than genetics?

The reason for selecting the oldest heifers is that generally they have greater opportunity to reach puberty, cycle more times prior to the start of the breeding season, and then conceive earlier and at higher rates than younger heifers. The birthdate selection criteria also assumes cows calving early in the calving season are more fertile because they are conceived earlier in the breeding season. However, the success of the current breeding season starts the year before, as cows calving early and in adequate body condition (BCS 5 or greater) are more likely to re-breed in a controlled breeding season.

There are several ways a cow achieves this adequate body condition the year before she conceived that replacement heifer early in the breeding season. The following scenarios can all lead to a cow with adequate condition at calving: she was fed high-quality forage during gestation, was supplemented before calving, her previous calf was early weaned, she didn’t wean a calf last year or she’s a low milk-producing cow.

If all the cows are in equal body condition at calving, then cow age, milk production and  weaning age of the previous calf would be the three most influential factors influencing body condition during the breeding season. If we select replacement heifers using only relative birth date, are we indirectly selecting heifers from lower milk-producing cows that hold condition?

Using relative birth date to select replacements is rooted in puberty research. Early data[1] suggested heifers had to achieve a certain age before being pubertal. Work that followed¹ suggested a combination of age and a target weight concept, where heifers should be managed to achieve 65% of mature weight to ensure puberty prior to the breeding season. This work started during an era when researchers focused on moving from calving 3-year-old heifers to the 2-year-old system used today at a time when the later maturing, continental breeds were still new. Management and genetics have changed significantly since then and heifers managed with adequate nutrition are rarely challenged by age at puberty in the modern beef herd.

Single trait selection for relative birth date in females is often supported by the idea that genetic improvement in commercial herds should be driven exclusively by the sire. On one level this makes sense, since sire selection is the most important aspect of genetic improvement. If “born-early” is so important, why do seedstock breeders talk about the maternal genetics of the bull yet omit when his mother was born relative to other calves? Alternatively do seedstock providers not retain any late-born females based on genetic merit?

A common refrain is that genetics do not matter if she doesn’t breed. Here is where the puberty discussion gets lost in the current recommendation. Puberty and heifer pregnancy are not the same as cow fertility.  Replacement heifers are not cows; heifers are not carrying the burden of calving late the prior year. From weaning until breeding, you have ~7 months to provide adequate management to develop a replacement to breed on the first day of the breeding season. There are several management practices available – and effective genetic selection tools now exist - to help ensure a heifer born on day 65 of the calving season is cycling similarly to the first-calf born, and just as apt to conceive early in the first breeding season.

The replacements you keep this year may be the most expensive cows in your herd in 5 years. Is her relative birthdate the most important factor to evaluate her merit as a replacement? Genetic potential is unchangeable and impacts the cow and her calf from the first to the last day of production. Consider expanding your selection criteria for the future of the cow herd beyond relative birth date.