A tool for selecting breeding herd replacement animals to improve fertility and longevity in large, low input production bison herds
One of the highest costs of commercial bison herd production is that of replacement breeding animals. Herds that are more fertile require fewer young replacements, which are expensive to develop to breeding age. Achieving and maintaining a high reproductive rate (mature females are bred every year) is an important factor in profitability. Reproductive rates affect not only the number of calves born, but the number of replacements needed to maintain a stable herd size when non-pregnant mature cows are normally culled. Those individual animals that annually breed and wean a calf are the most “fit” for the environment and the production system being used. Accordingly, the most profitable females in the herd are the mature breeding cows that wean a calf every year. Selection of replacement males and females with the highest probability of reproductive longevity within a herd is a critical component to achieve optimum fertility and “stay-ability”, which is the characteristic of a male or female to remain physically and reproductively sound for their full adult life, reproducing every year.
The challenge to improving stay-ability in commercial bison herds is the selection of replacements. How do we select replacements that have a higher probability for physical and reproductive soundness (ultimately leading to longevity over their herd mates)? Typically, the only tools for selection of young replacements are phenotype (How do they look?) and weight (How big are they compared to their herd mates at the time of selection?). In a low input production system (minimal or no outside feed and supplements to the breeding herd), females with higher nutrient demands (larger body size; increased growth; high milk production) often fail to rebreed early in their careers. These individuals are not “balanced” and won’t produce consistently over their lifetime without being in a high input system (outside feeds and forages supplemented on a regular basis). Identifying “trait balanced” individuals that will fit its environment is very challenging. While selecting individuals by phenotype and size on selection day is better than judgment based on no physical attributes, this method does not accurately determine stay-ability, as stay-ability is comprised of multiple traits and not based on aesthetics alone. On the normal low input bison ranch with a single breeding herd, the probability of selecting a calf out of a cow that will not have stay-ability is higher than selecting one that is from an aged cow that has stay-ability. Perhaps in large breeding herds, the use of genetically-based parentage testing is the only tool available for use as a multi-trait selection criterion based on past breeding performance of dams.
Geneseek, a company in Lincoln, Nebraska has developed a powerful, single nucleotide polymorphism (SNP)-based parentage tool specifically for bison. McGinley Ranch, a Turner property, implemented this tool on its bison herd in 2017. This initial application proved that the rigor and sensitivity of the new test was more than enough to distinguish sires and dams from a large multi sire breeding herd (1750 cows, 135 bulls). In fact, we were able to assign greater than 95% of the calves back to their dams with near 100% certainty of the pairing in the first year.
In 2018, we used this parentage information at McGinley Ranch to select replacement bulls from dams 1) with at least 10 calves and 2) had never been open; and then refined our selection by selecting the bulls within this group that had the best average daily gain on grass from 11-18 months of age. We did the same for replacement heifers; however, we had to drop the age of dams to those that weaned at least six calves, never being open. We are convinced that the use of parentage information for selecting replacements will increase herd reproductive efficiency over time, lowering the average number of replacements needed as average stay-ability increases in the cow herd. We offered a portion of the top end of these genetically and grass performance-tested heifers and bulls at the 2019 Turner Ranches Prairie Performance Auction, and will do the same in at the 2020 auction.
For more information on the science behind the development of the genetic test and the results from our first year, please see the linked video presentation below, which was delivered at the 2018 National Bison Association Winter Conference.