HAIR SHEDDING SCORES IN BRANGUS CATTLE

A TOOL TO FINE-TUNE HEAT TOLERANCE

by Harly J. Durbin, Ph.D., postdoctoral fellow, University of Missouri and Jared E. Decker, Ph.D., associate professor, state beef extension specialist, and Wurdack chair in animal genetics, University of Missouri

INTRODUCTION

The Brangus breed was created by mixing Brahman and Angus bloodlines with an eye towards improved heat tolerance. As we move towards improved beef production, increasing heat tolerance continues to be a focus. One tool producers in heat-stressed environments can use to identify well-adapted cattle is early-summer hair shedding scoring.

Early-summer hair shedding, or the rate at which the winter coat is lost, is an adaptive trait that prepares animals to better cope with the heat. Cattle that shed their winter coat earlier are less stressed and can direct more energy toward production. Hair shedding scores are used to evaluate earlysummer shedding of the winter coat, as opposed to hair coat scores which evaluate whether the hair is short and smooth versus long and shaggy. For example, Bos indicus cattle will almost always have shorter, less dense hair than most Bos taurus cattle and therefore have more desirable coat scores on average. However, both Bos indicus and Bos taurus cattle shed winter coats and can be evaluated for hair shedding score. Even within Bos indicus-influenced breeds like Brangus, there is ample variation in hair shedding.

Consistently, we find that a hair shedding score is about 35-40% heritable. Of note, this value is about halfway between the heritability of weaning weight (about 25%) and carcass traits like marbling (about 45%). Of all the variation observed for hair shedding, about 40% is due to genetic differences between individuals. The remaining 60% is due to differences in environment, management, other non-genetic factors and interactions among these effects. This suggests that hair shedding scores, like body condition scores, could be a good benchmark to adjust our cattle management.

HOW TO COLLECT SCORES

When collecting hair shedding scores, cattle are evaluated using a 1-to-5 visual scale. On this scale, a 1 is completely slick (100% shed off), a 2 is 75% shed off, a 3 is 50% shed off, a 4 is 25% shed off, and a 5 still has a complete winter coat (0% shed off). While there is some variability in shedding patterns between individuals, cattle tend to shed from front to back and top to bottom. So, compared to a 5 (0% shed off), a 4 (25% shed off) will usually have shed only on her head and neck. A 3 (50% shed) has additionally lost hair along her topline, shoulder, and further down her brisket. A 2 (75% shed) usually only retains winter coat on her flanks and around her belly. Hair shedding scores are easy to collect and can be recorded chute-side or in the pasture.

It is recommended that producers hair shedding score their herd when they observe the most variation between individuals. Timing varies based on latitude (north to south), management, and other factors, but most producers in the Midwest and South will observe the most variation in late April or early May. Herds should be scored when there are lots of 3s, a few 1s, and a few 5s. If possible, all cattle in the herd should be hair shedding scored on the same day or within a few days of one another. This allows for large, accurate contemporary groups and ensures that all cattle are compared against the same standard. Cattle should be at least yearlings at the time of scoring.

Since 2016, producers participating in the Mizzou Hair Shedding Project have collected just under 37,000 hair shedding scores on cattle across the nation, including about 2,000 hair shedding scores on registered Brangus and Ultrablack cattle (Figure 1). Most hair shedding scores were recorded in heat-stressed environments and/or on cattle grazing endophyte-infected (toxic) fescue. Analysis of this data has allowed us to create genetic evaluations of hair

Figure 1

shedding score so producers can identify heat-tolerant cattle. It has also helped us to better understand the genetics and biology of hair shedding in general.

HAIR SHEDDING SCORE AND MOTHERING ABILITY ARE GENETICALLY CORRELATED

To evaluate hair shedding as an indicator of cow productivity in heat-stressed environments, we examined the relationship between hair shedding score and weaning weight. Early hair shedding cows tend to wean an older and heavier calf.

We tested a model that created a direct tie between a cow’s hair shedding score and the weaning weight of the calf she raised that year. Using this model, we estimated the genetic correlation (shared genetic effects) between hair shedding and the maternal component of weaning weight. Though the maternal component of weaning weight is commonly referred to as “milk”, it includes everything a dam does to create an environment favorable to growth for her calf, including but not limited to milk production, behavioral characteristics, and health characteristics.

We found that there is a -20% to -30% genetic correlation between hair shedding score and the maternal component of weaning weight. This means that as dam hair shedding score decreases (becomes more desirable), her ability to provide a favorable environment for her calf tends to increase. Thus, weaning weight also increases.

HAIR SHEDDING SCORE VARIES ACROSS AN ANIMAL’S LIFETIME

One non-genetic factor that impacts hair shedding score is age. We found that 2- and 3-year olds tend to have the highest (least desirable) hair shedding scores compared to their more mature counterparts. When we consider that 2- and 3-year olds are still growing on top of experiencing the new stress of motherhood, this result makes sense. Surprisingly, the very oldest cows (12+) tend to have the most desirable hair shedding scores. Since most beef cows die or are culled by 12 years of age, this might suggest that early-shedding cows tend to remain productive later in their life due to superior environmental adaptation. To account for these differences, we fit the effect of age when estimating hair shedding score EPDs. In order to correctly account for changes in hair shedding across an animal’s lifespan, producers need to score their entire herd every year.

HAIR SHEDDING IS INFLUENCED BY DAY LENGTH AND AN ANIMAL’S ABILITY TO SENSE LIGHT

As winter fades, the days become longer and temperatures begin to rise. In response to these cues, nearly all animals undergo physiological changes to prepare for summer. For cattle and other mammals, these changes include shedding thick winter coats. Since days are shorter in length the further north you move, cattle living at more northern latitudes tend to begin shedding later in the year compared with cattle living further south. In our data, we find that with each hour increase in the average number of sunlight hours for the 30 days prior to scoring, there is a 0.45 decrease in the average hair shedding score. This means that the average hair shedding score of cattle that experienced about 12 hours of sunlight per day in the last month will be about half a score lower than cattle that experienced about 11 hours of sunlight per day in the last month.

When we search for genetic variants that cause differences in hair shedding between individuals, we find genes involved in creating the parts of the eye responsible for sensing light. This suggests that part of what makes early-shedding animals more desirable is an increased ability to sense changes in day length.

HAIR SHEDDING AND TOXIC FESCUE

As many producers in the Midwest and Southeast know, one of the side effects of grazing endophyte-infected KY-31 (toxic) tall fescue is a rough coat that doesn’t shed off. We found that the genetic correlation between hair shedding score while grazing toxic fescue and hair shedding score while grazing other forages is nearly 1. This means that when using a hair shedding score EPD, we’d select the same animals regardless of what they’ll be grazing. However, we found evidence suggesting that physiological damage caused by fescue toxicosis impacts shedding ability. We also found that on average, cattle grazing toxic fescue have hair shedding scores about half a score higher (less desirable) than cattle grazing other improved forages.

A caveat of these results is that we didn’t specifically measure ergot alkaloid levels (the chemicals that cause fescue toxicosis symptoms) as we only asked producers to report “yes” or “no” if cattle grazed toxic fescue in the spring of the recording year. However, it’s safe to say that hair shedding scores are a good evaluation of the heat stress symptoms of fescue toxicosis, and possibly reduced blood flow to the skin also caused by fescue toxicosis. Further study of the relationship between hair shedding score and other traits affected by fescue toxicosis, such as fertility, will be helpful in the future.

CONCLUSIONS

Hair shedding score is a new trait with a potentially large production impact for producers in heat-stressed environments. Many non-genetic factors contribute to variation in hair shedding score including age, plane of nutrition, and environmental variables like day length. However, about 40% of all variation in hair shedding score is due to differences in genetics. This moderate heritability suggests producers can improve heat tolerance in their herds by selecting for early hair shedding. As a trait that can be measured multiple times in an animal’s lifetime, hair shedding score also lends itself to an accurate EPD. Producers should start collecting hair shedding scores now to enable an IBBA EPD for hair shedding. Selection for improved hair shedding will allow us to identify more profitable cattle that are better adapted to the environment.

Our research would not have been possible without the contributions of producers enrolled in the Mizzou Hair Shedding Project, including producers part of the IBBA. Thank you!