The Heritability of Udder Traits
It's In the Genes!
Reading Time: 5 minutes
Many udder traits are hereditary. Because of this, underwhelming or ugly udders can improve by selection over generations of breeding. Just as they improve, they can also be compromised. Both bucks and does carry genetics for udder development. Ideally, selection occurs when choosing a buck and doe for breeding. Neither should have undesirable traits.
Perfect goats are rare. Often, we have a goat with many outstanding traits but a few weaknesses. Understanding the heritability of genetic traits can help us make breeding decisions to improve traits in their offspring. In the case of udders, most bucks do not have an udder to examine for soundness. To determine which traits he may be carrying, we need to look at siblings, offspring, and the dams in his line — both on his dam’s side and his sire’s side.
Selecting by pedigree alone can be risky. The term “winning the genetic lottery” has a basis in truth. Except in identical twins, offspring do not inherit the same combination of genes from their parents. While they all inherit half of their DNA from each parent, siblings can have less than half of the same genes and look nothing alike. There are 1,073,741,824 (2^30) possible combinations of the 60 chromosomes contributed by the sire and dam. To put that in perspective, humans only have 46 chromosomes, with 8,388,608 (2^23) possible combinations.
Look at these two does:
Their udders are entirely different. Expressed traits — what we see — are called phenotypes. An animal’s phenotype is a clue to its genotype, or genetic makeup, and that of its parents. Based on their udder phenotypes, you might think they aren’t closely related, and if they are, perhaps they are of different ages. They are twins. Somewhere in the lines of these does are the genetics for conical teats. We won’t know if the doe on the left inherited them unless we see them in her offspring, and even then, we will need to consider the possibility of the trait carried by the buck.
There are two types of genetic traits: qualitative and quantitative. Phenotypes that fall in specific categories, and are determined by a few genes, are qualitative traits. Examples of qualitative traits are eye color and blood type. Many genes influence quantitative or polygenic traits, as well as the environment. Milk production is a quantitative or polygenic trait.
All traits are not heritable to the same degree. The probability of inheriting certain polygenic traits is the topic of many studies. In dairy goats, producers measure milk production traits — quantity, fat content, and protein content — contributing data to the Dairy Herd Improvement program (DHI). Goats earn associated markers that are added to their pedigrees to identify their performance. In the late 1980s, this was taken a step further by the American Dairy Goat Association in developing the Linear Appraisal program to measure and include structure phenotype information in the data. DHI and Linear Appraisal data are used to determine the Predicted Transmitting Ability (PTA) of a trait.
The dairy goat trait heritability determinations below are based on 10,932 appraisals from the years 1988-1994 [Wiggans, G.R. and Hubbard, S.M., Genetic evaluation of yield and type traits of dairy goats in the United States. Journal of Dairy Science 84(E. Suppl): E69-E73, 2001.]
|Fore Udder Attachment||.25|
|Rear Udder Height||.25|
|Rear Udder Arch||.19|
|Medial Suspensory Ligament||.33|
Generally, the heritability of .20 or higher is accepted as indicating moderate heritability of a trait.
A 2016 chart by Langston University further indicated that milk yield has an estimated heritability of .25, milk protein content .50, and milk fat .55.
Heritability data can play an important role in herd improvement. It can help producers determine if it is more efficient to improve traits through management (such as nutrition) or breeding selection. Selecting stock for breeding based on a trait with low heritability is likely to produce disappointing results. But selecting stock expressing traits with high heritability will result in rapid changes in a herd.
The numbers read as percentages. Let’s look at the twin sisters. Teat diameter has a heritability of 38%. Management cannot improve teat diameter; breeders must improve it genetically. If we bred a buck and doe with this trait, there is a high probability that the offspring will also have conical teats.
We can improve our herd in two ways. Culling — removing this doe from our breeding program — is recommended when other animals in the herd are superior. If this is your only doe, or she has other traits with high heritability that are desirable, culling may not be your best option. In that case, identify a buck whose lines have preferred teat structure to improve the offspring’s chances of having correct teats.
While it is an automatic disqualification in dairy goats, and considered a fault in some meat breeds, many Boer goats express the trait of supernumerary teats. Unless fully functional, supernumerary teats can make nursing frustrating and even unproductive for kids and present an increased risk of mastitis. A Boer breed chart illustrates teat variations and categorizes them by preferred, acceptable, or disqualifying. Some Boer breeders select for standard teat structure: two teats, or 1×1. Others advocate for udders with multiple functional teats to increase productivity.
In Mississippi, Linda Owen, of Arrowhead Hills Farm, has a Boer cross doe, Ginger, with four teats from four functioning quarters. While Linda has seen multiple teats from the same supply, she hasn’t seen many with equal sizes in over 15 years of raising Boers. For many commercial breeders, Ginger’s udder is ideal. Linda finds that if the teats are separated and usable, they are quite an advantage. There is no fighting over teats.
Linda has had others in her herd with good extra teats but hasn’t tracked the genetics. She simply culls for “ugly udders.” While many believe the trait of supernumerary teats is very heritable, it is also very unpredictable. Ginger’s twin sister is 1×1 with another very small non/functional or “blind” teat. Ginger’s buck kids are 1×1 clean and 2×2 clean. The sire of the two does is 2×2, well separated, with another small, blind teat on one side. The dam was 1×1.
A study done on Alpine and Saanen breeds concluded that supernumerary teats are polygenetic, with an estimated heritability of .40 to .44. To make things even more difficult, the genetics are different in the two breeds (Martin, Palhiere, Tosser-Kopp and Rupp, 2016.) The researchers suggested that if the trait were routinely recorded and Estimated Breeding Values created, it would be possible to select against the trait with higher success to reduce the trait’s expression frequency.
While selection can help us improve udders, there can be unintended consequences. Focusing on a single trait can result in an exaggerated trait, or a goat out of balance as a whole. It can also result in an unexpected expression of those traits. Since udder genetics are carried by both does and bucks, both can express them. As breeders began selecting for higher levels of milk production, not only did the milk productivity of their does increase — so did their bucks. In some high-production lines, bucks develop udders capable of producing milk.
Selection isn’t just for show stock. Breeding goats with undesirable traits, even if “just for homestead/commercial use” perpetuates those traits — and problems. In the wild, survival of the fittest determines who breeds. When we choose breeding pairs, we have the same responsibility to remove or improve genetics that hinder domestic goats’ performance.
Originally published in the May/June 2021 issue of Goat Journal and regularly vetted for accuracy.