Linear Measurements Can Help Dairy Goat Breeders
Published on Tue, 05/19/2020 - 1:37pm
Linear Measurements Can Help Dairy Goat Breeders.
By Heather Smith Thomas - From an Article by Mark Baden
A few years ago Mark Baden (ADGA Board of Directors member, Licensed ADGA Judge and Linear Appraiser) wrote an article for the American Dairy Goat Association about the ADGA’s Linear Appraisal program. This program is very helpful for breeders who are trying to select the best genetics to produce dairy females.
This linear appraisal program provides ways to evaluate individual traits that affect structural and functional durability of lactating females, enabling breeders to take full advantage of the potential for genetic improvement through selective breeding. The program provides a framework for a uniform and accurate record system that can be useful for farm records (which can aid in making farm management decisions) as well as for educational programs and research, including the genetic evaluation of does and sires.
Individual breeders are the primary promoters of their animals’ Linear Appraisal scores and are the direct beneficiaries if this system. ADGA and sometimes local associations may also promote these scores for various sales, such as ADGA’s two annual dairy goat auctions, the Colorama Sale in June or July and the Spotlight Sale in October. The Linear Appraisal (LA) scores become part of the animals’ “Performance Pedigree,” with Linear Appraisal scores and milk production records added for each animal. These Performance Pedigrees are made available to breeders by ADGA.
The evaluation program can be used for promotion and sale of animals, and can help any dairy goat producer visualize an animal “by the numbers”. As descried by Baden, the term “linear” means that a scale is used to describe the biological range of each of the listed traits. In this particular evaluation system, the scale ranges from 0-50 for each trait. With the exception of stature and rump width, a linear trait score is an observation made by a trained appraiser rather than an actual measurement.
The various traits of each female are evaluated by the appraiser without regard for age, stage of lactation, farm management or environmental conditions. As stated by Baden, the biological traits used in the linear appraisal program are believed to have economic importance either in terms of increased longevity--which reduces culling rate-- or increased milk production. The traits (listed in Table 1 below) exhibit enough variation to provide a basis for selection in breeding decisions.
These traits are also heritable (genetically-controlled) enough so that progress or improvement can be made at an acceptable rate through the selection of sires.
Generally, heritability of .15 or higher is accepted as indicating moderate heritability of a trait. Dairy goat trait heritability determinations were based on 10,932 appraisals from the years 1988-1994 in a study by Wiggans, G.R. and Hubbard, S.M., entitled “Genetic evaluation of yield and type traits of dairy goats in the United States” and published in the Journal of Dairy Science 84(E. Suppl): E69-E73, 2001.
When a breeder becomes familiar with this linear appraisal system, it is possible to get a visualization or “mental picture” of an animal based entirely on the linear scores, without looking at the animal, and often these scores are more helpful than a photo. Many breeders use photographs when considering and choosing animals to utilize in a breeding program, but for a number of reasons, photographs may not always provide the most accurate, unbiased image. The camera angle, stance of the animal, background, etc. may make it difficult to accurately judge and evaluate certain points.
The linear appraisal program does not indicate a certain point (on a range of points on the scale for a linear trait) as “ideal,” nor are more points, fewer points, or midpoint on the range for a trait necessarily more desirable. As stated by Baden, the program is designed to objectively assess the condition of a trait that a sire passes on to his offspring. To provide an example of how this visualization might work, Baden examines two udder traits: rear udder arch and teat placement.
Rear udder arch (RUA) is evaluated by the appraiser as the shape at the top of the rear udder where the milk is carried. Teat placement (TP) is determined by observing where the center of the teat meets the udder floor.
In evaluating the linear scores of your own animals, or, perhaps the females in the pedigree of an animal you are considering as a herd sire, it is important to know the range of the trait, the heritability of the trait, the breed average for the trait, and the trait scores of the animals you are considering as possible genetic resources. TABLE 1 [see below] illustrates the heritability of each trait.
Each year, following completion of all the linear appraisal field sessions and data entry, ADGA publishes breed averages for each of the linear traits. Age-adjusted breed averages for the two example traits (RUA and TP) for the does scored in 2013 are shown on the next page in Table 2.
As explained by Baden, the trait mean (average) is determined by adding the age-adjusted scores for all does of the breed, then dividing that sum by the number of animals (N.) The standard deviation (SD) tells us the amount of variation. For example, the Alpine data tell us that 68.2% of the does scored had teat placement trait scores between 14.7 and 24.4 [19.6 +/- 4.9, or one standard deviation above/below the average.]
Calculating the range within two standard deviations would include 95% of all the animals scored. The information in Table 2 shows that for teat placement, the breed with the most variation in teat placement is LaMancha (SD=5.4) and the breeds with least variation in teat placement are Nigerian Dwarf and Nubian (SD=4.4.) Any ADGA-registered doe or buck evaluated in 2005 or later can be found by searching the ADGA Genetics website.
Linear history documents each appraisal and provides the scores for all the linear traits. From the example above, an Alpine doe with teat placement of 12 has teats placed farther to the outside of the udder than more than 68% of all Alpine does scored in 2013 (her teat placement score of 12 is more than one SD below the mean). If we compare the trait score of 12 to the range illustrated in Figure 2, we can visualize the teat placement as being about midway between the lower end of the range and the midpoint.
When looking at a buck’s type evaluation (not his own linear history) we can see the average scores of his daughters as well as the reliability of the scores. Reliability is based on the number of daughters appraised, number of appraisals, and the number of different herds and areas of the country where daughters were appraised. Bucks that have been used extensively will have higher reliability than those that are younger or have seen more limited use.
Each breeder has preferences regarding how does should look. Prior to a linear appraisal session, each herd owner is given a document illustrating the range for each linear trait. Comparing your doe’s scores with the illustrations will provide a better understanding of where that animal falls within the range. Being able to “visualize” an animal based on the linear trait scores provides an objective, unbiased tool. The ability to visualize where you are and where you want to go in your breeding program with respect to the linear traits helps a breeder make progress toward breeding a dairy goat that is sound and productive.