What is Heritability?
Figuring out which phenotypes are mostly due to genetics and which are mostly due to the environment is the most important consideration for breeders. Heritability is the measurement that allows breeders to quantify the relationship between genetics and the environment on different phenotypes. A high heritability (30% or higher) means it is more likely the phenotype will indicate the genotype. A moderate heritability (20-30%) will mean that the phenotype still reflects the genotype, but some environmental factors will also influence that trait. Low heritability (less than 10%) means that the trait is probably influenced more by environmental factors than by genotype.
Heritability can be used to measure the relationship between an animal’s Estimated Breeding Value (EBV) and its phenotype. Estimated breeding values given an objective measurement of what each animal can contribute to the herd. EBV’s are calculated for each trait, so that specific breeding strategies can be developed. If heritability is high, then you can have a high degree of confidence in EBV, since they will be more likely to pass on their phenotype to their offspring. Lower heritability values mean that other factors, besides EBV, will influence the phenotype of the offspring.
Here is an example looking at milk yields in dairy goats. Let’s say the projected milk yield on a first freshening dairy goat predicts that she will produce 200 lbs. more milk than the other goats she was reared with. Let’s assume that heritability of milk yield is 25% for all goats. This 25% is essentially a measurement of the expected change in breeding value per generation. Therefore, we would expect the estimated breeding value for this goat’s kids to be 25% of 200 lbs. or 50 lbs more than the herd average. Although this is a very simplified calculation, it can give a good estimate of what to expect - if you average the dam’s EBV with the sire’s, you can predict the progeny’s milk yield. Click here for a very detailed explanation of EBV calculations and an online calculator.
Research is always being done to determine the heritability of specific traits. Although a lot of research has been done with cattle, dairy goat research has been neglected. Currently there are databases being created to help better determine the heritability of traits through ADGA and the USDA (as well as many other international and national agencies). If you are breeding with animals registered through ADGA (http://www.adgagenetics.org), you can use their breeding planner to help estimate the predicted transmitting ability (PTA) of conformation and production (PTA = 1/2 EBV, since it will only contribute half of its genetics to offspring) from your dam and sire.
General guidelines to consider in dairy goats based on current research -
Feet and leg traits have low heritability
Udder and teat location traits have moderate heritability
Animal size has high heritability
Teat dimensions have high heritability
There are certain relationships that can be correlated –
Larger overall size will correlate with higher milk production (this can be troublesome with Nigerian Dwarf goats, since you need to keep the height below a certain level for showing purposes).
There is almost no correlation between 1) body, feet, and legs and 2) udder and teat traits – they must be considered separately when planning breedings.
Udder and teat traits can be separated out into suspensory ligaments, teat location, and teat dimension – these must also be considered separately.
Estimated Breeding Values (EBV) correlations are low between almost all traits except suspensory ligaments and milk yields.
These relationships become important in two cases –
Which traits are you willing to overlook in your goats when you choose them (you should have a strategy for correcting this problem already in mind)?
Which goats will you cull at the end of the year?
There are three websites that can provide incredibly useful data on the heritability of different traits:
http://kinne.net/heritcht.htm - This is a compilation of heritability rates for various livestock, with the assumption that dairy goats will have similar values.
http://aipl.arsusda.gov/publish/presentations/ADGA06/adga_grw06_type2.ppt - USDA data obtained from ADGA linear appraisals. This presentation gives a good overview of the genetic evaluations of small ruminants. The table below shows heritability and repeatability (a measure of the consistency of the relationship between repeated records for a trait in a population – high repeatability means that one production record is useful) values for dairy goats given in a 2006 report.
http://jds.fass.org/cgi/reprint/84/e-suppl/E69.pdf - An extensive study of the 7 major dairy goat breeds (excluding Nigerian Dwarf) and the key part of the study was the following table. For statistical purposes, the heritability values in the table below apply to all 7 breeds combined. The other two columns show the variation of EBV (SD of EBV gives an idea of variation in EBV for different traits) and the trend (annual genetic trend shows if a trait has improved or declined over the past year).
Below is an explanation of how EBV calculations are made:
Repeatability can be used to estimate future production of a doe on the basis of her past performance using the formula for the Probable Producing Ability (PPA): nr (Average adjusted deviation from herdmates) 1 + (n-1) r where n is the number of records, and r is repeatability of the trait. For example, the estimated future probable producing ability for milk yield of a doe with 3 lactation records averaging 200 lb above herdmates is +150 lb (assuming r = 0.50; 0.75 x 200 = 150). Not all does will produce as expected, but the fewest errors in estimation will be made when this method is used.
Repeatability (r) and heritability (hh) in a formula for the Estimated Breeding Value (EBV) can be used to estimate the genetic value of a doe on her past performance: nhh (Average adjusted deviation from herdmates) 1 + (n-1) r Assuming hh = 0.30 and r = 0.50 for milk yield, the EBV of the doe from the previous example is +90 lb (0.45 x 200 = 90). This procedure can be used to rank does conveniently for genetic value although they differ in number of lactations.
-Formulas from http://members.cox.net/foxcroft/genetics.htm
Weathertop Farm
Registered Nigerian Dwarf Goats
Johnsonville, New York 12094
Rensselaer County
Email us: weathertop@littlemilkers.com
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