Will R. Getz – Fort Valley State University

Making the environment as uniform as possible

This means managing animals in such a way that environmental effects on the performance of different animals are as similar as possible. This is the premise on which central test stations operate, but the same needs to be done on the home farm or ranch. What this does is to increase the proportion of total phenotypic variation in a population that is due to genetic differences among the goats. How do you manage animals so that environmental effects are as consistent as possible? The answer is to minimize the environmental advantages that some animals have over others. Key – minimizing environmental differences does not mean making the environment better. Rather, it means making the environment more uniform. There is no need for that to be an expensive proposition.

Accurate field measurements

This often means paying attention to small things. Examples include, being certain a scale is current in its adjustment; being certain there are no obstructions in the weighing machine; no animals or people touching the machine; getting animal identification right; accounting for differences in fill among early weighed animals and later weighed animals; calibrating measuring instruments such as ultrasound machines, monitoring the accumulation of manure on the platform, etc. Better measurement precision increases accuracy. It is of no enhancement to accuracy to insert decimal points beyond the units on the machine. For meat goats, weight measurements to the half pound or quarter kilo are generally considered adequate. Measurement to tenths is better if possible at chute side. It is not practical to get more precise for this kind of field animal data.

Mathematical adjustments help

Most environmental influences (pasture quality, level of feed intake) are difficult to quantify. These effects must be considered unknown, which means we cannot make mathematical adjustments for them. There are however some known effects. These are so consistent that research scientists have documented them and we can develop mathematical adjustment factors or adjustment procedures to account for them. For example, within breed:

• Birth weight is influenced by age of dam, and sex of kid.
• Weaning weight is influenced by age of kid, age of dam, type of birth, type of rearing, and sex.
• Postweaning weight gain is influenced by sex

Weaning weight in meat goats is a trait for which these adjustments can be illustrated. Weaning weights are normally adjusted for age of the kid first. The age-of-kid adjustment removes the weight advantage of older kids in a group weaned at the same time. The age-of-dam adjustment accounts for the increased milk production and better nutritional environment provided by older does.

Unfortunately there is very limited data available in meat goat circles for developing standardized industry accepted age-of-dam adjustments. Weaning weight is influenced also by birth weight, and so if that information is available it should be included in the adjustment. To illustrate the concept of adjustment, the following formula is used:

Adjusted Wean Weight = [(Actual Wt. minus Birth Wt.) / Age] x 100 days + (Birth Wt.) + (Age-of-Dam Factor)

This formula assumes the kids are all the same sex. If not then a sex-adjustment factor must be incorporated. The part of the formula in parentheses represents the growth rate of the kid from birth until weaning. This formula assumes a standard weaning age of 100 days. Multiplying the growth rate component by 100 and adding back the birth weight provides an estimate of the kid weight had it been exactly 100 days old when weighted. The age-of-dam factor would normally be a “book value” based on published data. That information is not readily available in the meat goat sector. Nonetheless this illustration should provide an idea of the process involved in creating and using accurate information in attempts to make genetic change.

Adjustment factors can be either additive or multiplicative. Adjustment factors often vary for different populations within a species. There is evidence of the need for breed-based adjustment factors in some cases. Much of this is lacking in the meat goat sector because it has not been a high priority under limited funding, and because of a shortage of time allocated to this kind of research for the breeders employed at institutions and agencies involved with meat goats.

Contemporary group techniques

Accuracy in selection and the rate of progress involved can be increased by considering deviations for a contemporary group mean. This is a useful approach when it is not possible or practical to mathematically adjust or by adjusting management considerations. A contemporary group is a group (sometimes within a group) of animals that have experienced similar environment with respect to the expression of a particular trait. Contemporaries are those performing in the same location, at the same time, are of the same sex, are of similar age, and have been managed alike.

For example a contemporary group for weaning weight in kids would commonly be defined by breed, year, season of birth, sex of kid, type of birth and rearing, and feeding regime. A classic temptation in situations where deviation from contemporary group mean is not utilized to create comparative information, is to select out those really nice looking, larger single doelings for replacement females without due consideration to type of birth and rearing. Twin does might make better replacements if considered on a contemporary basis. Contemporary groups are most commonly used to account for environment differences between groups of goats. They can be used as an alternative to the standard mathematical adjustments for known environmental effects.

The trait ratio option

By using the contemporary-group mean as a basis, individual level of performance can be expressed as a ratio of that individual record divided by the group mean. The advantage of this method is that it puts the performance number on an easily understandable basis. It allows a comparison among several different traits that may be measured in different units. Like a deviation from a contemporary group mean, a trait ration is an expression of relative performance.

To calculate a trait ratio, the individual animal’s performance record and the performance records of its contemporaries are first adjusted for known environmental effects. Then the ratio is calculated as:

Individual Ratio = (Individual Phenotypic Value / Phenotypic Value of Group) x 100

As an example, if a young buck had a 100-day adjusted weaning weight of 45 pounds, and the average adjusted weaning weight for all kids is 42.5 pounds, his weaning weight ration would be:

This would suggest that this buck was about 6 percent above the contemporary group mean. Values below 100, e.g. 92, would be 8 percent below the contemporary group mean. Values slightly below the mean do not necessarily indicate that the animal is undesirable. High merit in other traits may compensate for somewhat lower production in alternative traits.

Trait ratios are simple to understand. Their interpretation does not depend on knowing the trait involved, and it does not require any knowledge of the variability of the trait. Although convenient to use on the farm, there are more useful tools for the meat goat sector and breed populations. Examples include Estimated Breeding Values (EBV) and Expected Progeny Difference (EPD).