---

NSIF-FS6

Fact Sheet #6

Purdue University

Cooperative Extension Service

West Lafayette, IN 47907

Reviewed by

Larry Young, MARC

Charles Christians, University of Minnesota

Clifford Bussler, Minnesota Duroc breeder.

Introduction

Observed differences in performance among individuals or groups of individuals are a result of either genetic or environmental effects. As discussed in other factsheets, the genetic component is solely a function of the genes that an animal possesses. Selection for improved genetic ability is the primary tool for making improvement in breeding stock and only the genetic component is utilized in these selection programs. Therefore, it seems reasonable to ask what is the environmental component and does it provide us with useful information to make genetic improvement?

The environmental effects on observed performance of animals are broadly classified as the sum of all nongenetic effects, This simply means any effect which cannot be attributed to the genetic value or genes of the individual are termed environmental effects. Several factors can be included in the environmental component of any specific trait. Using litter weaning weight as an example, it is influenced by such nongenetic factors as parity of the sow, season of birth, level of nutrition and age at weaning just to name a few. Environmental effects can be quite numerous and all exhibit varying levels of effect on the trait of interest.

Selection only operates on the genetic component of a trait because the environmental effects are not passed on from parent to progeny. For example a boar could exhibit an exceptional growth rate mainly because he is kept under optimal conditions compared to other boars. These optimal conditions may include a high level of nutrition and excellent management etc., but he will not pass on these environmental benefits. In fact, he may have poorer genetic ability compared to other boars, but a better performance record, solely because of the excellent environment provided him. Therefore, environmental effects may become a problem since they mask genetic ability and are an obstacle in our attempts to measure true genetic effects.

Known and Unknown Environmental Effects

Attempting to account for all environmental effects is generally not possible. There is no way of accounting for an outbreak of scours in the nursery or a feed mixing problem in the postweaning diet. We have to assume that factors of this type affect all animals similarly and, therefore, simply ignore their effects. These types of effects are described as unknown environmental effects, and there is simply no accurate method of accounting for them. However, there are other factors that affect an animal's performance for which we can adjust. These are called known environmental effects.

The known environmental effects will vary depending upon which traits are of interest. For example, litter weight at 21 days should be adjusted for age of the litter when it was weighed. All litters cannot be weighed at exactly 21 days of age, and since age is known to affect litter weight, this trait should be adjusted to a constant 21 days of age. Litter weight should also be adjusted for number of pigs allowed to nurse since this will affect the amount of milk that can be produced by the gilt or sow and, therefore, the total litter weight. Growth traits, specifically days to 230 pounds, are adjusted for age and weight at weighing. Carcass traits such as backfat, carcass length and loin muscle area are adjusted to a constant body weight. These are some examples of effects that are known to influence the performance of an animal but are nongenetic in nature.

Adjustment Factors

The purpose of adjustment factors, as noted above, is to negate as much of the environmental component of a performance record as possible. The adjusted record can then be used to determine the best estimate of an individual's genetic value for that trait. Adjustment factors for all economically important traits are listed in Guidelines for Uniform Swine Improvement Programs (USDA Program Aid 1157). In example 1, the traits days to 230 pounds and backfat at 230 pounds. will be calculated to compare adjusted and unadjusted values.

Example 1. Adjusted days to 230 pounds and backfat on four boats,

Boar    Age at   Actual   Probed    Adjusted days  Adjusted backfat
ID      weighing weight   backfat   to 230 lb.a     at 230 lb.b
---------------------------------------------------------------------
       days       lb.      in.       days              in.

 1      145       235       .75        143              .73
 2      175       280       .95        151              .76
 3      135       220       .68        139              .61
 4      140       230       .71        140              .71
---------------------------------------------------------------------
a Adjusted day a 230 lbs = actual age (230 - actual weigh) x (actual age - 38) / actual weight))
b  Adjusted backfat at 230 lbs =  actual backfat + (230 - actual weight) / (actual backfat - 
(actual weight - 25))

The boars in this example represent quite a wide range of possible performance records. This serves the purpose of demonstrating the effect that known environmental factors can have on a performance record. Some of the unadjusted or actual values changed considerably once they were adjusted and others, such as boar 4, did not change at all since he was weighed at exactly 230 pounds. This demonstrates that any individual that is measured at exactly the age or body weight to which adjustments are made will not have to receive any adjustment. However, this is not always practical or possible for all animals. Therefore, adjustment factors help to make performance recording more practical and useful.

Once records have been adjusted, the adjusted records should be used to calculate a suitable measure of genetic values for each individual. Comparisons can then be made between pigs for selection purposes, using the genetic evaluations. Thus one can transform a raw performance record into a genetic evaluation by first adjusting the record for known environmental effects and then utilizing a method to compute the animal's genetic value.

Summary

Performance records are comprised of a genetic and an environmental component. The genetic component must be predicted so that selections and matings can be made in a genetic improvement program. Environmental effects are not passed on from parent to progeny and, therefore, provide no value in a genetic improvement program. Unknown environmental effects cannot be removed from performance records but are assumed to have an equal effect on all pigs being measured. Known environmental effects on performance records can be accounted for using adjustment factors that have been derived from large amounts of data. The purpose of adjustment factors is to remove, as much as possible, the influence of known environmental effects on a trait so that the best estimate of the genetic component can be found. Adjusted performance records are used to compute a genetic evaluation for each individual. Genetic evaluations are used to make selection and mating decisions which are the backbone of a successful genetic improvement program.