National sire evaluation--Programs of sire evaluation conducted by breed associations to compare sires on a progeny test basis. Carefully conducted national reference sire evaluation programs give unbiased estimates of expected progeny differences. Sire evaluations based on field data rely on large numbers of progeny per sire to compensate for possible favoritism or bias for sires within herds.

Nonadditive gene effects--Favorable effects or actions produced by specific gene pairs or combinations. Nonadditive gene action is the primary cause of heterosis. Nonadditive gene action occurs when the heterozygous genotype is not intermediate in phenotypic value to the two homozygous genotypes.

Number of contemporaries--The number of animals of similar breed, sex, and age, against which an animal was compared in performance tests. The greater the number of contemporaries, the greater the accuracy of comparisons.

Open--A term commonly used to indicate a non-pregnant female.

Optimum level of performance--The most profitable or favorable ranges in levels of performance for the economically important traits in a given environment and management system. For example, although many cows produce too little milk, in every management system there is a point beyond which higher levels of milk production may reduce fertility and decrease profit.

Outcrossing--Mating of individuals that are less closely related than the average of the breed. Commercial breeders and some purebred breeders should be outcrossing by periodically adding new sires that are unrelated to their cow herd. This outcrossing should reduce the possibility of loss of vigor due to inbreeding.

Ovulation--Release of the female germ cell (egg) by the ovary. Cows usually ovulate several hours (up to 15 hours) after the end of estrus or standing heat.

Parturition--The act of giving birth; calving.

Pedigree--A tabulation of names of ancestors, usually only those of the three to five closest generations.

Performance data--The record of the individual animal for reproduction, production, and possibly carcass merit. Traits included would be birth, weaning and yearling weights, calving ease, calving interval, milk production, etc.

Performance pedigree--A pedigree that includes performance records of ancestors, half and full sibs, and progeny in addition to the usual pedigree information. Also, the performance information is systematically combined to list estimated breeding values on the pedigrees by some breed associations.