Strategies to manage inbreeding accumulation in swine breeding company nucleus herds: Some case studies

Abstract

Modern swine breeding is undergoing a series of changes that affect the underlying strategies for genetic improvement. Increased competition among fewer breeding companies requires rapid genetic improvement to maintain competitive advantage. The result is intense selection pressure within swine industry nucleus populations. Selection pressure is enhanced through the use of artificial insemination, embryo transfer, and, potentially, marker-assisted selection. This increased selection pressure raises concerns due to the more rapid accumulation of inbreeding within these populations. The current strategies used to control the accumulation of inbreeding are somewhat simplistic. The methods often include arbitrary restrictions on the percentage of sows mated to a single sire, the number of sires retained from a full and half-sib family, the avoidance of matings between closely related individuals, and elimination of families presumed to be segregating for alleles with deleterious effects on fitness. As demonstrated by classic genetic theory, when the previously listed methods to minimize inbreeding are used, inbreeding is delayed, but it is inevitably accumulated within nucleus herds. The rate of accumulation typically is between 0.3 and 0.6% per annum. Composite lines addressing specific market needs, particularly for terminal sires, may offer short-term solutions for nucleus level inbreeding. Most parent and terminal products are produced from line crosses wherein inbreeding is eliminated in the resulting progeny. Understanding the causes of inbreeding depression continues to be important in order to improve the performance of purelines in multiplication, leading to more efficient transfer of genetic improvement.