Abstract:
Genetic and economic efficiency of alternative schemes breeding for resistance to gastrointestinal (GI) helminths in meat sheep was evaluated using deterministic simulation. Four breeding objectives and schemes were assessed. The first breeding objective simulated a situation where the flock size cannot be increased due to non-feed related constraints (FLOCK). The second specifically assumed that the flock size is restricted due to limited amount of feed resources (FEED). The third and fourth objectives assumed that sheep performed only tangible roles (TR) and both tangible and intangible roles (IR) in the production system, respectively. Within these breeding objectives, four breeding schemes that differed in the measures available for use as selection criteria were compared. The schemes ranged from one that utilised birth weight, weaning weight, yearling weight, litter size and lambing interval (scheme 1) to one that included two measurements of faecal egg count (FEC, eggs/g) in young rams immediately after weaning (scheme 4). For scheme 1, resistance to GI helminths was not included in the breeding objectives. A two-stage selection process was assumed in the selection of rams to be used in the nucleus. The annual monetary genetic gain and profit per ewe for all schemes varied within breeding objectives but were highest in TR. Within each breeding objective, the annual monetary genetic gain and profit per ewe was highest for the breeding scheme with the highest level of recording (scheme 4). In all objectives, the difference in the profit per ewe between a scheme that included records on FEC measured once in rams immediately after weaning (scheme 3) and scheme 4 was small (1.3–3.7%) indicating that there is little benefit taking a second measurement of FEC. The optimal size of the nucleus was determined by the breeding objective. In schemes 3 and 4, profit per ewe was optimal when the top 5%, 5%, 10% and 10% of rams were selected in the first selection stage for FEC measurement in FLOCK, FEED, TR and IR, respectively. The practical implications of these results are discussed.