Moving towards low-chemical farming with sheep and cattle: the potential of a breeding approach.

摘要

On livestock farms in New Zealand, some farmers are changing their sheep, beef cattle or dairy cattle enterprises towards specialization in low-chemical or organic systems. A vital part of the animal's ability to perform under these systems is having the disease resistance genes available (or some other means to cope), as well as having the production and quality traits. In practice, animals are likely to be under a number of disease challenges at any one time, so breeders must concentrate their genetic selection practices on the most important health problems. Other (non-genetic) means may also be critical in helping to protect the animal. This paper reviews the New Zealand evidence for genetic differences in resistance to various important diseases or metabolic stresses in sheep and cattle and shows that genetic selection could be applied successfully if required. The paper also discusses some of the challenges in realizing the goal of low chemical farming, such as finding a source of rams, bulls or semen. The diseases/stresses that are covered are: in sheep, nematode parasites, dags, fly strike, facial eczema (FE) and ryegrass (Lolium perenne) staggers (RGS); in dairy cattle, mastitis, bloat, FE, RGS, nematode parasites and hypomagnesaemia and in beef cattle, RGS, nematode parasites and hypomagnesaemia. Some breed differences are mentioned but at present the main New Zealand evidence of genetic variation is derived from heritability estimates. In sheep, average heritability estimates are: nematode parasites in lambs, 0.23 (repeatability 0.30); dags, 0.20 (repeatability 0.4); fly strike, 0.26 (Australian data); FE, 0.45 and RGS, up to 0.17, depending on the challenge level (repeatability 0.28). Corresponding heritability estimates in cattleare: clinical mastitis 0.06 (or somatic cell count 0.09); bloat 0.19 (repeatability 0.45); FE 0.56; RGS no published estimate; nematode parasites in calves 0.32 and hypomagnesaemia 0.15. The simple mean of heritability estimates for 10 traits in sheep and/or cattle was 0.26 (0.28 excluding the outlier, mastitis, at 0.06), suggesting that responses should be achieved if directional selection is applied after performance or progeny testing. Knowing the genetic correlations among these disease traits is also important, because of the multiple disease challenges likely but few of these correlations have yet been documented. Opportunities for ranking sires using the principles of Group Breeding Schemes or Sire Reference Schemes are discussed.