Moving towards low-chemical farming with sheep and cattle: the
potential of a breeding approach
C.A. MORRIS AND A.D. MACKAY
AgResearch, Ruakura Research Centre, PB 3123, Hamilton, New
Zealand
NZSAP 2002 Abstract No. 22
Proceedings of the New Zealand Society of Animal Production 62:
81-85
On livestock farms in New Zealand, some farmers are changing
their sheep, beef cattle or dairy cattle enterprises towards
specialisation 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 realising 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/i>, nematode
parasites, dags, fly strike, facial eczema (FE) and ryegrass staggers
(RGS); in dairy cattle, mastitis, bloat, FE, RGS, nematode
parasites and hypomagnesaemia; 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 cattle are: 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;
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.
Keywords: NZSAPAB;
genetics; heritability; sheep; cattle; sires; low chemical
farming
Last Updated 30/07/2002
