Genetic Changes in Breed Performance Over Time

J.P. Gibson

Projects such as the OPCAP provide estimates of breed differences at a particular point in time. These projects have many advantages over standard recording programs, particularly in comparing diverse breeding stock in a uniform rearing and testing environment, and in providing detailed information on many more traits of economic importance. It must be borne in mind, however, that such projects provide a snapshot in time and that breed differences may change over time.

The national recording and genetic evaluation programs for growth (days to 100 kg) and backfat (depth in mm) allow genetic trends for different breeds to be followed over time. Figures 1a and 1b below show these genetic trends for Yorkshire, Landrace, Hampshire and Duroc in Ontario from 1980 to 1995. These trends are shown on a scale where the breed differences in 1992/1993 match those estimated in the OPCAP. (1992/93 is the middle of the OPCAP and is when the average pig in the OPCAP was born.) It should be noted that the breed differences in OPCAP are based on intact males, barrows and gilts. There was evidence that sex differences were not the same for all four breeds, particularly for growth rate. The spread between breeds would be reduced by about three days to 100 kg if only barrows and gilts were included.

The genetic trends indicate that breed ranking for growth has not changed over the past 15 years, but the differences between breeds are getting larger. In 1980 the spread between Landrace and Hampshire was 7.2 days to 100 kg, compared to 10.6 days in 1992/93 and 12.2 days by 1995. The differences between breeds for backfat have been decreasing over time, and breeds have changed ranking a couple of times since 1980 and may change ranking again in the near future.

The consequences of these different rates of change in different breeds is that choice of breed to match a particular production and marketing system might change over time. While we can monitor breed changes for backfat and growth over time through existing genetic evaluation programs, we have no estimate of changes over time for the majority of economic traits measured in the OPCAP. Ideally, projects such as the OPCAP should be repeated on a regular basis to monitor changes in all traits of economic importance over time. Alternatively, a continuous testing program for detailed assessment of carcass and meat quality would accumulate information over time allowing continuous monitoring of changes. Such a program could also accumulate information on stocks from different sources within each breed to give accurate assessments of each stock's merits; something which was beyond the scope of the OPCAP.

Figure 1a/b. Genetic trends for days to 100 kg and backfat depth adjusted to 100kg (mm) in Ontario from 1980 to 1995.

Footnote: Genetic trends provided by Canadian Centre for Swine Improvement. Breed means have been adjusted to equal least squares means from OPCAP in 1992/93, shown by the vertical line on each figure.