Where your breeder goes, you go

01-07-2010 | Mark Young

By sticking with your breeder of rams or bulls, you get the same rate of genetic gain they are making, maybe even more. As your breeder makes genetic improvement in their flock or herd, you ride along on their coat-tails.

Let's look at how this works with a sheep example. We will assume you have selected a ram breeder that has long-term breeding goals aligned with your needs.

You can achieve the same rate of gain as your breeder's flock if you return there to buy your rams every year, but your flock will "lag" behind their flock somewhat in terms of average genetic merit.

Understanding the concept of "genetic lag" leads some to conclude that it's more important in the long-term to buy rams from a breeder consistently making genetic gains than it is to get the first pick of rams from a ram breeder. To see why, let's look at what "genetic lag" is.

Genetic lag occurs because a ram breeder is expected to be using sires with a higher average for genetic merit than those available for sale to their ram buyers.

The selection decisions the ram breeder makes flow through to their ram-buying clients over time.

A ram passes on only half his merit, so how can genetic gain in your flock keep up with that in your ram breeder's flock?

It is a case of where the gains come from. Yes, the ram passes on only half his genetic merit but that is the same for you and your ram breeder.

You get the returns over the next few years as ewes enter your flock with higher average merit than that of older age groups. Since the rams you buy are most likely a lower average for genetic merit than those your ram breeder uses, your flock reaches a given level of genetic merit a few years later. The main determinant of genetic lag is the difference in average genetic merit of the rams you use from those your breeder uses.

If you begin at a lower level of genetic merit, the rate of genetic gain in your flock should exceed the rate in your ram breeder's flock as you narrow the gap between your flocks. However the gap will not close as long as the average merit of rams you use is lower than that used by your ram breeder in the same year.

Most importantly, the two flocks will move forward at the same rate of genetic gain once a "steady state" of genetic lag is reached.

The graph below illustrates this. A commercial flock starts at a lower level of genetic merit but reduces this gap by 75% after about eight years.

Thereafter we see further small reductions in the gap as the commercial flock tracks along below the ram breeding flock. This example yielded a figure of just under three years for genetic lag. That is the time between points A and B marked on the figure.

How can you reduce genetic lag between your flock and your ram breeder's flock? Buy rams of higher genetic merit.

The higher the average merit of rams you buy, the less the gap (genetic lag) between the two flocks.

If you could use the same rams as your ram breeder, and your ewe flock age structure was similar to theirs, the gap would be close to zero.

Of course, usually high-merit rams cost more. There's no such thing as a free lunch!

Other strategies to reduce genetic lag further include replacing rams more frequently and using ram lambs.

So where your ram breeder goes, you go. You can rely on getting the same rate of genetic gain they are getting but you should expect your flock will lag behind the breeder's flock in terms of average genetic merit by a few years.

Locking in these year-by-year gains may be more important than trying to get further up the list for picking rams from a breeder.

For more information on this topic please contact us at silhelp@sheepimprovement.co.nz or 0800 silhelp (0800-745-435).

• Dr Mark Young is the SIL Geneticist for Meat & Wool New Zealand. SIL is part-funded by your levy.

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