The mating game

By Nicola Dennis

Last year I did a bad job of mating selection with my stock. Distracted by a small child and all that comes with keeping small children alive, my mating allocation (which would usually involve poring over a variety of excel spreadsheets) boiled down to using the sires that were athletic enough to take the initiative to go find the ladies by themselves.

It is embarrassing to admit, but when I looked out into the paddock (procrastinating about opening my spreadsheets), there were already a couple of likely looking lads in there. I was tired, so I took the “pray and walk away” approach. I put a few extra sires in the paddock, for good luck, and hoped they would somehow magically assign themselves according to performance and relatedness.

Incidentally, this year’s offspring show no signs of being mutants which reminds me of that old joke “if it works call it linebreeding, if it doesn’t, call it inbreeding”. However, there is a very good chance that they are not as good as they could have been, had I put some extra effort in.

I am just a lowly hobby farmer. I reserve the right to make stupid decisions every now and then, but how many proper farmers can honestly say that mating allocation is the highlight of the farming calendar? Good matings essentially boil down to three elements; you need to have (1) the right sires and dams, (2) for the right price, (3) in the right paddock. How many are really hitting the trifecta?

It is tricky. One problem is that there is a bit of a trade-off between genetic merit and genetic diversity. The thing about great animals is they have a tendency to be quite related to each other. You can arrange a series of “royal weddings” between your very best sires and dams, but then you risk contending with unfavourable effects from inbreeding.

The most dramatic effect of inbreeding is the accumulation of undesirable recessive genes which can increase in inbreed populations (e.g. Queen Victoria of England’s mutation for haemophilia might have been an anomaly within the royal family if her descendants had not interbred so prolifically). Less dramatic effects such as inbreeding depression (the opposite of hybrid vigour) can lead to poorer performance and are a little harder to recognise.

So, ideally, we should not put closely related males and females in the same paddock. But, it can be mind-boggling to work out how closely related two animals actually are. It is easy enough to avoid parent-offspring matings or brother-sister matings…even the royal family worked that one out. Assigning mating by age (i.e. putting all the young replacements with the freshly purchased sires) and crossbreeding are two low-tech but highly effective approaches for avoiding close family incest.

But, if you want to prevent the kissing cousins from accumulating, it usually takes a bit more work than this. It requires some dedicated recording and/or genotyping to nut out the ancestry of each animal. After that there is some number crunching of the pedigree to calculate how much inbreeding has already occurred and how much inbreeding is acceptable for future generations.

There is usually always some degree of inbreeding, I mean linebreeding, occurring because there is not an endless supply of family lines. The idea is to maximise the overall genetic potential of the offspring while keeping inbreeding to an acceptable level.

This, like most genetic problems, descends into complex maths equations when there are a lot of animals to consider. Some genetics providers and databases, particularly for large animals, will do the hard maths for you and offer reports on inbreeding or alerts if matings are arranged between animals that are too closely related.

If these services are not available for you, fear not! Your friendly genetics consultant (hint, hint) can also optimise mating allocation using software programs such as AniMate.

Pondering inbreeding and co-ancestry coefficients is generally the domain of stud breeders, but genetic diversity is something we can all think about. Whether you are a commercial farmer wondering if you would do better sourcing sires from another breeder (or another breed entirely). Or, if like me, you are too cheap to discard a perfectly good recycled Romeo or a homebred Harry just because he has a few relatives on-farm.