By Dr Clive Dalton
- The earlier list of breeds would be considered as “pure breeds” or purebreds.
- This does not mean that they are “genetically pure” as some breeders in the past would have had us believe. It just means that they have easily-recognised common features that persist when rams of that breed are mated to ewes of that breed.
- Genetically, all breeds have similar genes, the exception being the Merino with some genes that are a bit different such as the genes for superfine wool and wrinkles.
- When you look at DNA, then even different species of animal share the same DNA and even with humans!
This is the same as pure breeding but the pedigrees of all purebred animals are registered in the breed organisations flock book.
Strains within breeds
Within breeds there are different “strains” of sheep that may have different physical traits or may perform a bit differently. But again, their genes do not differ much. The Rare Breeds in the list above would be good examples of different strains, and the different wool types in the Merino (superfine and fine) are different strains.
“Crossbreds” come from mating rams and the ewes of different breeds. Here are some basic principles:
- When you crossbreed, then genetic variation is increased.
- Crossbreds are generally more productive than purebreds, and generally (but not always) exhibit “hybrid vigour” or “heterosis”. Hybrid vigour is where performance is better than the average of both parents. This is “positive heterosis” but it can be also be “negative” and worse than the average. Nobody talks about negative heterosis and always assumes it is positive which they call hybrid vigour.
- Don’t expect the crossbred to be better than both parents. It happens in plants when crossing highly inbred lines of maize for example which you cannot achieve with animals.
- This hybrid vigour is usually higher (as much as 10-13% extra) production in the first cross (called the F1), but then it declines more in the F2 and is all but gone by the F3 unless you take steps to maintain it.
- If you keep bringing in outside breeds to cross with your ewes which are different genetically, then you can keep a fair amount of the hybrid vigour going.
- The wider the genetic differences between the breeds crossed, the greater is the hybrid vigour. So if you cross two breeds that are very similar, don’t expect much hybrid vigour. You’ll end up with offspring that are an average of both parent breeds.
This is the main concern with crossbreeding. It’s easy to produce the first cross – then what? Here are some options:
- Keep re-mating the purebred parent breeds, and slaughter all the first-cross lambs.
- When you want to replace the purebred parents, then buy them in. This is the simplest thing to do.
- Or produce replacement parents yourself, but you’ll need flocks of each parent breed – a large flock to breed the purebred ewes and a small flock to breed purebred rams. This can be a lot of work and is a big-farm operation if you want to do the job properly, as you only get good crossbreds by mating good purebreds. That is worth remembering.
- Slaughter all the F1 male lambs for meat, but keep the F1 females as dams for further breeding. These F1 females are generally great sheep and make fertile and good milking ewes so their lambs grow well to weaning.
- Mate these F1 females to a specialist meat breed sire called a “terminal sire” as all the progeny go for slaughter.
This is using crossbreeding to change from one breed to another. Here’s the usual process:
- Start off with “foundation” animals of one breed or type. Usually there are no rules about what they look like, but it would pay to check if they need to be inspected and approved by the breed organisation of your choice.
- Mate them with a purebred pedigree registered ram of the breed you want to change to, and this will produce F1 offspring. These will have 1/2 the genes of the new breed you want.
- Mate these F1 females to a purebred pedigree ram again. They may need to be approved by an inspector from the breed society or association before proceeding to breed from them so check this out.
- This produces the F2 that have 3/4 of the new breed.
- Carry on mating the females born each generation to produce 7/8 and 15/16 offspring.
- Some breed organisations will allow you to register 7/8 as “purebreds” but others only accept the next generation of 15/16.
- This can be a very long process as only half the lambs born each generation are females, and along with losses, some animals may not turn out to be acceptable and not meet your or the breed association’s standards.
- It’s a funny thing that when you are grading up and want ewe lambs so badly, how often you seem to have an endless run of ram lambs confounding the laws of nature. The 50/50 rule doesn’t seem to work.
- Some pedigree breeders over the years have always been concerned that these newly graded-up purebreds could have introduced some “rogue” genes that will have had bad effects on the breed. Some geneticists consider this to have been an asset to the breed by introducing genetic variation to allow better selection.
When you get to the stage where things are becoming confusing and you don’t know what stage of cross the animal is, it’s maybe time to forget about where you are at and start “interbreeding” among the population, and ignore breed and select simply on performance. Here are a few key points:
- Note this is not inbreeding (see below).
- You will have to do some basic recording to identify good-performing animals.
- Keep the best ewes on performance and keep their female offspring as replacements. Cull heavily on performance and structural soundness.
- Keep the best ram lambs each year from the best ewes as potential flock sires– making sure they don’t mate their own daughters or grand-daughters.
- Traditional pure breeders in the past called this “mongrelising”. It’s the way new breeds were formed in the past and is the way modern commercial strains, types or “composites” are being produced by selection from a “gene pool”.
- It’s the way the New Zealand Huntaway was produced – and that’s the best example in the world of an animal selected primarily on performance as nobody cared what they looked like. Anything that did not perform was euthanased as nobody wanted them as pets.
- If you get to a stage where you are totally confused and don’t want to get involved in more basic recording, then start again by buying pure-bred stock and produce F1s or buy them in.
Inbreeding and linebreeding
In crossbreeding, animals are mated that are not related, so you end up producing more genetic variation in the flock. In direct contrast, inbreeding is where you mate animals that are related, i.e. have common ancestors somewhere in their pedigree. It’s sometimes called “close breeding”. Here are some of the key facts about inbreeding and linebreeding:
- If related animals are mated, their offspring will have more of their parent’s genes in common – whether they are good genes or bad ones, so genetic variation will decrease.
- If you mate animals that are very closely related like sons back on mothers or sires on daughters, you’ll increase relationships (called homozygosity) very rapidly indeed, and this is called “intense inbreeding”.
- If you do it more slowly when mating cousins, then homozygosity will build up more slowly and this is called “linebreeding”.
- So the difference between inbreeding and linebreeding is simply how fast you proceed towards homozygosity; or the state where everyone had the same genes as in clones.
- You don’t get very far with inbreeding in sheep compared to what’s possible in plants, as they usually become infertile or show very poor survival and die out.
- As inbreeding starts to build up, you see minor defects appearing like undershot jaws, extra limbs or odd colours. But then as it increases further, “inbreeding depression” takes hold and you find weak offspring at birth, poor survival and poor growth. Highly inbred animals get to breeding age they have low fertility.
- Embryo transfer is now regularly used in sheep with high genetic potential.
- Artificial insemination is not in common use yet.
- Ovine Single Nucleotide Polymorphisms (SNP) chip to make finding the genes in the sheep genome easier. Called the Ovine SNP50 Bead Chip it's a tool to characterise genetic variation to find important commercial traits.
This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.