July 2016
Stop Selecting Sissy Sheep (and Goats)!
Dr. Gareth Bath
Emeritus Professor
Faculty of Veterinary Science
University of Pretoria
SOUTH AFRICA
How are animals selected in the wild? The world of nature is hard and dispassionate. Those animals best suited to their environment and able to survive and flourish are the ones able to reproduce and spread their genes to future generations. In this way, a vigorous and well-adapted population is assured. Yet when it comes to domestic animals, mostly this is no longer possible.
It has been practical to select animals for breeding on many factors that have little or nothing to do with suitability for the environment. Looks, size, growth, fancy points and current fashions are all too often the decisive factors that determine which animals will be used for breeding. The ability of breeding stock to withstand infections and parasites is usually totally ignored. But one of the rules of nature in genetic terms is summarized by the phrase “use it or lose it.” The absence of any selection pressure for animals able to cope with parasites means that over time the population group becomes more susceptible to these organisms, as the weak ones have to be supported by dewormers to be able to produce and reproduce satisfactorily. That in turn leads to farmers becoming more and more reliant on dewormer drugs to control their parasite problems, and of course over-reliance on drugs is a certain recipe for drug resistance over a period of time. Since the second half of the 20th Century, the availability of many effective, safe wormers made it possible to select sheep and goats on several other factors, and deal with parasites by using these drugs.
However, that era has come to an end, with drug resistance at a level where some farmers have practically no drugs left that are effective against worms. Now, every farmer should have a selection and culling plan to breed not only animals that have the ability to produce well with regards to essential traits such as growth and reproduction, but are also able to cope well with parasites within the grazing system in use on their farms. There is a difference between selection and culling. Selection means identifying the best animals for breeding, while culling means eliminating the worst ones. Because of the small numbers of rams in relation to ewes used for breeding on farms, this means in practice that we can concentrate on strict selection of the best rams rams from the relatively large numbers of male offspring available, while we have to rely more on culling for the females since about 70% of the females bred are required to produce and raise lambs/kids.
Has the heritability of coping with worms been proven in sheep, and is it high enough to make a difference in a reasonable period of time? The answer to both these questions is a definite yes. Research done independently in many parts of the world has proven conclusively that coping with worms is partly inherited. Coping means breeding for both resistance (the ability to prevent being infected) and resilience (the ability to withstand the effects of infection with worms), and there seems to be a genetic link between these two characteristics. There is some evidence that breeding animals for resistance or resilience against one parasite like Barber’s Pole Worm (Haemonchus contortus) can also assist with making them able to cope with other worms like Black Scour Worm (Trichostrongylus species). This is not surprising since much the same immunological coping mechanisms are involved. Not only do we know that ability to cope is partly inherited, but we also know that the heritability is high enough to make meaningful progress within a few years.
Although 25 to 30% heritability may not sound much, it is enough to confidently predict that if selection of rams and culling of ewes for resistance and resilience against parasites is done consistently and correctly, the breeder should see a noticeable difference in the flock within 3 years, and will see a significant improvement within 5 years. There are outstanding examples of high levels of success, such as the Rylington Merino Stud in Australia that is able to produce excellently despite close to total withdrawal of dewormers, even in the fact of well-known severe worm challenge in the region. For those who say “but that’s a long time!” the response is “all the more reason to start straight away!”. While it is true that most scientific investigations have involved sheep, there are sufficient indications to conclude that selection and culling will also work with goats. Sustainable parasite control relies in large part on breeding the right animals for the environment and grazing system in use, and the FAMACHA© system has been shown to be applicable to this end.
How can we select and cull under practical farm conditions? The answer to this one is relatively straightforward, it is definitely achievable in most situations, especially with Barber’s Pole Worm, in which case FAMACHA© makes it possible, for the farmer trained in FAMACHA© application to apply him- or herself, without the need for recourse to lab sample evaluation. First, the rams. Distinguishing between copers and non-copers is only possible when the worms are active: for the Barber’s Pole Worm this would usually be from mid to late summer, even early fall. The young replacement candidate rams must be exposed to pastures which are contaminated with worm larvae. For this worm, regular FAMACHA© checks are a surprisingly good way to sort the strong from the weak sheep.
The FAMACHA© scores of all rams in the group are recorded every 1-2 weeks, and all those needing treatment (scores 3, 4 or 5) at any stage are eliminated. Then after four or more FAMACHA© evaluation occasions, average these – you then have a list of best (most commonly FAMACHA© average category 1) to worst (usually FAMACHA© average 2) and only the best (strongest FAMACHA© 1) should be selected for breeding. Note that even in animals in FAMACHA© category 1 (i.e. the best group of animals) there is usually a range of intensities of red coloration of the mucous membranes of the eyelids, from the extremely intense red of the outstanding animals, to a less intense red colour in those that are not quite as good, despite also qualifying for FAMACHA© category 1. In this way the men can be separated from the boys .
Fecal egg counts (dung samples) can also be used for selecting rams; the lower the better. We have to know which type of worm is dominant at the time to decide when to do the test. For Barber’s Pole Worm, it is usually safe to do this when the average flock count is at least a thousand or more, but preferably more. For Black Scour Worm and others it should not exceed a few hundred. The decision on the optimum timing for the test is best discussed with a knowledgeable local vet or parasitologist.
Once we have egg counts from all qualifying candidates (never treated) we can rank them from worst (highest) to best (lowest), usually as an index, which is the percentage above or below the average count. Preferably repeat this at least once. We can afford to be strict with rams since there many candidates but only place for 2 to 4 % of the breeding ewe flock. For the ewes, we have to concentrate on culling since many more ewe lambs have to be retained for replacements. On many farms we recommend a number equal to about 30% of the ewe breeding flock. Ewes that score 4 or 5 are definite candidates for elimination, and so are those that have to be treated more often than the average; the higher the number of treatments required, the stronger the case for culling.
It is sometimes said that use of methods of Targeted Selective Treatment (treatment of only affected individuals, instead of all animals whenever a few of them become overwhelmed by worm challenge, as with application of FAMACHA©) will result in less productive sheep and goats, but research has consistently shown that this need not be so, with either no or a small effect on wool, growth, or reproduction. In fact, because sickly sheep will suffer in real situations where they are exposed to parasites, their performance will be worse than that of tougher animals. The weaklings can only perform well if protected against parasites by worming; this is not a sustainable system!
Farmers in many countries who strictly select their rams and cull their ewes based on their ability to cope with worms see the benefits which progressively increase over the years. In many cases they not only solve their own problem, but also end up selling their proven worm-tough rams and ewes to other producers at a premium. That’s why we say “Stop Selecting Sissy Sheep!” – or if you like, “Strong Sheep Seldom Succumb!”
Ask your local ACSRPC advisor for further guidance on how to improve your flock’s genetics.