Wednesday, July 6, 2022

Be forever watchful

I was in a social gathering over the New Year and happened upon a discussion amongst some cat lovers. I listened politely and feigned interest but was nevertheless fascinated. The link between coat colour, eye colour and deafness came up which was news to me. Ginger cats being usually males was the extent of my cat knowledge. But that association never got into the conversation. Maybe ginger cats are not the show stoppers. This conversation prompted my thoughts on our livestock selection programmes in which a trait or small number of traits have been strongly selected for and other less desirable traits have been co-selected for. There have been lots of examples of this in pig and poultry breeding programmes over the years. Usually there are health issue consequences from intense selection for a single or very small number of traits.

When the profit of an enterprise is highly influenced by one production parameter, such as growth rate or feed conversion efficiency, it seems that it is easy for an undesirable trait to emerge at the same time.

On our doorstep was the fall in the fertility of Friesian milking cows when milk yield dominated the selection pressure. Of course, when one measurable trait is selected for at the expense of another, there is substitution of the progress of one trait compared to another. Intense selection of one trait is not necessarily exposing another trait.

So, those sheep breeders who bravely set out on selection for sheep that did not get facial eczema many years ago could not put the same selection pressure on their sheep for other productive traits. As a consequence the progress they made in the other standard sheep production traits slowed up. Unfortunately the stigma of those sheep being not that fertile remained long after they got over that hump and had caught up again on lambing percentage etc.

But in selecting so aggressively for such a specific trait it would not be surprising if something unexpected showed up. It still intrigues me how the Finn sheep turned up from the other side of the world with the same trait, obviously unintentionally selected for in another environment. They certainly had not sacrificed lambing performance as a consequence of being tolerant to facial eczema.

We have examples, though, of intense selection for one trait also selecting a negative trait. The Massey University wool breeding programme over many years in which selection was based only on wool weight made huge progress in selecting for high wool weights as well as a whole bunch of other wool features. But these sheep were more susceptible to worms than sheep not selected just on wool weight. If that susceptibility to worms was measured purely by having an increased faecal egg count it does not necessarily imply increased susceptibility to disease. My memory, though, is that these sheep did get wormier by our traditional definition. I often think of the implications of that for our modern sheep.

For many years the only selection criteria for ewe hoggets was their spring wool weight. Before ewe pregnancy scanning it was not practically possible on commercial farms to identify ewes producing twins. But any farmer could select ewe hoggets on their fleece weight. The boffins tell us today that in selecting sheep just on lambing percentage, wool weight and weaning weight we are increasing susceptibility to worms by 2% a year. I do struggle with this definition of susceptibility though.

Another lot of brave breeders many years ago began breeding for worm-resistant sheep. Initially this was based purely on faecal egg count data of the lambs. With this as a major selection bias there were increased dags, lowered weight gains and lowered fecundity. The association of low egg counts and dags and weight gain has been well defined and was probably due to more nutrients being diverted to fight the worms. Any lowered fecundity would have been associated with less selection pressure.

But more comprehensive selection programmes that followed those early times reduced any negative impact of selecting on low egg counts and there are now selected lines of sheep that also perform at a high level for the standard production parameters.

Advanced technology allowing selection for traits that previously we could not select for, even though they were desirable, could be creating the opportunity for selection for negative traits.

So, for example, putting ram hoggets through a CT scanner to measure meat yield or meat distribution or fat cover is now a standard selection tool for some breeders. Selecting intensively for various carcase characteristics is relatively new territory although there is one long-standing breeding programme in New Zealand that has selected very successfully using this technology and there do not appear to be any negative consequences. But this is a terminal sire programme and potential impact on breeding parameters could be being missed. This is still an area that deserves to be studied.

In doing some background checks for this I was reminded of a 1950s trial in Russia that I was told about very early in my career but had long forgotten. Foxes were selected as pups on nothing but tameness as part of domesticating them. Over time the foxes got very tame but their coat colour changed, their conformation changed and the coat shedding pattern changed. In fact, they looked and behaved like border collies. What a great example of the consequences of single trait selection, in this case based on a subjective selection tool.

How powerful is it when the selection tool is very specific – such as back fat depth? We must be forever watchful. 

Trevor Cook

Manawatu Veterinarian

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