Interesting developments in breeding for worm resistance
Two facts have emerged from my data over the past 12 months that could have a profound effect on our thinking on breeding for this trait.
First, the impact on another trait of breeding for the worm resistant trait.
It has long been accepted that breeding for this trait would have a negative impact on production traits, especially growth. The argument advanced to support this theory is that to achieve meaningful worm resistance would require energy to strengthen the immune system. This energy would have to be diverted from production traits. Therefore the net result would be that as worm resistance increases, the productivity of the flock would decrease. This, it was emphasised, would be most noticeable in growth figures. This theory assumes that the immune system is limited and not capable of increasing. I call it a theory because I have seen no experimental result to prove the case. Indeed, no trials would be possible as, up until now, there have been no highly resistant sheep available to conduct trials. Sadly this theory has been advanced for so long by many respected scientifically trained personnel that it was universally accepted as a proven fact. Even more sadly, it was a major deterrent to ram breeders to breed for this resistant trait. Imagine the difference if these authorities had strongly encouraged the breeding for this genetic option.
Over the years, I have not seen any negative impact on growth, and I have carefully studied all the data. In fact the reverse seems to be the case, as some highly worm resistant sires are also top in growth. One such sire had 7 of the top 10 ranking for growth out of 400 lambs evaluated.
To see what was happening in our own flock with regard to growth, I asked an independent professional to plot genetic trends of these traits. She traced the genetic trends for facial eczema, worm resistance and growth over the last twenty years and showed it in graphs. All graphs showed a similar positive pattern. This positive trend for growth is even more significant because unlike FE and worm resistance, I have never positively selected for growth, as I considered survival to be more important.
Clearly more research is essential to see what impact worm resistance has on other traits. My gut feeling is that a stronger immune system equates to healthier animals which would result in better productivity.
Finally, I must stress that this is not a trial but simply the genetic trends on one flock, in one environment.
The second development was both surprising and exciting.
This was the result of last summer’s faecal egg counting – FEC – of 397 ram lambs. The first count in the third week of January had a large average count of 3733 with a high of 25550 and a low of 105. The second FEC taken a month later, in the third week of February, resulted in an unbelievable reduction in the average to just 122, with all lambs reducing. The lamb with the 25550 count decreased to 140. This was totally unexpected, as we have been monitoring the reduction between testing for many years. In the early days, scientists at Ruakura stated that a reduction between the first and second count, was desirable. This was because the second count was at the peak of the worm challenge, and would be higher. In 2015 85% of lambs recorded a significant drop in FEC between the first and second count. 10% recorded an increase and about 5% remained the same. This result I found very pleasing. Imagine my shock when four years later, all 397 lambs tested showed a high drop in FEC. Having some knowledge of genetics, from a practical perspective, this was not genetically possible in such a time frame. But figures don’t lie, so other factors had to be at play. The environment would have to be the only factor. The past year has been unprecedented for exceptional for sheep health, especially for lambs. Virtually no pneumonia, which commercial farmers also noted, no pink eye which is normally prevalent over the late summer period, and no grass staggers. Inexplicably there was an absence of cattle tick and like barbers’ pole worm, suck blood, the lifeline of the immune system, were noticeably absent. This exceptional sheep health has thankfully extended into this year, with the ewes in better condition than I have seen since childhood, when my father had a low stocking rate in sheep. Without any crops this year, the lambs are very active and healthy without ever being drenched. My theory on the dramatic FEC drop is therefore as follow:
Normally the immune system faces many challenges over the late summer-autumn period, obviously worms, and also other health issues, principally pneumonia. Then there are other issues that weaken the immune responses like toxins produced by fungi in the pasture and cattle tick. So my theory is that the immune system did not have challenges from other diseases and parasites and therefore was better able to fight the worm challenge, and it blitzed a significant worm challenge in 4 weeks, and maybe less time.
I want to see if this is repeated this year. I have written to top scientists to see what their opinion is to the theory I am advancing.
Clearly this situation demands more research, to monitor worm challenges in animals that are bred for resistance, and gauge the responses of the immune system to such challenges. After the Kiki dispersal sale, when worm resistant sheep are in different ram breeding operations, and in different environments, would be the ideal time to conduct such trials.
What about the future of breeding for worm resistance.
The rapid spread of super worms that are resistant to all chemical drenches signals the end of the chemical control of worm challenges. There are no new drenches available and in the foreseeable future, partly due to concerns of the financial viability especially with falling sheep numbers worldwide. Farmers in the future will have to seek new options if they are to continue sheep farming. There are two options available. First to reinstate the methods employed by our forefathers before modern drenches were available. Trevor Cook has already designed farming strategies to reduce the intake of worm larvae to a point where worm challenges are controlled by an average immune responses.
The second option is to breed sheep that have immune responses that can control any worm challenge in all environments. That has been our policy for the past 34 years. This is the best, in my opinion, long term solution.
Currently the DPF – the SIL measure for worm resistance – of our flock has an average of around 650, with our 2019 lambs averaging 716. One sire had 15 of his 28 ram lambs over 1000. There were 4 outside rams used. I would anticipate this year’s ram lambs – with all Kiki sires – to average well over 800 DPF
All this begs the question. How far do we need the DPF figure to increase to ensure total protection from any worm challenge? I believe that an average DPF figure of 900 plus, but not more than 1000, is easily achievable and would be at a level to ensure that the immune system would give protection from any challenge. At this DPF average, the range would vary from about 700 to 1300. If the present increase continues, this would take 4 to 5 years. In the meantime, the current average DPF of around 700 would give total protection in most areas of the country.
The other area that needs improvement is to breed for an immune system that will react earlier. Anything is achievable in genetics because of the principle of genetic variation. I regard an average FEC of 3733 that we had in the ram lambs in the third week of January to be acceptable. The genetic variation was a low of 105 and a high of 25550. The ram lamb that had a figure of 105 was a good lamb, so he was used over a few ewes. He was subsequently sold to Murray Quinn a ram breeder at Kaikohe. The FECs of his ram lambs will be interesting.
Reflection
Initially the fact that our flock would have to be sold was very disappointing because a competent stud manager could not be found. However, I have come to believe that it is in the national interest to have other ram breeders complete the task of making worm resistance a dominant trait in their flocks. I have never been motivated by money, but have sought satisfaction in setting goals and trying to achieve them; not always successful. Breeding for worm resistance has been a huge learning experience with both disappointment and exciting moments. I have never doubted success, but the time frame was unknown.
Two facts I have found encouraging. First, parasitologists, more than 35 years ago stated that it would be possible to breed sheep with worm resistance given a long time frame. Secondly, French settlers took sheep with them when they arrived in the sub-tropical US state of Louisiana. Some sheep escaped and went feral. Sheep farmers face a huge barbers’ pole challenge and have to drench young unweaned lambs to keep them alive. However the feral sheep are totally unaffected. Another example of nature’s principle of ‘survival of the fittest’.
My earnest hope is that other ram breeders will continue with the programme that has been successful, and build on the 34 years of ground work that has been done. It is in the national interest they do so. The genetic option ticks all the boxes. An enhanced immune system will not only control worm challenges, but ensure better overall health of sheep. The farmer will benefit with less costs and labour input. To have fewer chemicals entering the food chain would be a huge plus for consumers.
As long as I am able, I will give advice where it is requested.
Gordon Levet
February 2021
Below are links to Flock 151 Trend Analysis and faecal egg counts referred to above