How breeding for worm resistance has evolved
In the early 1960’s I came to believe that there were genetic factors involved in foot diseases. The conventional thinking at the time was that these disease problems had to be treated because one could not breed against diseases caused by germs. However, I had noticed that some strains of sheep – by different sires – were less affected by this problem. I therefore concluded that some strains were more predisposed to foot diseases than other strains, and this predisposition would have to involve genetic factors. As a result of these conclusions, I decided to stop treating foot problems and embarked on a programme of selecting sheep that had no foot problems, and culling those with the worst feet. After 20 years, foot problems were virtually eliminated.
Over the early 1980’s I noted that some lambs were seriously affected by the blood sucking Barbers’ Pole worm, while others appeared to be unaffected. Having succeeded in eliminating foot problems from my flock, I started to wonder if the same genetic technique could be applied to controlling worms. Eventually in 1986, I travelled to Ruakura to discuss my theories with parasitologist, Dr Tom Watson. The outcome was that he drew up protocols to start breeding for worm resistance.
Basically this programme was as follows:
Over the years, there has been little change to these protocols in breeding for worm resistance.
I found that over the first 10 years progress was slow because most sheep were susceptible to the high worm population which comes with intensive farming. Fortunately, a breakthrough came in 1999 when I selected a sire which was outstanding for the worm resistance trait. By using this sire fairly intensively, and using several sons and grandsons, some real progress was made.
In 1990, three years after I started my programme, a national programme was introduced under the name Worm FEC. About 30 ram breeders joined the scheme, but many dropped out after 3 or 4 years when they realised that there was a lot of work and considerable cost involved, but no return in the value of their rams. Those who believed in the concept carried on regardless.
Parasitologists stated 30 years ago that it should be possible to breed meaningful worm resistance into a flock in about 25 years. I guess this was based on selecting mainly for this trait. In the real world of stud breeding, single trait selection is not an option as farmers require many traits both physical and productive.
So why did the concept of breeding for worm resistance not take off as the selection for FE tolerance has? After all, the cost to farmers is probably greater than FE as it occurs over the whole country every year, whereas FE only affects part of the country in some years.
There are many reasons. First, it is relatively easy to drench lambs at a reasonable cost. Secondly, there has been little pressure by farmers on stud breeders to pursue this trait. Thirdly, various alternatives have cropped up over the years, which have been diversionary. The resilience concept was the first. Some ram breeders and scientists have advocated this practice. Basically, resistance is breeding for lower levels of worms in the sheep and fewer eggs and larvae on the pasture. Resilience does not take into account worm levels and is based on the growth rates of lambs regardless of worm levels. So resilience is the ability to be unaffected by worm challenges. However, where the Barber’s Pole worm is dominant, high levels of this worm result in severe weight loss, dehydration and many deaths. On four occasions over the last 25 years I have visited the CSIRO Agricultural Research Centre at Armadale in Northern New South Wales where I discussed breeding techniques for controlling worms. On each occasion, I enquired as to whether they were carrying out research on resilience. The answer on each occasion was that resilience was not considered an option where the Barbers’ Pole worm was their major problem. Breeding for resistance was considered the only option.
About 1998 another method of breeding for resistance was promoted, particularly by scientists based at Wallaceville. This involved examining the antibodies in the blood. A simple blood sample attracted a number of ram breeders to pay good money to use this method to breed for resistance. Australian scientists examined the research and decided not to recommend it as an alternative to FEC testing. After several years, this method was withdrawn by Wallacevillle. More recently the “Worm Star” gene was discovered and hailed as a major breakthrough that could, if bred for, reduce the incidence of worms carried by sheep by up to 20%. Again, ram breeders shelled out to participate in this programme. However, now scientists have stated that breeding for this gene is of little value.
Another recent method of breeding for resistance is the Carla test.
This involves using saliva to detect elements which indicate worm resistance. I understand that a number of ram breeders are using this method. However, I also understand that some scientists have reservations. This test may well show which sheep have developed an immunity by autumn but I cannot see how it will identify sheep with an early developing immunity, which is the aim of my breeding programme.
The latest method of identifying sheep carrying worn resistance genes is the DNA (5K) test. I am reliably informed that this blood evaluation should be carried as an adjunct to FEC’s – not replacing it.
All these diversions over the past 25 years have, in my opinion, “muddied the waters” and left both farmers and ram breeders in a quandary as to the best method to adopt to breed sheep less susceptible to internal parasites. Most leading scientists and parasitologists here and in Australia believe that breeding for resistance via the Worm FEC or similar programmes is the best formula to breed a worm resistance flock.
Sadly over the years, the Worm FEC programme suffered from some bad press. Some ran breeders were highly critical, stating that it was just a ploy to sell more rams, and that the end result would be a flock of small daggy sheep. Unfortunately, they did not understand the genetics involved, nor had they discussed the subject with parasitologists.
More recently, another reason why breeding for resistance is a bad idea has gained traction, especially amongst academics. That is, that it requires energy to breed an enhanced immune system to give worm resistance, and that energy will be diverted away from productivity traits. In other words, you can’t have a productive animal as well as worm resistance. I know of no sound scientific evidence to prove this theory. In practice, I have found that this theory is wrong and that the opposite is the case. What I have found is that the sires that have produced the top ranking sons for productive traits are also tops for worm resistance.
Over the past 30 years, all of my 186 sires used over that period have been ranked for all productive, disease and parasite resistant traits. All these traits are combined in what is known as a DPO – Dual Purpose Overall. Then you also have a ranking for individual traits. My current top sire is ranked 4th for DPO; 18th for growth; 4th for FE and 2nd for worn resistance. These placings are from a total of 186 sires. This year’s sons of this top sire had the top 12 rankings for DPO and also the top 4 rankings for worm resistance, out of a total of 487 rams evaluated. On these facts a strong argument could be made that sheep with strong immune systems enabling them to withstand worm challenges are more healthy, enabling them to have better growth and grow more wool.
This year, for the first time in New Zealand, rams will be sold by public auction that have never been drenched. At this stage – before cataloguing – it is estimated that more than 80% will be in the “never been drenched” category. This has been achieved after 30 years of collecting approximately 30,000 dung samples for faecal egg counting. The approximate cost of this is $200,000 which does not take into account about 1500 hours of work. It should also be borne in mind that this has been achieved in Northern New Zealand where the warm, damp conditions give high worm challenges. Faecal egg counts of 50,000 per gram are not uncommon.
Over recent years, the rate of progress in resistance to worms seems to be accelerating. Unlike all other measureable traits, worm resistance is measured as a minus. So, the bigger the minus, the greater the resistance. Thus a sheep measuring -20.00 would have a moderate degree of resistance to worms. In 2003 after 17 years of breeding for resistance, I had 5 rams out of 304 that were better than -40.00 with the best one being -47.00. Six years later – 2009 – 188 were better than -40.00 out of a total of 325. This included 8 better than -50.00, with a high of -60.02.
In the 2015 born ram lambs, 122 were better than -60.00 out of a total of 427, and for the first time, we cracked -70.00 with 11 rams and a top of -74.65. Even more pleasing is the sons of our top sire, 99 in total averaged marginally lower than – 62.00. To think we only cracked the -60.00 mark in 2009 is hard to believe. How far we can go in the future is unknown.
New Zealand farmers, and particularly ram breeders need to have a critical look at their breeding programmes now that breeding for worn resistance has been proved to be a viable option. Now, instead of taking three decades to breed meaningful worm resistance into a flock, this time can be at least halved by introducing resistant sires.
Or do we continue along the same pathway of needing new drenches or combinations every decade because of drench resistance? Do we realise that since the advent of very effective drenches 60 years ago that our national flock has become more susceptible to worms, because even the most worn susceptible ewe lambs will thrive and be retained for breeding. We understand in our breeding programmes that we need to select the best and cull the worst. But do we realize that with over drenching programmes we are effectively culling the 99% of susceptible worms and retaining the 1% of the most resilient for breeding purposes. Most farmers now realize that the genetic option for reducing the impact of FE in our flocks is the best long term solution. Should we not now consider that there is a genetic solution to the worm problem?
The world’s consumers are demanding less input of chemicals into the food chain,, so less drenching is in part meeting that demand. The fact that the British food chain, Sainsburys, has invested £200,000 into New Zealand for research into and promotion of breeding for worm resistance must send a strong message to all sheep farmers.
Gordon Levet
October 2016
In the early 1960’s I came to believe that there were genetic factors involved in foot diseases. The conventional thinking at the time was that these disease problems had to be treated because one could not breed against diseases caused by germs. However, I had noticed that some strains of sheep – by different sires – were less affected by this problem. I therefore concluded that some strains were more predisposed to foot diseases than other strains, and this predisposition would have to involve genetic factors. As a result of these conclusions, I decided to stop treating foot problems and embarked on a programme of selecting sheep that had no foot problems, and culling those with the worst feet. After 20 years, foot problems were virtually eliminated.
Over the early 1980’s I noted that some lambs were seriously affected by the blood sucking Barbers’ Pole worm, while others appeared to be unaffected. Having succeeded in eliminating foot problems from my flock, I started to wonder if the same genetic technique could be applied to controlling worms. Eventually in 1986, I travelled to Ruakura to discuss my theories with parasitologist, Dr Tom Watson. The outcome was that he drew up protocols to start breeding for worm resistance.
Basically this programme was as follows:
- Drench all lambs at weaning or soon after and monitor the FEC (faecal egg counts). When the average count reached approximately 1500, all lambs should be sampled and a FEC carried out on each lamb.
- This process was to be repeated to gain greater accuracy.
- The average FEC of each sires progeny was then calculated.
- A low count ram from the sire with the lowest average count would then be used.
Over the years, there has been little change to these protocols in breeding for worm resistance.
I found that over the first 10 years progress was slow because most sheep were susceptible to the high worm population which comes with intensive farming. Fortunately, a breakthrough came in 1999 when I selected a sire which was outstanding for the worm resistance trait. By using this sire fairly intensively, and using several sons and grandsons, some real progress was made.
In 1990, three years after I started my programme, a national programme was introduced under the name Worm FEC. About 30 ram breeders joined the scheme, but many dropped out after 3 or 4 years when they realised that there was a lot of work and considerable cost involved, but no return in the value of their rams. Those who believed in the concept carried on regardless.
Parasitologists stated 30 years ago that it should be possible to breed meaningful worm resistance into a flock in about 25 years. I guess this was based on selecting mainly for this trait. In the real world of stud breeding, single trait selection is not an option as farmers require many traits both physical and productive.
So why did the concept of breeding for worm resistance not take off as the selection for FE tolerance has? After all, the cost to farmers is probably greater than FE as it occurs over the whole country every year, whereas FE only affects part of the country in some years.
There are many reasons. First, it is relatively easy to drench lambs at a reasonable cost. Secondly, there has been little pressure by farmers on stud breeders to pursue this trait. Thirdly, various alternatives have cropped up over the years, which have been diversionary. The resilience concept was the first. Some ram breeders and scientists have advocated this practice. Basically, resistance is breeding for lower levels of worms in the sheep and fewer eggs and larvae on the pasture. Resilience does not take into account worm levels and is based on the growth rates of lambs regardless of worm levels. So resilience is the ability to be unaffected by worm challenges. However, where the Barber’s Pole worm is dominant, high levels of this worm result in severe weight loss, dehydration and many deaths. On four occasions over the last 25 years I have visited the CSIRO Agricultural Research Centre at Armadale in Northern New South Wales where I discussed breeding techniques for controlling worms. On each occasion, I enquired as to whether they were carrying out research on resilience. The answer on each occasion was that resilience was not considered an option where the Barbers’ Pole worm was their major problem. Breeding for resistance was considered the only option.
About 1998 another method of breeding for resistance was promoted, particularly by scientists based at Wallaceville. This involved examining the antibodies in the blood. A simple blood sample attracted a number of ram breeders to pay good money to use this method to breed for resistance. Australian scientists examined the research and decided not to recommend it as an alternative to FEC testing. After several years, this method was withdrawn by Wallacevillle. More recently the “Worm Star” gene was discovered and hailed as a major breakthrough that could, if bred for, reduce the incidence of worms carried by sheep by up to 20%. Again, ram breeders shelled out to participate in this programme. However, now scientists have stated that breeding for this gene is of little value.
Another recent method of breeding for resistance is the Carla test.
This involves using saliva to detect elements which indicate worm resistance. I understand that a number of ram breeders are using this method. However, I also understand that some scientists have reservations. This test may well show which sheep have developed an immunity by autumn but I cannot see how it will identify sheep with an early developing immunity, which is the aim of my breeding programme.
The latest method of identifying sheep carrying worn resistance genes is the DNA (5K) test. I am reliably informed that this blood evaluation should be carried as an adjunct to FEC’s – not replacing it.
All these diversions over the past 25 years have, in my opinion, “muddied the waters” and left both farmers and ram breeders in a quandary as to the best method to adopt to breed sheep less susceptible to internal parasites. Most leading scientists and parasitologists here and in Australia believe that breeding for resistance via the Worm FEC or similar programmes is the best formula to breed a worm resistance flock.
Sadly over the years, the Worm FEC programme suffered from some bad press. Some ran breeders were highly critical, stating that it was just a ploy to sell more rams, and that the end result would be a flock of small daggy sheep. Unfortunately, they did not understand the genetics involved, nor had they discussed the subject with parasitologists.
More recently, another reason why breeding for resistance is a bad idea has gained traction, especially amongst academics. That is, that it requires energy to breed an enhanced immune system to give worm resistance, and that energy will be diverted away from productivity traits. In other words, you can’t have a productive animal as well as worm resistance. I know of no sound scientific evidence to prove this theory. In practice, I have found that this theory is wrong and that the opposite is the case. What I have found is that the sires that have produced the top ranking sons for productive traits are also tops for worm resistance.
Over the past 30 years, all of my 186 sires used over that period have been ranked for all productive, disease and parasite resistant traits. All these traits are combined in what is known as a DPO – Dual Purpose Overall. Then you also have a ranking for individual traits. My current top sire is ranked 4th for DPO; 18th for growth; 4th for FE and 2nd for worn resistance. These placings are from a total of 186 sires. This year’s sons of this top sire had the top 12 rankings for DPO and also the top 4 rankings for worm resistance, out of a total of 487 rams evaluated. On these facts a strong argument could be made that sheep with strong immune systems enabling them to withstand worm challenges are more healthy, enabling them to have better growth and grow more wool.
This year, for the first time in New Zealand, rams will be sold by public auction that have never been drenched. At this stage – before cataloguing – it is estimated that more than 80% will be in the “never been drenched” category. This has been achieved after 30 years of collecting approximately 30,000 dung samples for faecal egg counting. The approximate cost of this is $200,000 which does not take into account about 1500 hours of work. It should also be borne in mind that this has been achieved in Northern New Zealand where the warm, damp conditions give high worm challenges. Faecal egg counts of 50,000 per gram are not uncommon.
Over recent years, the rate of progress in resistance to worms seems to be accelerating. Unlike all other measureable traits, worm resistance is measured as a minus. So, the bigger the minus, the greater the resistance. Thus a sheep measuring -20.00 would have a moderate degree of resistance to worms. In 2003 after 17 years of breeding for resistance, I had 5 rams out of 304 that were better than -40.00 with the best one being -47.00. Six years later – 2009 – 188 were better than -40.00 out of a total of 325. This included 8 better than -50.00, with a high of -60.02.
In the 2015 born ram lambs, 122 were better than -60.00 out of a total of 427, and for the first time, we cracked -70.00 with 11 rams and a top of -74.65. Even more pleasing is the sons of our top sire, 99 in total averaged marginally lower than – 62.00. To think we only cracked the -60.00 mark in 2009 is hard to believe. How far we can go in the future is unknown.
New Zealand farmers, and particularly ram breeders need to have a critical look at their breeding programmes now that breeding for worn resistance has been proved to be a viable option. Now, instead of taking three decades to breed meaningful worm resistance into a flock, this time can be at least halved by introducing resistant sires.
Or do we continue along the same pathway of needing new drenches or combinations every decade because of drench resistance? Do we realise that since the advent of very effective drenches 60 years ago that our national flock has become more susceptible to worms, because even the most worn susceptible ewe lambs will thrive and be retained for breeding. We understand in our breeding programmes that we need to select the best and cull the worst. But do we realize that with over drenching programmes we are effectively culling the 99% of susceptible worms and retaining the 1% of the most resilient for breeding purposes. Most farmers now realize that the genetic option for reducing the impact of FE in our flocks is the best long term solution. Should we not now consider that there is a genetic solution to the worm problem?
The world’s consumers are demanding less input of chemicals into the food chain,, so less drenching is in part meeting that demand. The fact that the British food chain, Sainsburys, has invested £200,000 into New Zealand for research into and promotion of breeding for worm resistance must send a strong message to all sheep farmers.
Gordon Levet
October 2016