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resistance

4 September 1999

Pitching against

resistance

In the second of the series, Tom Allen-Stevens finds out how one distributor is using trials results to determine the best way to overcome resistance.

A FIELD riddled with resistant blackgrass is an unsavoury nightmare for most growers, but for those wishing to carry out herbicide trial work its a gift. United Agri Products (UAP) has descended on one such field at Caenby in Lincolnshire, where five-star resistance to fops and dims has been confirmed. Various trials have been carried out to tackle the problem, looking at both chemical and cultural control.

"We have other sensitive blackgrass trial sites in various locations in the UK where we have been looking at lower doses of IPU and its alternatives. At Caenby, we are taking the opportunity to pitch a number of resistant blackgrass control strategies against each other and assess the outcome," says Eastern regional technical manager Brin Hughes.

Three cultivation programmes are being explored:

&#8226 Ploughing late to enable chitted seed to be buried

&#8226 Stale seedbed technique using glyphosate to kill chitted seed prior to drilling

&#8226 Stale seedbed technique with no total herbicide used.

"It became apparent early on that a stale seedbed technique was a fairly hopeless approach unless glyphosate was used. Ploughing is a useful way of burying chitted and unchitted seed, but what tends to happen is any remaining blackgrass will emerge over a relatively long period of time. The best opportunity for following up with chemical control came after making a sterile seedbed using glyphosate," reports Mr Hughes.

The recommended way to produce a stale seedbed is to disc and press the field shortly after harvest. If a straw-chopper is used, care should be taken to ensure the chaff is spread as evenly as possible behind the combine. After discing, the field should be left for 5-6 weeks so that seeds can chit. The green material is then sprayed off with Round-Up (glyphosate) for example, and the next crop drilled using a power-harrow/drill combination. At Caenby, the initial seedbed was brought about with a plough, followed by power-harrow and rolls – more expensive, but just as effective.

Eight post emergence spray treatments were then applied to the plots (see table). Most of the treatments were applied on November 25. The straight Lexus (flupyrsulfuron-methyl) was applied early (November 3), when the weeds were at GS11, and the Topik (clodinafop) on March 16.

In the plots that were late ploughed, control of the blackgrass varies considerably under the different treatments. "Isoproturon (IPU) proves to be a useful chemical still, but bear in mind the predominant resistance on this sight is to fops and dims and you have to get a good seedbed for it to work right. Reducing the rate and adding Stomp (pendimethalin) appears to have little benefit. Of the residual herbicides, Harlequin (IPU and simazine) and trifluralin is the best here," sums up Mr Hughes.

Effective

The late Topik has very little effect and the residuals work better than the contact herbicides overall. But Mr Hughes warns that a blackgrass control programme can only begin to be effective if it knocks out 98% of the weeds, and the Harlequin/trifluralin mix takes out only 78%. If more than 2% of the population goes to seed, enough will be returned to the seed bank to bring about a full population in the following year.

A pre-emergence dose of Avadex (tri-allate) helps all treatments enormously: "You can actually see the 12m stripe in the field where it was applied," notes Mr Hughes. But all plots still elude the magical 98% control – 96% is the best achieved with the Harlequin/trifluralin mix.

So the only way to control blackgrass effectively is to start with a sterile seedbed, using glyphosate to burn off any chitted weed seed. Overall the treatments prove to be much more effective following this cultivation programme, with Lexus + Hawk (clodinafop and trifluralin) + oil being the best performing with 98.8% control. Across the board, pre-em Avadex improves results further, though offering minimal change to the Lexus/Hawk/oil mix. Following Avadex, the Puma(fenoxaprop and IPU) + trifluralin + oil mix also gives more than 98% control.

UAP has also carried out trials on Cyanamids pendimethlin-based AC210. The new chemical performed at least as well as the best of the old, but its early days.

Mr Hughes is also impressed with Monsantos new brome killer, Mon375, out next spring. UAP trials have shown that the best of the current chemistry (Avadex, IPU and Fortrol – cyanazine) manages to quell less than half the population of rye brome in the plots. A full treatment of Mon375 in the spring has succeeded in giving an impressive 100% clear-up rate. "It looks really good; its far less affected by soil type and trash content and does surprisingly well in difficult conditions. But again its early days and it needs more evaluation particularly against the more difficult to control species – sterile brome. We need to try sequences with Avadex and split timings," adds Mr Hughes.

However, Mr Hughes sounds a timely warning for those growers wishing to mix and match new chemistry, "Mon375, along with Lexus, Eagle (amidosulfuron) and Ally (metsulfuron-methyl) are all sulphonylureas. Only one of these chemicals can be used in a crop each year."

How eight post-em herbicides fared on blackgrass


Treatment Rate(s) Cost % blackgrass controlled

(litres/ha) (£/ha) Late plough Sterile seedbed

Pre-em Avadex – 22 + – + –

IPU 5.0 16 71 80 97 94

IPU + Stomp 2.5 +3.3 36 17 33 81 86

Harlequin + trifluralin 4.0 + 2.0 26 78 96 78 86

Hawk + oil 2.5 +1.0 30 44 86 33 75

Lexus 20g 16 22 89 89 96

Lexus + Hawk + oil 20g + 2.5 + 1.0 46 78 94 98.8 98.8

Puma + trifluralin + oil 4.0 + 2.0 + 1.0 36.50 58 86 94 98.2

Topik + oil 125ml + 1.0 40.40 6 86 0 50

Key: + = with Avadex; – = without Avadex. Source: UAP trials

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RESISTANCE

25 July 1997

RACE FOR INBUILT SCRAPIE

RESISTANCE

Scrapie genotyping is a technological advance the sheep industry can no longer afford to ignore. But how should it be used for the best advantage of the industry? Sue Rider reports

THE race to breed scrapie resistance into UK sheep flocks is now on.

Mike Dawson, of MAFFs Central Veterinary Laboratory and who, with his colleague Linda Hoinville, is studying scrapie in several flocks in England and Wales, reports a significant upturn in demand for genetic testing.

Increased interest is both from individual breeders keen to have rams tested ahead of this summers sales, and breed societies, especially the Swaledale, Shetland, and Cheviots. Demand is driven by premiums likely to be available for high indexing rams with a scrapie resistant tag, says Mr Dawson.

"More and more producers recognise the value of a resistant animal and are keen to take blood samples, mainly from rams, to send off for the gene test. Demand is such that well be busy this summer, but its encouraging that we can now do something positive about controlling scrapie incidence in the national flock."

The scrapie gene test fingerprints the gene which encodes for PrP, the protein involved in the development of scrapie. Fingerprinting identifies those forms of the gene linked to scrapie susceptibility and those linked to its resistance.

Commercial testing is available through the CVL or the SAC, but the concept and early technology was pioneered at the Institute of Animal Healths neuropathogenesis unit in Edinburgh, explains Mr Dawson.

IAH work has identified 256 positions, or codons, on the PrP gene. Each codes for an amino acid – the building blocks of the PrP protein.

The PrP gene, like all genes in mammals, is composed of two strands, or alleles, one from each parent. At least five variations of PrP alleles have been identified in sheep, but not all have been found in every breed studied so far.

Differences between the five alleles are due to variations in the amino acids specified at codons 136, 154 and 171 (see box).

As mentioned previously, a gene comprises a pair of alleles, and the options for pairing are determined by the alleles carried by ewe and ram.

Pairing can be of like or unlike alleles, says Mr Dawson. For example, VRQ could pair with another VRQ, giving the genotype V/V at 136 – R/R at 154 – Q/Q at 171, or VRQ could pair with ARR, giving V/A at 136 – R/R at 154 – Q/R at 171.

Indicates susceptibility

The VRQ allele indicates scrapie susceptibility in many breeds, such as the Swaledale, Shetland, Welsh Mountain, Cheviot, Charollais, Texel and Bleu du Maine.

In Suffolks, the VRQ allele is very rare: in this breed scrapie susceptibility is linked to the ARQ allele. As for AHQ, there is evidence that it provides some scrapie resistance in breeds where scrapie is linked to VRQ.

ARH is common in Texels, but its significance is not clear: It may have a neutral effect on disease susceptibility. ARR seems to be associated with scrapie resistance in all breeds studied to date, says Mr Dawson. But whether this will apply to all breeds of sheep and all strains of scrapie remains to be seen.

"Strategies for developing scrapie resistance require negative selection for those alleles linked to scrapie susceptibility and positive selection for those linked to resistance," he adds. "Selection will also have to take into account impact on other breed traits, but it may be that development of scrapie resistance should be a priority."

The common alleles in the Suffolk are ARQ and ARR: There is no variation at 136 and 154 so testing is only needed for 171. Mr Dawson and his team are currently developing a one codon test for breeds with variation at 171 only to enable cheaper testing.

"In Suffolks, increasing the genetic resistance to scrapie is straightforward. Avoid using 171Q/Q rams and where possible use 171R/R rams in preference to 171Q/Rs.

"By using 171 R/R rams year-on-year the level of resistance will increase. Progeny of such rams will at least be 171Q/R and have a significant degree of protection against clinical disease caused by all known strains of scrapie," says Mr Dawson.

Mike Dawson and colleague Saira Hamid analyse the results of blood tests – which are in increasing demand – to determine scrapie genotype.


Scrapie genotypes

136154171

VRQ

ARQ

AHQ

ARH

ARR

V = valine, A = alanine, R = arginine, Q = glutamine and &#42 = histidine.

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