It’s not every year that…

14 Jul 2014

Tour de France in Essex 2014It’s not every year that you can walk ten minutes down to the end of your street and see the Tour de France go by. It is really a spectacle. We were lucky enough to see all the first three stages as one of our daughters lives in a part of Yorkshire that’s very close to where the first two stages passed. I must admit there appeared to be more enthusiasm in Yorkshire during the build up for Le Tour, but the citizens of Cambridge did turn up in huge numbers to see the start of the third stage. We went down to the centre of Cambridge early on the morning of the start and I heard an announcer say that France was the host country of the Tour de France. I guess we all say something daft on occasions.

It’s also not every year that has good conditions for the control of perennial broad-leaved weeds with pre-harvest glyphosate, but they do come round far more regularly than the Tour de France visiting Cambridge. This year in many parts of the country we’ve had sufficient moisture to encourage the active weed growth that is so necessary for good control with glyphosate.

The pre-harvest recommendation, announced in 1981, played a critical role in the development of UK farming systems during the last couple of decades of the last century. Before then, we had to wait until the couch and other perennial weeds grew to a sufficient size after harvest. This delayed cultivations and in many cases there was never enough growth of perennial broad-leaved weeds for effective control. In some years an early frost removed the green growth of creeping thistle.

You have to remember that there were many more couch and perennial broad-leaved weed infestations around when glyphosate was introduced. It was really the pre-harvest recommendation that did for these weeds. I’m often asked whether herbicides have been so efficient that they have destroyed their own market by eradicating all their target weeds. The only example I can think of is that perennial weed populations have dramatically been reduced by glyphosate and are no longer a common occurrence.

Pre-harvest glyphosateThe main reasons for the application of pre-harvest glyphosate nowadays seem to be general weed control rather than perennial weed control and to ‘aid harvest’ of weed free crops. I do have problems with the latter because all the independent trials I have seen suggest that the application does not bring forward the wheat harvest but it can bring forward the spring barley harvest by a day or so.

The claim for glyphosate ‘evening-up’ the ripening of wheat can be precarious. Pre-harvest glyphosate application can start when the bulk moisture content of the grain is 30%. In a very variably ripening crop a significant proportion of the wheat may not be fully ripe and continuing to put on weight despite the average bulk grain moisture being 30%. Hence, the application of glyphosate may not only reduce the yield but also, glyphosate residues in the grain may be increased.

This is not a theoretical notion; there is trial data recording yield decreases in such situations. Hence, pre-harvest application should start only where the bulk grain moisture content in the latest part of the field is at or below 30% and at this moisture content, as I said earlier, application will not bring forward the wheat harvest.

I’m also not sure whether the pre-harvest application to a weed free crop will make harvesting any easier. All the trials I’ve seen suggest that such applications don’t reduce the moisture content of the straw by the time that the grain is sufficiently dry to harvest. I suppose that it may be an advantage regarding the moisture content of the straw a couple of weeks or so after application but trial data also suggests that treated straw can become wetter than untreated straw where the harvest is significantly delayed by rain. This is logical because the treated straw may be degrading more quickly and therefore is more likely to absorb moisture.

So whilst I recognise the huge contribution that glyphosate makes to modern cropping systems there is always a need to question whether each application will produce the desired results.