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Van't Hoff's rule, Spawning Salmon & Propyzamide

Date Added: 09/11/2016

Blog: by Richard Elsdon, Technical Consultant, United Oilseeds

A very long time ago, I studied chemistry at a school in the North East of England, an area somewhat colder than where I now live in Wiltshire.  Amongst other things on the syllabus was Van’t Hoffs rule which, if I recall correctly, stated that for every10 degrees C increase in temperature in the normal range for a reaction, biological activity doubles. So an increase in temperature makes things go quicker? Well not always.

Look at this increase in temperature rule from the viewpoint of a salmon keen to spawn. Relatively high temperatures create a seriously unfavourable situation for spawning, as the warmer the water, the less dissolved oxygen it contains. This means that the highly active salmon finds it harder to make its way upstream and  fight against strong currents mixed in with the occasional set of rapids. In this case, a cold temperature is definitely required.


For the herbicide propyzamide to work, cold is vital. The instructions from Dow say that the soil temperature at 30cms should be 10 deg C and falling and, in addition, the soil moisture deficit must be less than 50mm and falling. Now a quick check on these two criteria shows that the soil temperature at present is just below 12 deg C and that the soil moisture deficit, as expected, is lower in the west at anywhere between 0 and 30mm. But the eastern side of the country is still very dry, with the highest deficit of 121mm in Essex.

Now I know what you are thinking “it may be dry now but once it starts raining it will never stop and I will not be able to travel with my sprayer.” I cannot forecast the weather but I can forecast disappointment if this herbicide is applied when the soil is either too dry or too warm.

There are two herbicides containing propyzamide. One is straight propyzamide while the other is co-formulated with aminopyralid which controls a larger range of broadleaved weeds. As far as application is concerned, there is no difference. Both need cold and dampish soil to give the best results.

Whilst on the subject of results, every year some growers are disappointed and claim that their black grass is resistant to propyzamide. But the scientists tell us that this is not the case – so what is happening? Propyzamide is relatively immobile in the soil and rarely travels deeper than 50mm (two inches). Why is this important? If cultivation has taken place deeper than 50mm, then it is likely that some black grass will germinate from depth and, as is the way of black grass, the roots emerge from the base of the stem which will be lower than the herbicide layer. This will allow the black grass to survive the herbicide and give rise to the ‘resistance’ tag.

Cold and wet are important for a successful outcome with propyzamide and that may mean you must wait until conditions are right for the best outcome. Rather like a spawning salmon waiting for a flood and cold weather to enable it to reach the gravel beds to lay its eggs.

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