Sarah Cook
Plenty of grass weeds are now visible above the canopy with broad-leaved weeds lurking below. The key question is did my herbicide programme work and if not why not? Did I use the correct actives for the weeds present? Do I suspect herbicide resistance? Have I imported a new weed in manure or compost?
Weed control always starts with prevention, this begins with planning the rotation. You should have collected good information on what weeds are present in the field and the location of patches.
Sometimes the weed species will dictate changes in planned cropping. It is easier to use certain cultural techniques in some crops than others. For example, it is not really suitable to plough before oilseed rape, but before cereals it is. Cereals are more suited to delayed drilling, but break crops give you the chance to use different herbicides. Barley and oats are more competitive than wheat. Wheat and barley have more herbicide choices than oats.
Once you have selected the crop consider all the cultural techniques (IPM) that can be deployed to reduce weed numbers. What were the weed numbers in the current crop? If there are not any then minimise cultivations to prevent anymore weed seeds being pulled up from the seedbank. If there was a high weed population then consider burying them by ploughing or allowing them to germinate and spraying off before drilling later.
It is key to reduce the pressure that the herbicides will face so aim to reduce the weed population before drilling. It is all a question of maths:
10 weed plants with 95% control from a herbicide equals leaves 0.5 weeds.
1000 weed plants with 95% control from a herbicide equals leaves 50 weeds.
If the herbicides did not work in the previous crop, it is not necessarily as a result of herbicide resistance. It could be due to the incorrect identification of the weed, the wrong product or rate, incorrect timing (weeds too big, too small or not all emerged at the time of spraying), lack of a wetter or too many weeds at the time of spraying.
If it is herbicide resistance then we can help. We offer a full service of herbicide resistance testing for both grass and broad-leaved weeds.
More information can be found here https://www.adas.uk/Service/herbicide-resistance-testing
Black-grass dormancy
We will be testing seed this year for black-grass dormancy. It is difficult to predict what this might be now in winter wheat, as we are currently in the period when it is determined. At the moment it is hot/warm so maybe dormancy will be low in some areas.
For black-grass in spring cereals the dormancy determining period will be later in the season as the seed ripens later than in winter crops. The same weather rules apply for black-grass in winter wheat.
