Be aware of digestate destiny from clopyralid treated maize

Maize has increased its area from around 160,000 hectares in 2010 to close to 200,000 hectares anticipated this year. This increase may be as a consequence of the Three-Crop Rule or to spread the workload or to introduce another break crop on the farm in order to get rid of difficult to control weeds. The majority of maize, according to the June Census 2014, is fodder maize, but there is an increasing area of maize that is grown for AD plants. Last year 17% of the total maize planted (or 29,373 hectares) was grown for this purpose.

“Growers are only too well aware that maize is a weak plant during its establishment phase and that maize seedlings can easily be overwhelmed by weed competition. Weeds colonise bare soil and take advantage of the wide rows and upright growth habit of the maize crop. Early weed removal is essential to achieve yield and quality. Crops are generally sprayed two or three times with a herbicide, pre and early post-emergence, to ensure the crop is able to grow through this vulnerable stage,” says Peter Waite of Dow AgroSciences.

Dow Shield 400 has a full label recommendation for forage maize and is a useful product to control some high biomass, highly competitive weeds such as sow-thistle, mayweeds, groundsel and corn marigold. It is applied post-emergence to all varieties of forage maize at 0.25 l per ha from the 3 leaf stage up to the 9 true leaves of the crop.

“Maize forms the base feedstock for most on-farm AD plants, with yield being the key driver. Growers need to be aware, however, that when approved products containing clopyralid are used in maize destined for the anaerobic digester, it takes 6 months for the chemical to breakdown and so the digestate should not be spread onto bare land where susceptible crops are to be grown. Contaminated crop residues can be a problem in some circumstances. If the digestate is spread onto land destined for grass, cereals, oilseed rape or maize, there are no worries. But if it is spread onto land where susceptible crops such as beans or peas are to be planted, there could be an issue, depending on how much treated maize went into the digester and how much time has elapsed between spreading and the susceptible crop being planted. It is better to be safe than sorry and follow the advice on the label,” says Peter.

The label text says ‘Dow Shield 400 residues in plant tissue (including digestate) which have not completely decayed may affect succeeding susceptible crops. If treated crop remains have not fully decayed by the time of planting following crops, avoid planting peas, beans, other legumes, carrots, potatoes, lettuce, glasshouse and protected crops. Winter beans and peas should not be planted in the same year as treatment. Where susceptible crops are to be planted the following spring, do not apply Dow Shield 400 later than the end of July of the previous season.’


Peter says that other herbicides, both pre-emergence and post-emergence, used in maize do not permit peas and beans as following crops either. “Growers, especially those new to the crop, need to make sure they read the labels thoroughly before use, in particular the restrictions on following crops.”