Spring Crop Weed Control

According to Michael Hennessy, Teagasc Tillage Specialist, excellent weed control can be achieved by all growers – but some basic knowledge is required. When inspecting the crop bring a picture of weeds at various growth stages with you when walking crops and identify these weeds as you go.

Pictures of these weeds can be found on the Teagasc client website. It is quite easy to identify weeds if you exercise a little patience in the field during inspection.

Alternatively, many farmers use an agronomist to help with the selection of an herbicide. The decision to use specific herbicide should be made jointly as it is impossible to walk every square meter of the field.

The best an agronomist can do is to walk the field to get a general sense of the weeds and use knowledge from previous years, but small patches of troublesome weeds may not be immediately obvious. This is where a farmers’ knowledge comes in.

When a grower gets a recommendation, it is prudent for the farmer to chat to the agronomist to make sure concerns about certain weeds are addressed. Deciding on the chemical to be used will be based on the herbicide or combination of herbicides which will control the majority of weeds identified in the field.

Herbicide selection is important but it isn’t everything. Sowing date, crop density and general growing conditions all impact on the success of weed control in cereals. It is fundamental to work with the growing conditions around the spray timing to optimise weed control and reduce costs.

Trials carried out in Teagasc Oak Park over the past two years have shown a reduced rate of herbicide, applied early (four leaf to early tillering), will out-perform (in terms of yield) a full rate herbicide, applied late (early stem extension).

These trials also show that it is best to delay the application of herbicides when crops are under stress due to moisture, harsh weather, etc. As trials have shown yield reductions due to the extra stress the herbicide puts onto the crop. This underlines the fact that good growing conditions before the application of herbicides are very important.

Herbicide Choice

Sulfonylurea’s (Ally Max, Cameo Max, etc.) form the backbone of broad leafed weed control in spring cereals. However, hormones (CMPP, HBN, etc.) are still needed to fill gaps in the SU herbicide susceptibility profile.

The range of herbicides available offer good value for money as they contain up to 100% more active ingredients for a small increase price over older straight products like Ally (Finy, Lorate) or Cameo (Thor).

The mixing and increased concentration of these SU actives in one product brings increased control to a broader range of weeds than using one SU product on its own. Due to the increased availability of products like Ally Max SX, Cameo Max SX, Harmony M Max, etc., these are the most commonly used herbicides in spring crops and offer better insurance for fields where you are unsure of the weeds present.

There is a tendency to use these products in a similar fashion to the old SU products e.g. reduce the rate to 80% of a full rate but as these products are more expensive, costs inevitably increase.  Rates can be stretched in the correct circumstances to half rates or lower.