An important reminder when using Group 4 (old Group I) herbicides is to choose the right product at the right time. Group 4 herbicides work by mimicking indol-3-yl-acetic acid (IAA). IAA is responsible for cell division, differentiation and elongation, among other functions. An application of a Group 4 will turn on all the growth ‘switches’ in a plant, causing it to twist and distort as rapid growth occurs that the plant can’t keep up with, causing death. Cereal crops get their selectivity as they increase in maturity, hence why it is so important to get the product and rate right.
Early in the season, we prefer to use MCPA products when doing broadleaf sprays in cereals as they are softer on crops. These products can be mixed with a range of other products to suit your weed spectrum in the paddock. Normally, the rate range will increase as the plant matures and the number of leaves on it increases. Later in the season, you can switch from MCPA products to 2,4D products once the first node can be felt at the base.
The other differences between these products are whether they are ester or amine formulation. Esters have higher vapor pressures than amines, meaning there will be increased volatilization. Whether either provides better weed control depends on a lot of variables. However, ester formulations are typically more active on weeds due to the fact they are more soluble when in contact with the plant’s waxy cuticle.
Here are a couple of key tips to remember going forward:
- If you find yourself in a situation where MCPA is not readily available, it’s important not to rush into using a 2,4D product too early. Doing so can lead to significant crop injury.
- Always follow the label for rates, as well as to make sure you are using the right Group I mix partner. If you have any questions don’t hesitate to call our Agronomy Team.
- Remember to use the correct nozzles when applying 2,4D products.
- Don’t double up. Avoid spraying 2 applications of Group 4 herbicides alone onto the same lot of weeds in the same season—try rotating your actives to avoid resistance.
- As a reminder, Group 4 herbicides include:
- 2,4-D
- 2,4-DB (Buttress)
- MCPA 750
- LVE MCPA 570
- Dicamba
- Clopyralid (Lontrel)
- Fluroxypyr
- As a reminder, Group 4 herbicides include:
