The aim is to apply herbicide as early as possible in the growing season, as this reduces the herbicide rate required and the amount of weed biomass that must be removed by grazing.
However, this needs to be balanced against the maturity of the companion clovers. To avoid significant herbicide damage to clover, at least three trifoliate leaves need to have grown on sub-clover when applying MCPA® Amine at the low rate and generally eight leaves for balansa and Persian clover. When using 2,4-D Amine, white clover should also have at least eight trifoliate leaves. Adequate clover maturity is commonly reached 6–8 weeks after germination.
The ideal size of the target weed is about 20cm (hand diameter). Plants greater than 30cm diameter (dinner plate sized) become too difficult to control with this technique, as the additional rate will result in significant damage to the clover.
Spraying too late in the season can also have negative effects. Firstly, sub-clover seed production can be compromised. Secondly, there is insufficient time for the desirable species to grow in the gaps left by the decaying weeds before the season finishes.
The impact of spray-grazing timing on pasture recovery. Spray-grazed June (top) and early September (bottom).
The herbicide rate increases as the target plants become larger. Therefore, the earlier spraying can mean less herbicide is required. Rates may vary depending on the herbicide used, so always follow label recommendations.
How do I get the grazing right?
Timely and intensive grazing is critical to the success of this technique. An advantage of spray-grazing is the opportunity to use the treated weed as part of the feed supply. One challenge is having sufficient stock numbers to complete the grazing within the desired period. The less feed on offer when grazing starts and the smaller the weed, the easier this is to achieve.