Destroying a good clover ley to eradicate docks may seem like an extreme step, but dock infestation can reach a stage where milk or beef production is seriously compromised.

This is the case when dock levels reach more than 15% in a clover ley, says Duncan Connabeer, Dow regional business manager. “A pasture producing 10t/ha DM with an ME of 11MJ/kg DM and a dock infestation of 15% can result in reduced milk yields of 1090 litres/ha,” he points out.

“Whether using a clover-safe spray is the correct strategy for dock control will depend on the extent of dock infestation, the amount of clover in the sward and the cost of ammonium nitrate.”

To help farmers decide, Dow has developed a weed impact calculator in association with Kingshay. The calculator demonstrates that a point is reached where controlling docks at the expense of clover can bring significant benefits in milk yield or beef production.

Figures show that the more expensive ammonium nitrate gets and the higher the level of clover, the more valuable clover is in the sward and there is less cost benefit in using a non-clover safe spray, says Mr Connabeer.

Beef farmer Edward Dean, Carlisle, has been working with Mr Connabeer to control docks as part of a project supported by the Grass Right Group.

“With dock levels at 10-25%, they have always been our main weed problem, but I did not realise how much they were costing until I used the weed calculator,” says Mr Dean.

“My ultimate aim is to increase clover leys and maximise fertiliser efficiency as much as possible, so initially I was reluctant to kill off clover. However, I was surprised how small a percentage of docks is needed to give a cost benefit of using a non-clover safe spray and reintroducing clover.”

Mr Dean has used the calculator to decide where this strategy is economically justified. “We have decided to bite the bullet, use a non-clover safe spray as soon as docks are at the correct stage, and reintroduce clover after first or second cut,” he says.

To get maximum control before first-cut silage, April and early May are the best times for dock control, says Mr Connabeer. “Weeds need to be healthy, with good height uniformity throughout.”

Timing is key if you are going to destroy a clover ley, he adds. “When dock levels are above 15% in a clover ley, it is time to use a non-clover safe spray at full rate, but only if weeds are of good quality and at the right growth stage.

“If weeds are not healthy and of different sizes, using a split treatment may be the correct strategy. Most products can be used at half rates to weaken plants in spring and then apply a second treatment in late summer.

“Alternatively, docks can be treated after first cut. This means weeds have been cut down to the same height and will grow back uniformly to maximise weed control efficiency.”

Clover can be stitched back in six weeks after treatment and grass four weeks after, he says.

The key to maintaining a weed-free clover sward is good management. Once a ley has established, maintaining good grass and clover cover and minimising poaching will prevent weeds from spreading. “A good, dense sward is the best herbicide,” adds Mr Connabeer.