Features Agronomy Weed Management
Use fertilizer to help improve weed control on pastures and hayland

Managing weeds is one of the main challenges in maintaining maximum grazing capacity and forage production on hayland and pastures. Weeds can reduce yield, nutritional value, palatability and marketability of established forage stands. And poisonous weeds can be especially costly for a producer.

“Weeds are often an indicator of forage health and can cause economic losses if not addressed,” says weed specialist, Dan Cole with the Crop Diversification Centre North, a division of Alberta Agriculture, Food and Rural Development (AAFRD) at Edmonton. “Weed infestations are usually more prevalent in stressed or disturbed stands and on low fertility soils.”

Cole says that broadleaf weeds such as ox-eye daisy, dandelion, tall buttercup, yellow toadflax and white cockle are especially difficult to control. Logically, he says any practice that encourages a healthy and competitive forage stand will assist in controlling, suppressing or managing weeds.

Figure 1. Integrated management of dandelion. Average percent control (treatment vs. check plant counts). One year after herbicide and fertilizer application at four locations. Source: Cole, AAFRD.

“We have found that a combination of herbicide and fertilizer can control ox-eye daisy, common tansy, wild caraway, dandelion and yellow toadflax in hayland,” says Cole.

Fertilizer helps forage choke out weeds

Cole says AAFRD has conducted several Integrated Weed Management (IWM) trials over the past decade and found that fertilizing forage stands can be effective in reducing or eliminating weeds. “In fact, two years of spring, surface-applied fertilizer can remove ox-eye daisy from a forage stand,” he says.

In those trials, an annual mid-May application of fertilizer in two consecutive years, consisting of 100kg N, 45kg P₂O₅, 44kg K₂O and 15kg S per hectare resulted in the removal of ox-eye daisy. However, the tall buttercup population was not removed from the forage stand with fertilizer alone, likely because it has large leaves that grow above the forage canopy, allowing it to compete with the forage stand.

Based on those results, Cole initiated trials that looked at how fertilizer and herbicide applications impact on weed control and forage stands. He compared various herbicide applications with and without fertilizer at recommended rates from soil sampling test results, usually 100kg N, 45kg P₂O₅ and 15kg S per hectare in the areas where his trials were conducted.

Looking at ox-eye daisy, he found that when 2,4-D and other broadleaf weed herbicides were applied to a grass-legume stand, legumes were suppressed, which caused an increase in ox-eye daisy growth and plant populations. This was due to the more open forage canopy and less forage competition because some of the legumes in the stand were suppressed or killed. However, fertilizer application counteracted this negative effect by stimulating grass growth and preventing the further spread of ox-eye daisy. Fairly heavy manure applications also helped reduce ox-eye daisy populations in pastures. “I’m quite impressed with how fast alfalfa comes back with the sulfonylureas, like Ally or Refine Extra,” says Cole. “They don’t seem to kill the base or root of the alfalfa, so they come back from the growing point.”

On dandelion, Cole says that several herbicides, including Ally and Ally+2,4-D ester tank-mix, provided good control of seedling and established dandelion for one to three years after application in grass dominated stands. Control was enhanced with the application of fertilizer. “Ally plus surface-applied fertilizer is a cost effective option for grass hayland and pastures,” he says.

White cockle is mainly a problem in leguminous forage and mixed hay crops. It is especially difficult to control in forage stands containing alfalfa or other legumes where few herbicides are registered and when herbicide application may injure the legume. Cole says that since white cockle is controlled by herbicides such as 2,4-D and MCPA, it is important to use an integrated approach to managing
this weed in order to get the best results, since these herbicides may also have an impact on legumes in the forage crop.

In Cole’s research trials, Express herbicide looks promising for white cockle control in grass pastures and hayland, especially when fertilizer to soil test recommendation is surface-applied about the same time as the Express is applied. Sixteen grams per acre of Express was required to reduce white cockle dry weight by 75 percent in the unfertilized plots, but only two grams per acre of Express was required to reduce white cockle dry weight by 87 percent in the fertilized plots.

“The tolerance of the different grass seed crops to Express still needs to be determined,” says Cole. “But preliminary results indicate that tall fescue may be tolerant to Express applications while timothy may not be as tolerant. Express can damage legume crops.”

Surface-applied fertilizer also enhanced the control of tansy when herbicides and mowing was also included. Mowing and spraying the tansy re-growth improved the herbicide control. The combination of Escort in the first year and three years of annual fertilizer application provided the most effective long-term tansy management.

Considering economics, Cole explains that the fertilizer often pays by increasing the yield of the forage as well. In the ox-eye daisy trials, for example, he says that the fertilizer application resulted in an $89 per acre increase in forage yield. “The fertilizer not only paid for itself, but also resulted in better weed control and a cleaner field,” he says.

Research is also being conducted on fall dormant spraying of mixed grass/alfalfa hayland. Cole says Ally worked fairly well on broadleaf weeds, and a minor use permit is being applied for this application.

Cole’s research provides a timely message for producers who may be struggling to rejuvenate pastures that have suffered the effects of drought in the early 2000s. And with the cattle market slowly recovering from border closures, now might be the time to rebuild pasture health by eliminating weeds and improving soil fertility.

It should be pointed out, however, that surface-applied fertilizer on pastures and hayland needs moisture to be effective in enhancing the forage growth.

A final caution is that some of the herbicides used in Cole’s trials are not registered for use in some forage, grass and legume crops. Producers should consult and follow label recommendations to avoid damaging forage crops.