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Controlling Canada thistle in pastures

Canada thistles in pastures don’t just look ugly, they cause economic harm with yield losses approaching nearly two to one. That’s two pounds of desirable forage biomass lost for every pound of thistle biomass.

The good news is that you will gain the forage back if you remove thistles, says Dr. Edward Bork, professor of rangeland ecology and management at the University of Alberta, Edmonton.

“A bundling approach including fertility with a judicious herbicide regime coupled with rotational grazing in an integrated manner will get this weed and probably lots of others down to manageable levels over the long term,” he says.

Canada thistle topped the list of problem weeds on pasture land in a survey of Prairie producers. Its notorious prickles can render otherwise grazeable land useless. The deep, creeping roots send new shoots popping up along lengths that extend up to 20 feet, making the weed difficult to control once it takes hold.

Bork and his students put Canada thistle to the test in perennial pastures to derive recommendations that avoid having to rip out the forage stand.

Field studies were conducted in co-operation with Alberta producers who provided thistle-infested tame grass and grass-legume pastures as well as cattle for grazing as needed.

Broadcast applications of 29-13-3-4 fertilizer at the rate of 335 pounds per acre (lbs./ac.) in each of three years increased productivity of desirable forage by an average of 76 per cent. Unfortunately, thistle productivity also increased by 26 per cent, on average.

“So, if you have lots of thistle, and are going to fertilize and do nothing else, you might make the problem worse and be wasting your valuable fertilizer,” Bork explains.

The fertilizer rate, which works out to 100 pounds actual nitrogen per acre, was chosen to remove variable soil fertility as a recovery factor for desirable forages during the study. In practice, the rate may be considered quite high and could be scaled back, especially after the first year, because the big boost in forage production came in the first year.

Four broadleaf herbicides were compared for effectiveness in removal of thistles from permanent pasture: Grazon, Dyvel DS, Lontrel and 2,4-D. Each was applied singularly on pasture plots in early to mid-July. Two months later, all products had reduced the thistle population compared to the unsprayed checks. Two years later, all sites that had received herbicide treatments still had lower thistle densities compared to the checks. The lowest thistle densities were in areas sprayed with Grazon and Lontrel.

This didn’t come as a surprise because Grazon and Lontrel are highly translocated throughout the plant, leading to effective control of roots and shoots. Additionally, both are bioactives with residual properties in the soil and, therefore, capable of providing longer-term control of thistle plants as they grow, Bork explains.

The recommended time for application is when the majority of thistles are at the bud or early-flowering stage. This is when transport of herbicide to the roots will be most efficient and plant energy reserves will be low, weakening its ability to rebound and reducing its vigour. Control will be most effective when there is adequate leaf area to absorb the herbicide.

Though none of the herbicides require removal of beef cattle from pastures, some do have restrictions on removal of animals from treated fields prior to slaughter and the restrictions are tighter for dairy cattle immediately following treatment, Bork cautions, advising producers to follow label recommendations.

“As for forage response, we were definitely getting forage back by removing the weed,” Bork confirms. Without fertilization, the herbicide treatments resulted in an average 36 per cent increase in forage production, which translated into 985 lbs./ac. of additional forage. Forage production on fertilized sites was highest and increased by 1,234 lbs./ac. following herbicide treatment.

A strategy known as weed-and-feed takes advantage of the strong synergies detected between fertilization and herbicides to improve weed suppression in the long term, Bork adds. Weakening the weed population with herbicide allows the grasses to get the lion’s share of the benefit from fertilizer and compete rigorously to further suppress the weed.

Weed removal by grazing

If carefully managed, rotational grazing may, in itself, be enough to bring a thistle infestation under control. It is certainly the key to maintaining effective thistle control.

Cattle are selective grazers by nature and their preferences for consuming certain plants and avoiding others will cause changes in the composition of species within a pasture. Grazing only desirable forages around weeds weakens and reduces the competitiveness of the desired species, leaving more light, water and nutrients for the weeds.

Bork’s team looked at how Canada thistle responds to the intensity and frequency of defoliation of surrounding plants. Four grazing strategies were implemented on fertilized and unfertilized sites and cumulative grass production was measured for three years.

One part of the study involved clipping forages around thistles to isolate the effect of selective grazing.

Continuous clipping down to a height of approximately one inch every 14 days mimicked maximum stress on forages and resulted in the lowest grass production on fertilized and unfertilized pastures.

Low-intensity/high-frequency (LIHF) sites clipped to a height of approximately four inches every two weeks resulted in the second-lowest grass production.

High-intensity/low-frequency (HILF) sites were clipped to approximately one inch every six weeks and provided the second-highest grass production. This showed that rest is critical to rebuilding forage leaf area, root mass and stems.

The highest forage biomass came from fertilized and unfertilized plots in the deferred grazing sites, which were left to grow for the entire summer.

“These findings demonstrate the potential of deferred grazing and even changes in the intensity and frequency of defoliation to maximize grazing opportunities,” Bork says.

Thistle response to defoliation of surrounding plants was the exact reverse. Thistle biomass was highest in the continuous system, followed by the LIHF, HILF and deferred systems, suggesting strong competition between the weed population and adjacent forage.

The second part of the study compared HILF, LIHF and continuous systems grazed by cattle at four locations for three years.

Thistle density remained stable in excess of 32 stems per square metre when cattle had repeated access to plants in the continuous system.

LIHF grazing was managed to achieve 40 per cent utilization with two weeks of rest between grazing periods. It provided limited thistle control with 24 thistle stems per square metre by the end of the third year.

Cattle in the HILF system were allowed to graze 70 to 80 per cent of the forage, which took about five days, and the pasture was then rested for six weeks. This provided the best thistle suppression with fewer than two thistle plants per square metre remaining after three years.

Bork says the HILF system was most effective because the recovery period was sufficient for the grasses to remain competitive and cattle didn’t selectively graze around thistles to the same extent as in the other systems. In fact, they actually removed thistle by consuming it at a rate of 1,397 lbs./ac.!

Cattle didn’t reject thistles under HILF grazing because periodic defoliation kept most of them at the early growth stage (rosette) when they are most palatable and nutritious, with
crude protein content of 18.6 per cent and total digestible nutrients of 83 per cent.

They consumed some thistle in the LIHF system, but not nearly enough to provide effective control.

Cattle didn’t touch the thistles in the continuous grazing system, letting them advance to the late-flowering-to-fluff stage. By this time, the weed has more stem, is pricklier and becomes harder for animals to eat and digest than young thistles.

The final question was to determine whether superior thistle control achieved with the HILF system had provided only temporary control of the shoots, or had controlled the extensive root system as well.

“A year after producers had reverted back to continuous grazing, the thistles had not recovered on the HILF pastures, suggesting that control was achieved above and below ground,” Bork says. “The thistle population was struggling to recover and a big increase in total forage productivity was still evident. This is a rare win-win situation because the greatest thistle control coincided with the grazing system that gave the best production and best utilization of thistle.”

For more information or to receive an electronic version of the booklet, How to Win the War on Canada Thistle, contact Bork at 403-492-3843 or [email protected].

— Debbie Furber is a field editor for Canadian Cattlemen at Tisdale, Sask. This article appeared in the May 2013 issue.

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