Bovine solution for eating into a Canada thistle infestation

Cows will avoid eating the prickly weed if they have a choice, 
but keep them in a paddock longer and they’ll control the invader

There’s 10 million square kilometres of free — and highly nutritious — feed spread across North America.

But because it’s Canada thistle, you need a special grazing system to get cattle to eat it.

“You can use managed grazing to keep weeds, including Canada thistle, at tolerable levels and increase forage production,” said Sue De Bruijn, who studied thistle control as a grad student at the University of Alberta and looked at the impact of different grazing regimes with U of A professor, Edward Bork. She presented her findings in a recent Saskatchewan Agriculture webinar.

The study compared continuous grazing to more intensively managed grazing management systems at four locations in central Alberta. In one of these, cattle were moved every day or two after grazing about half the available forage and the pastures rested about four weeks (low-intensity, high-frequency grazing LIHF). In the other intensively managed system cattle grazed about 80 per cent of the available forage before moving. This heavy utilization of forage is not usually recommended, and those pastures needed eight weeks or more to recover.

However, Canada thistle decreased dramatically under this heavy use and rest regime, from 30 to 40 thistle stems per square metre at first to under 10 stems per square metre after one year, to five after Year 2 and then just one or two after Year 3. In the other pastures thistle shoot numbers didn’t change during the study. The LIHF pastures looked better but the numbers weren’t significantly different from continuously grazed pastures.

“This study was done in 2000, 2001 and 2002 — real drought years,” said De Bruijn. “The lack of moisture may have had some effect on the results, but they were very impressive. At one site during a year with little grass and the same cattle were used throughout the study, the cattle ate Canada thistle in the continuous grazing situation, where they had free choice. They didn’t eat as much as when they had no choice, but they had learned they could eat thistle.”

System differences

There were differences in what the cattle ate and the total amount of vegetation removed in the three grazing systems.

With continuous grazing, cattle come back to eat a plant as soon as it begins to regrow, even if it’s just a few centimetres. De Bruijn found cattle in this system ate almost entirely grass and consumed about 2,900 kilograms worth per hectare. With low-intensity, high-frequency grazing (the system that generally gives the highest animal weight gains), they ate about 70 per cent grass, a small amount of forbs, and almost 30 per cent Canada thistle, just over 2,000 kg/ha.

“In that system, they seemed to trample Canada thistle rather than eating it,” says De Bruijn.

During the three or four days the cows were on the high-intensity, low-frequency treatment, grass made up just over half of the forage removed, almost a third was Canada thistle and 10 per cent was forbs, for a total of 4,400 kg/ha of forage consumed.

“After the hard grazing that it takes to make the cattle eat Canada thistle, the pasture looks terrible. After such intense defoliation it needs to be rested, rested, rested. I’d say seven or eight weeks at least.”

The effects on Canada thistle persisted into the following year, when none of the co-operating farmers maintained the controlled grazing systems. A year after the grazing trial ended, the continuously grazed paddock area averaged 18 Canada thistle stems per square metre and the low-intensity grazing had nine. But there were no thistle stems on the heavily grazed pastures.

Total forage production was higher on those areas, too — 4,500 kg/ha, compared to 4,000 for LIHF grazing, and 2,900 kg/ha for the continuously grazed area.

As well, thistles in the high-intensity, low-frequency paddocks were all in the rosette stage. That suggested to De Bruijn that the cattle had eaten them and thistle had to regrow from the roots, compared with the other pastures where thistles were at the normal stage and almost all had fluffy seed heads late in the grazing season.

“I wouldn’t use this system with stockers, where you’re being paid for weight gain,” says De Bruijn. “We used cow-calf pairs so it’s not critical if they stay on an area with little feed for a short period — at most a day.

“They ate just about everything they could out of those fields. And they learned they could eat thistle. It was a bit of work at first, but it got easier each time.”

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