Leaving short pasture heights in fall can lead to higher burdock populations in the following growing season, say University of Wisconsin-Madison researchers.

Weed scientist Mark Renz and Agroecology researcher Marie Schmidt found that retaining a 6- 8” residual height in the fall through the start of the following grazing season can decrease common burdock establishment by an average of 82% compared to shorter residual heights.

The research was done at Arlington and New Glarus in southern Wisconsin. Pasture species common in the Arlington pastures were perennial ryegrass and tall fescue. Meadow fescue, perennial ryegrass, orchardgrass and Kentucky bluegrass made up the Franbrook pastures. In November 2008, the researchers clipped pasture plots to mimic grazing to five residual heights: 2, 4, 6 and 8” plus an unclipped area. Clipping treatments resumed the following May at Franbrook and June at Arlington, and continued through August.

In order for burdock to germinate in pastures, light needs to reach the ground, Renz and Schmidt point out. They measured the amount of light being intercepted by foliage and thus not reaching the ground. In April 2009, before grass or weed growth had started, residual heights affected how much light reached the ground in pastures at both sites.

At Arlington, forages in the 2” and 4” treatments intercepted 41% less light, on average, than forages in the 6”, 8” and unclipped treatments. At Franbrook, forages in the 4” treatment intercepted an average of 34% less light than the other plots. However, most of those differences diminished just before clipping resumed in May at Franbrook and June at Arlington.

Weed densities varied across treatments. At Arlington, reduced burdock density was observed with the 6” and 8” treatments compared to the 2” and 4” treatments. Fewer burdocks emerged in the unclipped plots compared to the plots clipped to 4”. Although similar trends were seen at Franbrook, the differences were not statistically significant.

While these results are specific to common burdock, other biennial pasture weeds such as bull thistle and common mullein may react similarly, say the researchers. More research is needed to determine if these results hold up under actual grazing conditions, they add.