Don't Fix What Ain't Broke

By Juanita Polegi, P Ag

Conservation Agrologist

In this the era when products are constantly being touted as "new and improved," some products don't need to change. Take for instance, the theme song from Hockey Night in Canada; the colour of the paint on John Deere equipment; Crown Royal in a velvet bag; and 2,4-D.

Like the others on the list, 2,4-D has been around a long time. In fact, in a weed control book published in 1961, it's stated that 2,4-D first appeared in the literature in 1941 and that by 1944, the USDA was reporting that 2,4-D applied to lawns did a good job of controlling dandelion, plantain and other weeds. For nearly 60 years 2,4-D has been whacking weeds.

If you thumbed through the Weed Control Guide this spring while filling the sprayer, you may have noticed the list of available herbicides continues to grow. In fact, there are over 100 products listed in the index. That makes for a lot of herbicide choices. For many reasons, having a variety of chemistries to choose from for weed control is good. But it's important to keep 2,4-D in your arsenal against weeds, especially for controlling seedlings of winter annuals and weeds such as dandelion.

During the long weekend in May, I left a couple of casseroles in the fridge and hubby on the tractor and then loaded the kids into the car and headed northwest along the Yellowhead. The further along #16 you went, the drier it got but that didn't seem to deter the dandelions. The ditches were pretty yellow the entire route. The dandelions seem to be thriving everywhere - in ditches, lawns, stands of forage and annual crops.

While there are a few in-crop herbicides that will provide control of dandelion, most achieve only suppression or top-growth control. To get the big old granddaddy dandelions, an application of pre-harvest Roundup is pretty effective. But even that treatment can be beefed up by an application of 2,4-D late in the fall, just prior to freeze-up. In the years 1992- 94, Dr. Doug Derksen applied 1.0 l/ac pre-harvest Roundup to lentils. Half the lentils each year received an additional application of 2,4-D late in the fall. In the spring of 1995, dandelion control was rated visually on all treatments from each of the 3 years. The fields that had received both pre-harvest Roundup and a post-harvest application of 2,4-D in each of the 3 years, had greater dandelion control than those that had received only the pre-harvest treatment of Roundup, even up to 3 years after applying the 2,4-D (See Table 1).

2,4-D does have some residue and that must be taken into consideration, especially if the succeeding crop is a broad leaf. Dr. Rick Holm assessed the effect of 2,4-D applied to broadleaf crops. Both soil type and soil zone play a role in the sensitivity of broad leaf crops to 2,4-D. As Table 2 illustrates, canola is not affected by high rates of 2,4-D applied in the fall or low rates in the spring. But at a high rate in the spring, the canola was damaged in the Dark Brown Soil Zone. Lentils are more sensitive to both fall and spring applications of 2,4-D than either canola or peas.

Table 3 illustrates how soil type influences the sensitivity of broad leaf crops to 2,4-D. Crops on Black soils and clay soils are generally less susceptible to the herbicide than those on sandy loam.

With the threat of weed resistance ever looming, it's not likely anyone will apply 2,4-D to the same field year-after-year. However, a study was conducted at Indian Head several years ago to see what would happen in the soil after repeated long term use (35 years) of the herbicide on one field. Although 2,4-D has a short-term soil residual, it is quickly broken down and the study found no build-up of 2,4-D residues over the course of the study. It was also concluded that 2,4-D did not interfere with nutrient cycling or have any adverse effects on the soil microbiological populations.

In the weed control "tool box," there are many tools available to hold the weeds in check. Although 2,4-D is an old product, it still has a place in the top tray of the box. While spraying in the very cool conditions typical of late fall is no one's idea of fun, an application of 2,4-D at that time of year may very well save the need for spending precious dollars on more expensive in-crop herbicides the following spring.

Table 1. Effect of Harvest Treatment in Lentil on Control of Dandelion (Visual Rating), 1995

Dr. Doug Derksen, AAFC, Brandon

Table 2. Fall vs. Spring 2,4-D Effect on Canola

Dr. Rick Holm, University of Saskatchewan

Table 3. Number of Years Yield Reduced by Spring 2,4-D @ 0.28 l/ac (Saskatoon)

Dr. Rick Holm, University of Saskatchewan