Management Strategies to Control Resistant Pigweed in Carrot Production

By: Kristen Obeid, OMAFRA Weed Management Program Lead – Horticulture and Clarence Swanton, University of Guelph

The Problem

Carrot growers in particular are struggling with resistant pigweed in Ontario fields, a problem that was studied extensively in 2011 and 2012. As far back as 1997, resistance to group 5 herbicides (prometryne) was noted. Then in 1998, resistance to group 2 (rimsulfuron) herbicides was noted. Resistance to group 7 (linuron) herbicides appeared in 1999. There are some weed populations with multiple resistance (i.e. resistance to both group 5 and group 7 herbicides or maybe even to three different herbicide groups).

Different pigweed species are showing resistance to herbicides, including:

• Redroot pigweed (Amaranthus retroflexus, L.)
• Green pigweed (Amaranthus powellii, S. Watson)
• Smooth pigweed (Amaranthus hybridus, L.)
• Common waterhemp (Amaranthus tuberculatus, syn, rudis)

Without effective herbicides, growers are using alternative methods to manage pigweed, including:

1. Wick-weeding with glyphosate (Figure 1).
2. Hand hoeing and removing pigweed from the field.

In 2013, we heard that 95% of producers in Simcoe County have resistant weeds on their farm. The majority of resistance is Group 7 (linuron) resistant pigweed, with some cases having multiple resistance to Group 5 (prometryne).­

Currently, the other herbicides registered for control of pigweed in carrots are:

Herbicide	Group	Pigweed Control	Application Restrictions
carfentrazone POST (Aim)	14	Good	Hooded sprayer between rows.
trifluralin PPI (Treflan, Rival, Bonanza)	3	Good	Do not apply on soil with > 15% organic matter.
metribuzin POST (SENCOR) *resistance may already be present.	5	Good	Applied when the 4th leaf is partially expanded on most carrot plants, followed by a 2nd application, if necessary, when the 5th leaf is partially expanded (about 6 days after the 1st application). Use ONLY on carrots previously treated with a PRE application of linuron. Do NOT apply to carrots grown on muck soils.
s-metolachlor PRE (Dual II Magnum)	15	Good	Applied after planting, prior to carrot and weed emergence. Make ONLY one application per year. Do NOT follow with a postemergence spray later in the season.
s-metolachlor POST (Dual II Magnum)	15	Good	Applied at the 3-5 leaf stage of carrots and before the 2nd leaf stage of weeds.

The best strategy to manage this herbicide resistant pigweed is to use multiple approaches, also known as Integrated Weed Management (IWM):

1. Crop Rotation
The more diverse the rotation the less selection pressure there will be on weeds to develop resistance because timing of tillage and other management practices will vary. Rotation to crops (e.g. onions, beets) where herbicides can be used to control pigweed can be beneficial. See table below for herbicides that can be used on common rotational crops of carrots. These are provided as a general outline only. Please refer to complete label before using these products.

Crop	Herbicide	Group
Onions	Chateau, Goal, Aim	14
	Prowl	3
	Pardner	6
	Frontier	15
Beets	Nortron (soil type restrictions)	16
	Upbeet	2

2. Minimizing the Depth of Tillage (when possible)
Deep tillage continually inverts the soil bringing weed seeds to the surface. If you shallowly till your fields (2-4 inches maximum) you will deplete the seed bank over time. This practice will result in a more even emergence of weeds, allowing better control from herbicide applications.

3. Removing Weeds from the Field – Sanitation
It is important to remove suspected resistant weeds from the field before they go to seed. This will prevent seed return to the soil. Effective removal of plants by hand requires more than just cutting them off. Plants must be uprooted and removed from the field. Paying a crew of people to remove plants in mid-season should be considered a viable solution, even at a relatively high cost. The result of not doing so could be a substantial loss of income in future years. It is best to burn these weeds instead of feeding them to livestock or disposing by other means. It has been shown that fire will kill viable seeds left on the seed head. Whereas, many weed seeds can survive through the digestive track of livestock and could potentially become a problem for the producer taking the manure for fertilizer.

4. Cleaning Equipment – Sanitation
Resistant weed seeds can be easily spread by any and all types of farm equipment entering the field. Take special care to clean all equipment before you enter another field or drive significant distances along roads or highways.

5. Herbicides that are Currently in the Pesticide Registration System
There are several projects currently in the Canadian Minor Use System for potential registration in the future (see Table below). Keep in mind that these products are not registered for these uses.

Herbicide	Group	Pigweed Control	State of Registration
Blazer Ultra aciflurofen	14	Excellent	2013 project. AAFC13-015.
Nortron ethofumesate	16	Fair to Good	2011 project. Efficacy, residue and crop tolerance data required.
Pardner bromoxynil	6	Fair to Good	Submitted 2013.
pyroxasulfone	15	Good to Excellent	Hope to obtain an A priority at the National Minor Use meeting, March 2015.

Overall, remember once you have a resistant weed on your farm, it is there to stay. For example, pigweed can produce on average 100,000 seeds per plant and can remain in the soil for as long as 60 years. It is best to use a combination of the above management techniques (integrated weed management) to help minimize the resistant seed return to the soil each year.