Crop Report – Week of September 18th, 2023

Three tips for weed management this fall.

Reflecting on some of the weed control issues that came up this past spring, below are three key items where management activities this fall can significantly improve control and/or financial return next season.

1. Fall management provides better dandelion control.

In the fall, resource allocation of perennial weeds shifts and carbohydrates move towards the root system, building root mass for overwintering. When dandelions are managed in the fall, whether it be by tillage or herbicides, resource allocation to the roots is disrupted and control is significantly better when compared to management in the spring (Table 1). A better seed bed is also established when dandelions are managed in the fall since all the vegetative growth has been desiccated by planting time (Figure 1).

Figure 1. The headlands of this field received a fall application to control dandelions, making for a nice seed bed to plant into. The rest of the field had no fall management, and the dandelion pressure is significant and will take much longer to control now.

Table 1. Control of dandelions at four different herbicide application timings (Adapted from Franssen and Kells, 2007).

Dandelion	Late fall	Early fall	Early spring	Late spring
Control (%)	80%	74%	65%	55%

When using glyphosate to manage dandelions, if they are larger than 6” (15 cm) in diameter, a higher rate is required according to the product label. Efficacy work conducted by the University of Guelph and presented in Table 2, is consistent with the labelled guidance.

Table 2. Comparing the average and range in control of dandelion over multiple trials evaluated at eight weeks after the application of glyphosate at two rates.

Product	Rate	Control (%)	Range (%)	# of Trials
Glyphosate (540 g/L)	0.67 L/acre	65	54-98	8
Glyphosate (540 g/L)	1.34 L/acre	83	65-95	8

2) Consider fall management in winter wheat if Canada fleabane is present.

Although winter wheat yield loss from weed competition is typically small, there are exceptions. High densities of weeds that emerge with, or shortly after the crop will have the most impact on yield loss. A recent study conducted by the University of Guelph (Ridgetown Campus) found that Canada fleabane interference reduced winter wheat yield by as much as 27%. Managing Canada fleabane in the fall, improved winter wheat yields by 10-15% when compared to waiting until the spring to manage Canada fleabane (Table 3).

Table 3. Control of Canada fleabane at different application timings and impact on the yield of winter wheat (Adapted from Soltani et al., 2023).

Product	Application Timing	Canada fleabane control (%)	Winter wheat yield (bu/acre)
No weed control	–	0	45
Infinity	Fall	83	59
	Fall + Spring	100	61
	Spring	99	53
Pixxaro	Fall	97	61
	Fall + Spring	100	57
	Spring	100	52

A complete list of herbicide options that are labelled for application to winter wheat in the fall can be found at the Ontario Crop Protection Hub.

3. When killing off an old stand of alfalfa with herbicides, make sure that the two-week weather forecast after application has warm air temperatures.

A study conducted at the University of Guelph (Elora Research Station) demonstrated that warm air temperatures after herbicide application, quantified by growing degree days, appeared to negatively effect the control of alfalfa. Since alfalfa has a base growing temperature of 5°C, when air temperatures following herbicide application rarely exceeded 5°C, alfalfa growth was stagnant, thereby reducing herbicide uptake and movement within the plant, and leading to unacceptable control (Table 4).

Table 4. Alfalfa control as affected by growing degree day accumulation in the two weeks following application.

Treatment	Application Date	Growing Degree Days (14 days after application)	Alfalfa Control (%)
Glyphosate + 2,4-D ester	Oct 3	84	98
Glyphosate + 2,4-D ester	Oct 30	9	30

Weather Data – Weekly September 11 – 17, 2023

Location	Year	Highest Temp (°C)	Lowest Temp (°C)	Rain (mm)	Rain (mm) April 1st	GDD 0C April 1st	GDD 5C April 1st	CHU May 1st
Harrow	2023	23.0	7.4	6.5	587	2867	2033	3134
	2022	27.2	10.6	25.1	412	3052	2229	3467
	10 YR Norm (11 – 20)	23.9	10.1	23.2	527	3094	2216	3527
Ridgetown	2023	24.0	5.9	8.5	513	2736	1910	2937
	2022	27.8	9.9	8.0	285	2900	2085	3220
	10 YR Avg. (2011-20)	23.9	8.4	21.8	479	2918	2045	3285
London	2023	22.6	5.6	16.3	561	2716	1892	2926
	2022	27.2	6.5	15.2	333	2820	2015	3136
	10 YR Avg. (2011-20)	23.4	8.9	23.3	505	2888	2020	3258
Brantford	2023	24.3	4.2	1.1	446	2696	1870	2891
	2022	27.3	6.2	23.3	311	2833	2017	3075
Welland	2023	24.3	6.1	1.4	507	2756	1924	2982
	2022	25.5	7.4	15.3	380	2885	2060	3223
	10 YR Avg. (2011-20)	24.1	9.2	16.8	447	2907	2038	3284
Elora	2023	22.9	4.4	33.9	478	2498	1689	2653
	2022	26.5	3.7	2.7	256	2605	1801	2827
	10 YR Avg. (2011-20)	22.5	7.3	21.7	498	2610	1754	2904
Mount Forest	2023	21.5	4.7	11.1	417	2492	1688	2656
	2022	25.3	4.5	5.0	379	2596	1799	2870
	10 YR Avg. (2011-20)	22.1	7.9	19.6	508	2601	1754	2930
Peterborough	2023	25.1	1.5	19.6	430	2561	1739	2686
	2022	24.1	1.8	18.6	342	2601	1783	2824
	10 YR Avg. (2011-20)	23.8	5.0	13.2	440	2637	1780	2889
Kemptville	2023	23.5	5.5	6.4	516	2718	1902	2895
	2022	24.6	6.0	32.3	488	2761	1924	3041
	10 YR Avg. (2011-20)	23.0	6.4	19.7	497	2772	1917	3094
Earlton	2023	22.0	4.2	20.6	408	2317	1548	2459
	2022	20.6	3.4	60.1	412	2333	1564	2568
	10 YR Avg. (2011-20)	20.6	5.4	24.2	448	2262	1490	2538
Sudbury	2023	21.9	6.5	14.5	530	2356	1588	2527
	2022	22.6	0.0	27.9	347	2394	1617	2695
	10 YR Avg. (2011-20)	20.9	6.3	31.3	483	2427	1633	2753
Thunder Bay	2023	21.1	-0.6	4.9	305	2154	1407	2320
	2022	23.6	0.7	78.9	556	2129	1388	2326
	10 YR Avg. (2011-20)	18.7	4.3	17.7	435	2177	1393	2389
Fort Frances	2023	18.9	-3.1	1.1	312	2310	1556	2508
	2022	25.6	2.1	46.7	622	2217	1478	2491
	10 YR Avg. (2011-20)	20.3	3.9	19.7	438	2303	1501	2540