Weed Emergence in Winter Wheat

Crop Staging and Weed Emergence - Week of May 9th, 2005

Crop Stage: The wheat crop has grown quite dramatically in the last 7 days (over 10 cm in height). Of the fields scouted this past week (see Figure 1) he majority of wheat ranged in height from 24-41 cm and was at the first node visible stage of growth (see Table 1).

*For more information on Zadok's scale refer to the OMAF Publication 811,
Agronomy Guide for Field Crops

Figure 1: Map of winter wheat locations scouted
for weeds throughout central and south-western Ontario.

New Weeds Emerging - Week of May 2nd, 2005

Two of the 6 locations received an herbicide application late last week and at this
point no new weeds had emerged at those two locations. Visual control of the weeds present at application is being observed (see below). Weed emergence occurring after these herbicide applications will be monitored over the next 2-3 weeks.

This week significant populations of lady's thumb, wild buckwheat (Figure 2) and prostrate knotweed (Figure 4) began to emerge across several locations. Perennial weeds like field bindweed (Figure 5) and field horsetail (Figure 3) also began to emerge for the first time. Wild carrot has continued to germinate at a rapid pace where present. Purslane speedwell (Figure 6) emerged at one location but is a weed with such a short life cycle that controlling this species is rarely advantageous unless at extremely high densities. Some of the early emerging winter annuals like stinkweed are at heights of 35 cm and approaching the top of the wheat canopy.

Figure 2: Wild buckwheat at the 1 leaf stage

Figure 3: Newly emerged Field horsetail

Figure 4: Prostrate knotweed

Figure 5: Field Bindweed

Figure 6: Purslane Speedwell

* Indicates newly emerged seedlings since the previous week of scouting

Weed Control Observations (2 locations):

dichlorprop/2,4-D (Estaprop, Dichlorprop D, Turboprop): Excellent activity on annual sow-thistle common ragweed, flixweed, dandelion (Figure 7), Canada Fleabane (Figure 8) and broadleaf plantain (Figure 9) has been observed. However some minor crop injury was observed throughout the field.

Figure 7: dichlorprop/2,4-D activity on dandelion

Figure 8: dichlorprop/2,4-D activity on Canada fleabane

Figure 9: dichlorprop/2,4-D activity on broadleaf plantain

bromoxynil/MCPA (Buctril M, Badge, Mextrol): As expected, control of dandelion was poor, but activity on other species like common ragweed, wild buckwheat, and common lamb's-quarters appears excellent thus far. Suppression of newly emerged field bindweed has also been observed (Figure 10).

Figure 10: bromoxynil/MCPA activity on field bindweed

Implications for Weed Management: 2 of the 6 locations have already received herbicide applications. Based on the emergence patterns and density of weeds in the remaining fields, weed control would be justified at another two locations (sites 2 and 5). The other two locations still have low weed populations and likely won't require an herbicide (although those fields will continue to be scouted up to the flag leaf stage). Of interest is the emergence of perennial weeds, most notably Field bindweed, which can present challenges when harvesting the crop and has also been know to lower yield and test weight. Herbicide applications made earlier would have missed this species, and although one cannot expect long term control of this species with any wheat herbicide, they can expect suppression to minimize harvest and yield effects.

The wheat crop is growing at a very fast past and therefore canopy closure is imminent. The necessity for weed control in winter wheat fields should be made immediately as the window for effective control is closing.

Related Links

• OMAF Publication 811 - Agronomy Guide for Field Crops

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