Since 1988 the number of resistant weeds identified in Manitoba has increased from two to four (Table 1). These include wild oat and green foxtail which are the province's worst annual grassy weeds. Both species have developed resistance to Group 1 herbicides. Group 1 products are also referred to as ACCase inhibitors and include herbicides registered in Canada such as Hoegrass, Poast, Excel, Puma, Triumph Plus and Select and in U.S. such as Hoelon, Cheyenne and Dakota.Table 1. Occurrence of herbicide resistant weeds in Manitoba.

Species/Herbicide Group	First Report Year	Current Confirmed Cases
Green Foxtail
Group 1	1991	20
Group 3 (Treflan, Edge)	1988	>100
Group 1 & 3	1992	2
Wild Oat
Group 1	1990	>100
Group 8 (Avadex, Avenge)	1989	2
Kochia
Group2 (Refine, Ally)	1988	3

Wild Mustard
Group 2 (Refine, Ally, Muster)	1992	1
Group 4 (21,4-D, MCPA, Dicamba	1991	9

The first documented case of Group 1 resistant wild oat was reported in 1990 in the Swan River area. The number of reported cases now exceeds well over 100. Most Group 1 resistant wild oat occur in fields that had been sprayed year after year with Group 1 herbicides. In some fields resistance occurred with as few as 3 successive applications.

The intent of this presentation is to provide an update of the current research being conducted at the University of Manitoba on Group 1 resistant wild oat. The objectives of the research are to answer to the two following questions:

1. What areas of Manitoba are at risk for Group 1 resistance?

2. How many fields in a high risk area have resistant wild oat?

With respect to the first question, we used the Manitoba Crop Insurance Database to determine what herbicides were applied in Manitoba during the period of 1981 to 1993. The database contains information on producer management practices on all insured fields. Herbicide use information is available on 0.75 of a million fields.

Group 1 herbicides were used on 15% of sprayed fields in 1981 (sprayed fields include all fields that received at least one herbicide application) (Figure 1). In the 1990's, half of Manitoba's insured fields were sprayed each year with a Group 1 herbicide.

Group 1 use was averaged over the 1989 to 1993 period by township (Figure 2). A township describes an area 6 miles by 6 miles. Townships with herbicide use averages above 50%, from 30 to 50% and less than 30% were classified as high, medium and low risk, respectively.

Areas at greatest risk (dark square) are Swan River, Dauphin, Treherne, north of the Red River valley, Killarney and isolated townships north of Brandon.

The majority of the townships are in the medium risk category. On average Group 1 herbicides are used once every two or three years in the medium risk areas. This still constitute a greater frequency than the one year in three recommendation by Manitoba Agriculture.

With respect to the second question, the township surveyed is located in the Treherne area. This township ranked 39th out of 770 townships in terms of Group 1 use from 1981 to 1993. The fields in this townships had an average of 54 % Group 1 use over the 1981-1993 period. This means that more than half of the fields have been sprayed every year with Group 1 herbicides since 1981.

A systematic survey of 30 randomly chosen wheat fields was conducted in the summer of 1993 in this township. The survey consisted of walking up and down a field and stopping at regular intervals (100 paces). At each stop the presence and density of wild oat was recorded and seed was sampled. We also sampled and recorded the location of any suspicious patches that were encountered between the regular stops. A total of 780 samples were collected from the 30 fields. The number of samples per field varied from 2 to 73.

Producers were asked to provide field herbicide histories prior to the initiation of the surveys. The 30 fields had an average Group 1 use of over 63%. In 1993 16 fields were sprayed with Group 1 herbicides; 9 fields received no herbicide application and 5 fields were treated with other graminicides. In no instance did a producer indicate that herbicide resistant weeds pose a problem in these fields.

Seed samples were screened for resistance in petri-dish bioassays with Excel and Poast. Resistant wild oat populations (R) were present in 21 of the 30 surveyed fields. We are uncertain (U) about Populations of wild oat from five fields. Samples from these fields are currently being retested to confirm the presence or absence of resistance. Only four fields positively had no resistant wild oat (s).

The probability of finding Group 1 resistant wild oat was greater in fields with a history of high Group I use compared to fields with low Group I use (Table 2). Only half of the fields sprayed with less than five Group 1 applications in an eleven year period had resistance. However all fields treated with more than eight applications of Group 1 in eleven years had resistance. This demonstrates that reduced frequency of Group 1 herbicides can be an effective tool in reducing the chances of selecting for resistant wild oat.

Table 2. Relation between the number of fields with Resistant, Uncertain and Susceptible wild oat, and Group 1 herbicide use from 1983 to 1993.

Years of Group 1 use from 1983-93	R	U	S
4	1	1	0
5	3	1	2
6	1	0	0
7	5	1	2
8	3	0	0
9	3	0	0
10	0	0	0
11	1	0	0

The probability of finding Group 1 resistant wild oat was also higher in fields sprayed with a Group 1 herbicide in 1993 than where no herbicides were applied (Table 3). The resistant wild oat "stood out" in fields sprayed with Group 1 herbicides because susceptible wild oat populations were eliminated. The susceptible wild oat populations diluted the resistant wild oat populations when no herbicide was applied. Therefore, there is a higher probability that uncontrolled patches of wild oat in a sprayed field are actually resistant Mechanical destruction of these patches could help to limit the spread of resistant weed seed throughout the field.

Table 3. Relation between the number of fields with Resistant, Uncertain and Susceptible wild oat, and herbicide use in 1993.

	Number of Fields
Herbicide in 1993	R	U	S
Group 1	13	2	1
None	4	3	2

CONCLUSIONS:

Wild oat populations resistant to Group 1 products are gaining ground. Jo stop them producers will have to rotate herbicides and use integrated weed management techniques.

Rotating herbicides should be interpreted as using a maximum of one Group 1 herbicide application in 3 years. Long intervals between Group 1 herbicide applications will impede resistance development in wild oat.

Management practices such as crop rotation and delayed seeding dates need to be integrated into an effective weed control system. Herbicide applications should be followed by scouting to evaluate herbicide efficacy. Sketching weed patches on field maps can help to survey the spread of these patches over the years. Mechanical elimination of wild oat patches that escaped control will significantly reduce the chances of spread over the whole field or farm.