INTRODUCTION

There is currently a growing trend in Manitoba towards the adoption of reduced and zero tillage practices. A reduction in tillage has resulted in changes to the way producers farm, requiring better residue management, fertilizer placement, and crop rotations for disease control. Weed management is another area affected by a reduction in tillage, requiring changes to the way producers have traditionally practiced weed control.

The combined effect of a producer's cropping, tillage and herbicide practices determine the weed problems present in the field. The adoption of reduced or zero tillage will require changes to crop rotations and herbicide use to compensate for the loss of tillage as a form of weed control.

Traditional thinking suggests that a reduction in tillage will result in an increase in certain perennial species (Canada thistle, sow thistle, quackgrass), wind-dispersed species (dandelion, foxtail barley) and volunteer crops, but certain summer annual species (green foxtail) may decrease. Recent research, however, does not entirely support this theory. Frick and Thomas (1992) found that although there were small differences among surveyed conventional, reduced and zero tillage fields in Southwestern Ontario, weed communities were, in general, quite similar despite the tillage system used. Likewise, Derksen et al. (1993) concluded that changes in weed communities over a four year study in Saskatchewan were more likely to depend upon species, location, and environment rather than solely on tillage systems.

In order to address the concerns of Manitoba producers interested in adopting and already s zero tillage, a field survey was conducted in 1994 to determine if differences in weed populations occur among zero and conventional tillage systems in Manitoba.

MATERIALS AND METHODS

Approximately 65 zero and 65 conventional tillage fields were surveyed in 1994. Selected zero tillage fields had to be in zero till production for a minimum of years (no tillage in the fall of 1991). Conventional till fields had to have a minimum of two tillage operations in the spring or fall. Zero and conventional till fields were "paired" such that a pair of fields had to be within close proximity of one another (approximately 2 miles) and have similar cropping histories. Wheat and canola fields were chosen for the survey, and both members of a pair had to be sown to the same crop. A maximum of three fields could be selected from the same producer to help ensure that management practices were randomized.

Field sampling was conducted by the Manitoba Weed Supervisors Association and by Manitoba Agriculture. Sampling was conducted in the spring prior to spraying and in the fall after tillage had occurred on conventional fields, but prior to spraying for winter annual weeds (mid-October). The sampling technique was based upon the "inverted W" method proposed by Thomas (1985) which utilized twenty weed counts with 0.5 by 0.5 m2 quadrats. A survey of the field borders was also conducted to determine if there were new weed problems encroaching from the field edges. An additional twenty counts were made along the field borders (3 m from the edge). Perennial species such as Canada thistle and quackgrass were counted as the number of shoots/m2, while annual species were counted as plants/m2.

A questionnaire was sent out to each producer participating in the survey to assess current management practices of zero tillage producers.

DATA ANALYSIS

A preliminary analysis was conducted on twelve of the surveyed fields. Six field pairs were chosen where the only difference between fields was the tillage system. In most of these cases a field was split in two to demonstrate the advantages and disadvantages of zero tillage. The same producer farmed both fields of the pair, thus production management and rotational histories were very similar. These fields were in zero tillage production for an average of four years. Results are presented for the spring survey only. Questionnaire results have not been compiled at the time of paper preparation.

The data from these six field pairs was combined and summarized using frequency, field uniformity, and density measures as described by Thomas (1985). Frequency is defined as the number of fields iii which a species occurred and is expressed as a percentage of the total number of fields. It reflects the geographic extent of the weed in the study area. Field uniformity is the number of quadrats in which a species occurred expressed as a percentage of the total number of quadrats and is an indication of the amount of land in the study area that contains a given species. Density is defined as the average number of plants per square metre. Mean field density (MFD) is obtained by summing the density of each weed species for all fields and dividing by the total number of fields. A relative abundance value was calculated by combining frequency, field uniformity and mean field density (Thomas, 1985). It is used to rank the contribution of individual species to the weed community and assumes that frequency, field uniformity, and mean field density are of equal importance in estimating the abundance of a species.

RESULTS

In general, the results from these twelve fields suggest that weed populations were similar under zero and conventional tillage systems. The total mean weed density per field (calculated by summing the total number of weeds in a field regardless of species) was 103 plants/m2 under zero tillage and 106 plants/m22 under conventional tillage for the in-field survey (Table 1). Zero tillage fields in the border survey had a slightly higher mean weed density, with 98 plants/m2 under zero tillage and 134 plants/m2 under conventional tillage (Table 2).

Regarding individual species, the two tillage systems had similar densities and field uniformities for both the in-field and border surveys. The only exception was wild mustard, which was consistently found at lower levels in the zero tillage fields. The mean field density of wild mustard was 2.7 and 37.3 plants/m22 in zero and conventional till fields, respectively, for the in-field survey, and 1.2 and 30.8 plants/m2 in zero and conventional till fields, respectively, for the border survey. Likewise, field uniformity for wild mustard was 24.2 and 47.5% (in-field survey) and 14.2 and 47.5% (border survey) for zero and conventional tillage systems.

The relative abundance ranking also indicates similar weed populations among tillage systems with the exception of wild mustard, which was consistently ranked lower under zero tillage (Tables 3 and 4).

Green foxtail was found at only slightly lower densities under zero tillage compared with conventional tillage fields (1.8 vs. 2.6 plants/m2, respectively) in the in-field survey (Table 1). In the border survey, green foxtail was present at 2.2 and 5.7 plants/m2 for zero and conventional tillage fields, respectively (Table 2).

There is an indication that zero tillage crop production may support a slightly more diverse weed community, particularly in the field borders. 32 weed species were present in the border survey of zero tillage fields while only 26 species were observed under conventional tillage. Both tillage systems hosted the same number of species for the in-field survey (24 species each). Weeds present in the border survey of zero tillage fields that were not found in conventional tillage fields included bromegrass, poplar, absinth, american dragonhead, ragweed, and broad-leaved plantain. These weeds were generally found at low densities (less than I plant/m2). Bromegrass was ranked twelve on the relative abundance index, and was present at a density of .7 shoots/m 2 in zero tillage fields.

DISCUSSION

Presented results are preliminary and may not be reflective of the larger field sample size. However, preliminary results indicate that weed populations were similar among tillage systems, with the exception of wild mustard. As well, there is an indication that zero till fields may host a more diverse weed community in the held borders. Further analysis is required to substantiate these preliminary results.

The observation that weed populations may be similar among tillage systems could be explained by a compensation in weed management strategies. The loss of tillage as a weed control technique can be compensated for by altering cropping practices and herbicide use. This undoubtedly occurs in Manitoba, as most zero till producers use a spring burnoff treatment in conjunction with fall spraying for winter annual weeds that would not normally be used under conventional tillage. Similar weed populations among tillage systems would indicate that Manitoba zero tillage producers are successful in managing their weed problems and preventing the occurrence of any adverse weed shifts.

REFERENCES

Derksen, D.A., Lafond, G.P., Thomas, A.G., Loeppky, H.A., and Swanton, C.J. 1993. Impact of agronomic practices on weed communities: tillage systems. Weed Sci. 41: 409-417.

Frick, B. and Thomas, A.G. 1992. Weed surveys in different tillage systems in southwestern Ontario field crops. Can. J. Plant Sci. 72: 1337-1347.

Thomas, A.G. 1985. Weed survey used in Saskatchewan for cereal and oilseed crops. Weed Sci. 33: 34-43.

ACKNOWLEDGEMENTS

Funding for this project was provided by the Sustainable Development Innovations Fund and was administered by the Manitoba - North Dakota Zero Tillage Farmer's Association.

Table 1. Mean field density (#1m2) and field uniformity (%) of the most common weeds in the in-field survey.
	Mean Field Density (#/m2)		Field Uniformity (%)
Weed	Zero Tillage	Conventional Tillage	Zero Tillage	Conventional Tillage
Volunteer Canola	33.9	32.9	60.8	64.2
Smartweed	16.4	5.8	32.5	28.3
Wild Oat	13.0	20.1	41.6	50.0
Wild Buckwheat	3.1	8.4	39.2	28.3
Canada thistle	6.3	3.6	30.8	21.7
Stinkweed	7.0	8.4	19.2	19.2
Green Foxtail	1.8	2.6	15.0	28.3
Wild Mustard	2.7	37.3	24.2	47.5
Sheperd'sPurse	2.8	0.8	16.7	5.8
Volunteer Wheat	4.8	1.8	16.7	15.8
Perennial Sow Thistle	1.7	0.7	10.8	12.5
Quackgrass	0.3	0.1	4.2	0.8
Sum of All Weeds (1)	103	106	365	400
Sum of all recorded weeds in each tillage system.

Table 2. Mean field density (#1m2) and field uniformity (%) of the most common weeds in the border survey.
	Mean Field Density (#/m2)		Field Uniformity (%)
Weed	Zero Tillage	Conventional Tillage	Zero Tillage	Conventional Tillage
Volunteer Canola	36.2	31.2	59.2	62.5
Wild Oat	7.9	25.8	49.2	59.2
Stinkweed	13.2	14.9	17.5	14.2
Canada Thistle	5.2	1.1	31.7	14.2
Wild Buckwheat	3.0	2.4	34.2	33.3
Lamb's Quarters	9.5	6.6	23.3	30.0
Quackgrass	3.6	1.5	19.2	1.7
Smartweed	2.0	1.7	17.5	14.2
Green Foxtail	2.2	5.7	20.0	41.7
Perennial Sow Thistle	3.4	0.5	19.2	7.5
Wild Mustard	1.2	30.8	14.2	47.5
Bromegrass	1.7	n/p	16.7	n/p
Sum of All Weeds (1)	98	134	405	395
Sum of all recorded weeds in each tillage system. n/p - Not present.

Table 3. The 15 most abundant weeds found in the in-field survey for each tillage system (ranking based on relative abundance index).
Zero Tillage	Ranking	Conventional Tillage
Volunteer Canola	1	Volunteer Canola
Smartweed	2	Wild Mustard
Wild Oat	3	Wild Oat
Wild Buckwheat	4	Wild Buckwheat
Canada thistle	5	Canada Thistle
Stinkweed	6	Smartweed
Green Foxtail	7	Stinkweed
Wild Mustard	8	Chickweed
Sheperd'sPurse	9	Ball Mustard
Volunteer Wheat	10	Lamb's Quarters
Perennial Sow Thistle	11	Green Foxtail
Quackgrass	12	Perennial Sow Thistle
Dandelion	13	Hemp Nettle
Barnyard Grass	14	Volunteer Wheat
Kochia	15	Redroot Pigweed

Table 4. The 15 most abundant weeds found in the border survey for each tillage system (ranking based on relative abundance index).
Zero Tillage	Ranking	Conventional Tillage
Volunteer Canola	1	Volunteer Canola
Wild Oat	2	Wild Oats
Stinkweed	3	Wild Mustard
Canada Thistle	4	Green Foxtail
Wild Buckwheat	5	Lamb's Quarters
Lamb's Quarters	6	Wild Buckwheat
Quackgrass	7	Stinkweed
Smartweed	8	Canada Thistle
Green Foxtail	9	Ball Mustard
Perennial Sow Thistle	10	Smartweed
Wild Mustard	11	Kochia
Bromegrass	12	Redroot Pigweed
Hemp Nettle	13	Chickweed
Cleavers	14	Volunteer Wheat
Chickweed	15	Hemp Nettle