Post-emergence field pea harrowing – rotary or tine?

B. Frick, E. Johnson - Scott Research Farm

Problem
Post-emergence harrowing generally results in low selectivity, which is the ratio between weed control and crop injury. Many organic growers have suggested that post-emergence harrowing’s selectivity could be improved with changes in implement design. This study’s objective was to determine if selectivity differed between a Phoenix rotary harrow and a tine harrow.

Background
The rotary harrow is manufactured in western Canada and is designed for granular herbicide incorporation and field leveling under high crop residues. The amount of soil disturbance caused by the rotary harrow is dependent on the angle of the harrow gangs in relation to the direction of travel. Soil disturbance is low when the harrow gangs are set perpendicular to the harrow hitch and increases as the angle approaches parallel. Organic growers commonly use postemergence harrowing for weed control; however it is not a practice commonly used by conventional growers. Fifty-two percent of organic growers surveyed in Saskatchewan used post-emergence harrowing as a weed control practice. There are very few published papers on postemergence harrowing in western Canada. The results of a 12-year study at Indian Head showed that the yield of barley and spring wheat grown under weed-free conditions was not reduced by a single harrow pass conducted at emergence, the 1.5 or 2.5 leaf stage. Kirkland reported that multiple postemergence harrowing passes reduced wild oat panicles and fresh weight in spring wheat in two years out of a three-year study. However, spring wheat yield was improved in only one year of the study. Three to four passes were required in order to obtain a reduction in wild oat fresh weight of 40 to 80%. Studies have shown that field pea can tolerate post-emergence tillage performed with a harrow or rotary hoe. Yield responses from post-emergence tillage in field pea have ranged from 0 to 18%.

Study description
The experiment was conducted on spring wheat stubble in 1998 and 1999 at the Scott Research Farm. Weeds were seeded in the early spring followed by a shallow cultivation to distribute weed seeds and crop residue. Field peas ( Grande) were seeded on May 13, 1998 and May 5, 1999. We used a hoe-opener plot seeder (22 cm row space) equipped with on-row packing. The experimental design was a randomized complete block and the treatment design was a 2x2x2 factorial. Factors included harrow type (rotary, tine), setting (low and high soil disturbance) and number of passes (single, double). Low and high soil disturbance settings are illustrated in Figure 1. Our experiment included two checks -- one untreated, one herbicide treated. Harrow treatments were carried out at field pea’s four - five node stage. The single pass harrowing operation was done in the same direction as the crop rows. In the double pass treatment, the second harrow pass was done in the opposite direction to the first. Harrowing speed was 6 km/hr . Plot size was 4x5 meters and each treatment was replicated four times. Crop burial was visually estimated on a scale of 0-100 per cent immediately after harrowing was done. We collected data on crop and weed density and biomass, and crop yield.

Major findings
The tine harrow buried more crop than the rotary harrow in both years, particularly when harrows were set at high soil disturbance level. The rotary harrow’s high disturbance setting and the tine harrow’s low disturbance setting (tines set at an angle of 45° backwards) buried similar amounts of crop. Post-emergence harrowing had significant effects on field pea density in both years. In both years, the tine harrow’s high disturbance setting reduced field pea density more than its low disturbance setting and both rotary harrow settings (Table 1). The tine harrow’s high disturbance setting resulted in a 25% (1998) and 35% (1999) decline in field pea density. The tine harrow’s low disturbance setting resulted in fewer problems with crop residue plugging than did the high disturbance setting. Residue attached to the bottom of the tines on the high disturbance setting caused more plant uprooting. The rotary harrow is designed to distribute crop residue and this may account for its lack of crop injury. In both years, harrow type had a significant effect on wild mustard density (Table 2). The tine harrow was more effective in reducing wild mustard density in both years. Harrow type had no effect on wild oat density in 1998; however, in 1999 the tine harrow’s high disturbance setting resulted in higher wild oat density and biomass, due to greater crop injury (Table 3). Increased plant covering effects from the tine harrow’s high disturbance setting was more effective in controlling wild mustard density but this was offset by higher crop injury. This agrees with Rasmuusen’s findings – that a flexible chain harrow was a more efficient weed killer than a spring-tine harrow but it also caused more crop damage in field pea and spring wheat. He concluded that similar results could be obtained with all harrow types, if their settings are adjusted to give similar plant covering effects. In 1998, there was no significant increase in crop yield due to post-emergence harrowing. In 1999, the highest yields were obtained with the tine harrow's low disturbance setting and the rotary harrow's high disturbance setting (Table 3). The rotary harrow's low disturbance setting did not improve crop yield since it didn't effectively control broadleaf weeds.

Conclusions
Selectivity did not vary with harrow type. There was opportunity to improve selectivity within both harrow types by adjusting the level of soil disturbance and crop burial caused by the harrow. A high level of soil disturbance and crop burial was counter-productive as significant crop injury offset any improvements in weed control. Since the tine harrow’s low disturbance setting resulted in lower crop injury without sacrificing weed control, producers would be advised to set their harrow tines at a 45° angle backwards. On the rotary harrow, a 45° angle provided both satisfactory weed control and crop tolerance.

Funding
Provided by the Canada-Saskatchewan Agri-Food Innovation Fund

Contact information
Brenda Frick, Ph.D., P.Ag.
Prairie Coordinator
Organic Agriculture Centre of Canada
c/o Department of Plant Sciences
University of Saskatchewan
51 Campus Drive, Saskatoon
Saskatchewan, Canada S7N 5A8
Tel: (306) 966-4975
Fax: (306) 966-5015
Email: brenda.frick@usask.ca


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