An assessment of the concept of early time of weed removal in field pea using natural weed populations

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1 An assessment of the concept of early time of weed removal in field pea using natural weed populations W. E. May 1, G. P. Lafond 1,7, E. N. Johnson 2, T. Hogg 3, A. M. Johnston 4, B. Nybo 5, N. Harker 6, and G. Clayton 6 1 Indian Head Research Farm, Box 760, Indian Head, Saskatchewan, Canada S0G 2K0; 2 Scott Research Farm, Box 10, Scott, Saskatchewan, Canada S0K 4A0; 3 Canada Saskatchewan Irrigation Diversification Centre, Box 700, Outlook, Saskatchewan, Canada S0L 2N0; 4 Potash and Phosphate Institute of Canada, Pinehouse Dr, Saskatoon, Saskatchewan, Canada S7K 5K2; 5 Wheatland Conservation Area Inc. Box 2015, Swift Current, Saskatchewan, Canada S9H 4M7; 6 Lacombe Research Centre, 6000 C&E Trail, Lacombe, Alberta, Canada T4L1W1. Received 17 June 2002, accepted 10 October May, W. E., Lafond, G. P., Johnson, E. N., Hogg, T., Johnston, A. M., Nybo, B., Harker, N. and Clayton, G An assessment of the concept of early time of weed removal in field pea using natural weed populations. Can. J. Plant Sci. 83: The economic and agronomic benefits of including annual grain legumes in cropping systems has resulted in a large increase in field pea production in Western Canada. Concerns have been raised about the large year-to-year variability in field pea grain yields and recent attention has focussed on weed management. A study was conducted to quantify the effects of time of post-emergence herbicide application to normal leafed and semi-leafless field pea types under natural weed populations and conservation tillage practices. Odyssey TM herbicide (50:50 imazethapyr:imazamox) was applied either 1, 2 or 3 wk after crop emergence with an untreated check included. The study was conducted at seven locations across the major agro-ecological zones of Saskatchewan in 1999 and The results of this study support the merits of early herbicide application 1 to 2 wk after crop emergence. Benefits of early time of weed removal occurred in 7 of 14 sites-years. There was no yield penalty recorded from early applications of herbicide, but some yield penalty was observed with later applications at certain locations. Spraying early rather than late minimizes the risks associated with delayed applications resulting from adverse weather conditions and a possible decrease in crop tolerance as was observed at the Swift Current site. Results support the early removal of weeds from field pea crops to optimize seed yield. Key words: Crop-weed interference, Pisum sativum L., weed management, timing of herbicide application May, W. E., Lafond, G. P., Johnson, E. N., Hogg, T., Johnston, A. M., Nybo, B., Harker, N. et Clayton, G Évaluation du désherbage hâtif dans les cultures de pois de plein champ avec peuplement naturel d adventices. Can. J. Plant Sci. 83: Les avantages agronomiques et économiques associés à l inclusion de légumineuses à graine annuelles aux systèmes culturaux a donné lieu à une importante hausse de la production de pois de grande culture dans l Ouest canadien. Néanmoins, la forte variation annuelle du rendement de cette culture inquiète, si bien qu on s est récemment interrogé sur la lutte contre les mauvaises herbes. Les auteurs ont entrepris une étude afin d établir quelle incidence l application d un herbicide après la levée aurait sur les variétés de pois normales et semi-aphylles en présence d une population naturelle d adventices et avec des pratiques de conservation du sol. Pour cela, ils ont appliqué de l herbicide Odyssey MC (50:50 imazéthapyr:imazamox) 1, 2 ou 3 semaines après la levée, en laissant une parcelle sans traitement comme témoin. L étude s est déroulée à sept endroits dans les principales zones agro-écologiques de la Saskatchewan, en 1999 et Les résultats confirment l utilité d une application hâtive de l herbicide 1 ou 2 semaines après la levée. Les avantages d une telle pratique ont été observés 7 années-site sur 14. Pareille pratique ne réduit pas le rendement alors qu un désherbage chimique plus tardif a l effet inverse à certains endroits. En pulvérisant rapidement au lieu d attendre, on atténue les risques associés à une application tardive du produit par mauvais temps et la possibilité d une moins bonne tolérance de la culture, comme cela s est produit au site de Swift Current. Les résultats montrent qu il est préférable de supprimer rapidement les adventices dans les cultures de pois afin d optimiser le rendement de ces dernières. Mots clés: Interférence entre culture et adventices, Pisum sativum L., moment du désherbage chimique Over the past 10 years, there has been a large increase in the production of field pea in Western Canada with 1.3 million ha seeded in 2001 (Statistics Canada 2001). This increase can be attributed to the economic benefits derived when including annual legumes in a cropping system (Zentner et al. 2002), to its rotational benefits (Stevenson and Van Kessel 1996; Miller et al. 2002) and to the yield increases with conservation tillage (Lafond et al. 1992). More recently it has been shown that including annual legumes in a 7 To whom correspondence should be addressed. 423 cropping system combined with conservation tillage can potentially increase the rate of carbon sequestration in the soil thereby providing some important environmental benefits (Fu 2000). Concerns have been documented over the large year-toyear variability observed with field pea yields (Harker et al. 2001). An assessment of yield losses due to weeds in Central Alberta revealed that 67% of the field pea fields sur- Abbreviations: FPT, field pea type; TWR, time of weed removal

2 424 CANADIAN JOURNAL OF PLANT SCIENCE veyed experienced yield losses due to weeds versus 40% for canola and 27% for barley (Harker 2001). It is well known that early emerging weeds have the largest impact in reducing crop yields as demonstrated with wild oats in wheat and barley (Peters 1984; O Donovan et al. 1985). Recent work by Harker et al. (2001), using small hand-weeded plots to maintain weed-free conditions in commercial fields for the remainder of the growing season, showed that early time of weed removal was critical for protecting field pea yields. In fact, the onset of yield loss occurred from 1 to 2 wk after pea emergence with yield losses averaging 45 kg ha 1 d 1 for the next 2 to 3 wk period, depending on the weed densities present. With the introduction of new post-emergence herbicides with good crop tolerance at very early crop stages (Blackshaw 1998), it is possible to test the concept of early time of weed removal in field pea as discussed by Harker et al. (2001). Producers are interested in knowing whether the concept of early time of weed removal is feasible when only one herbicide application is used for economic reasons. The objective of this study was to evaluate a single application of a post-emergence herbicide 1, 2 and 3 wk after field pea emergence using a semi-leafless and a normalleafed cultivar at various locations in Saskatchewan using natural weed populations. MATERIALS AND METHODS Field studies were carried out at seven locations in Saskatchewan across the major agro-ecological zones of the province. The sites were located near Swift Current in the Brown soil zone and near Outlook and Scott in the Dark Brown soil zone. The sites in the thin-black soil zone were located near Indian Head and Redvers and in the Black soil zone at Canora and Melfort. Dryland production techniques were used at all sites except for Outlook, which was irrigated. Conservation tillage practises that involved direct seeding into cereal stubble were employed at all locations except Outlook where a conventional tillage system was used. A description of the soil associations and textures is provided for each location in Table 1. The field studies consisted of two factors arranged in a randomized complete block design with four replicates. The factors were field pea type (FPT) and time of weed removal (TWR). The field pea types consisted of a semi-leafless cultivar (cv. Carneval or Swing depending on the year) and a normal-leafed cultivar (cv. Grande). The second factor consisted of three times of weed removal with herbicide; 1, 2 and 3 wk after crop emergence. A no-herbicide check was also included. The post-emergence herbicide used to test the concept of early time of weed removal was imazamox/imazethapyr (Odyssey TM ) at a rate of 30 g a.i. ha 1. This herbicide has some medium-term residual activity, which can control some later weed flushes. At Redvers in 1999, a combination of metribuzin (Sencor TM ) at a rate of 114 g a.e. ha 1 and MCPA at a rate of 141 g a.e. ha 1 was used. Emergence was deemed to have occurred when distinct rows of field pea were visible. Plot sizes varied with location and ranged from 10 to 100 m 2. All plots were sprayed with glyphosate either just prior to or immediately after seeding except at Outlook where the plots were cultivated just prior to seeding. Glyphosate rates varied from 440 to 880 g a.e. ha 1 depending on the weed species and densities present. Fertilizer phosphorus was applied at a rate of 20 kg P 2 O 5 ha 1 using monoammonium phosphate either seed-placed or side-banded, depending on the seeder used at the various locations. Granular peat inoculant (Soil Implant TM ) was applied with the seed at a rate of 5.6 kg ha 1 at all locations in both years except for Scott in 1999, where a seed-applied peat powder (LiphaTech TM ) was used at a rate of 4 g kg 1 of seed. Other pertinent agronomic information is provided in Table 1. Field pea populations were determined by counting plants in three 1-m row at 3 4 wk after crop emergence and expressed as plants m 2. Grain yield was determined for the entire plot using a clean weight basis corrected to 14.5% moisture and expressed as kg ha 1. A subsample was kept to determine 1000-seed weight expressed in grams. Weed densities were determined just prior to the in-crop herbicide application by counting the number of broadleaf and grassy weeds in five 0.25-m 2 quadrats. The weed counts on the check plots were recorded just prior to the last herbicide application, which was 3 wk after crop emergence. Data were analysed using the Proc GLM procedure in SAS (Littel et al. 1996). Treatment means were compared using a protected LSD. Linear and quadratic contrasts for response to weed removal were used after removing the data for the check treatment. Data were analysed separately for each year and location because of the confounding effects due to differences in weed densities and crop tolerance among the locations and years. RESULTS Swift Current In general, moisture is usually the greatest limiting factor during the growing season. However, for the 2 yr of the study, above-average precipitation and below-normal temperatures were observed (Table 2). The higher precipitation was reflected in the overall yields obtained (Table 3). Plant and weed densities were similar between both field pea types (FPT) in both years except for slightly higher grassy weed numbers with the normal-leaf type in It should be noted that the differences in weediness are due to the non-random distribution of weeds within a field and not FPT. Grain yield was highest for the semi-leafless type in both years. There was an interaction observed between FPT and TWR for seed weight. In 1999, an increase in seed weight was observed as TWR was delayed in the normal-leafed pea type, but not the semi-leafless type. The opposite was observed in A significant effect of TWR on grain yield was observed in both years. As TWR was delayed, a linear decrease in yield was observed. This decrease was observed even though the density of weeds was low and did not increase appreciably with TWR. The yield of the check (no herbicide) was higher than the later herbicide applications suggesting that the reduction in

3 MAY ET AL. EARLY WEED REMOVAL IN FIELD PEA 425 Table 1. Summary of pertinent agronomic information for each location in 1999 and 2000 Indian Swift Head Current Melfort Outlook Redvers Scott Canora Fox Valley Loam Soil Association Indian Head (1999) Swinton Clay Melfort Silty Bradwell Oxbow Canora and Texture Heavy Clay Loam (2000) Clay Loam Sandy Loam Clay Loam Scott Loam Light Loam 1999 Seeding date 26 May 4 May 27 May 7 May 9 June 18 May 26 May Seeding rate Carneval (kg ha 1 ) 208 NA z Grande (kg ha 1 ) 216 NA Emergence y (d) Row width (cm) Harvest date 18 Sept. NA 23 Sept. 30 Aug. 22 Sept. 4 Sept. NA 2000 Seeding date 28 Apr. 8 May 9 May 8 May 3 May 4 May 8 May Seeding rate Swing (kg ha 1) Grande (kg ha 1 ) Emergence (d) Row width (cm) Harvest date 23 Aug. 18 Aug. 30 Aug. 7 Sept. 16 Aug. 18 Aug. 12 Sept. z NA, information not available. y Emergence represents the time from seeding until the field pea rows are visible. Table 2. Mean monthly temperatures C and total monthly precipitation for each site year and long-term averages Mean Temperatures ( C) Monthly Precipitation (mm) Location Year April May June July August April May June July August Swift Current Long-term Average Outlook Long-term Average Scott Long-term Average Indian Head Long-term Average Redvers z Melfort Long-term Average Canora z z Long-term weather data are not available for those sites. Redvers would be very similar to Indian Head and Canora would be similar to Melfort. yield can be attributed to a loss in crop tolerance with the later applications of the herbicides. Outlook Temperatures were similar to the long-term average with monthly precipitation varying between years and with the long-term average (Table 2). The precipitation in July was 178% of normal for 1999 and 162% of normal for When combined with the irrigation events, this would have resulted in increased disease pressure possibly contributing to the lower reported yields (Table 4) than those observed at the dryland site at Swift Current (Table 2). Crop densities were different between the FPT in 1999 but not 2000 (Table 4). Weed densities were similar between the FPT and between the dates for TWR. Seed weight decreased with increased time between crop emergence and herbicide application in In 2000, there was an FPT TWR interaction for seed weight. There was an increase in seed weight with a delay in TWR for the semileafless FPT but no difference for the normal-leafed FPT. In 1999, there was a decrease in yield with delayed TWR and the decrease was greater between the second and third TWR than between the first and second TWR as indicated

4 426 CANADIAN JOURNAL OF PLANT SCIENCE Table 3. Time of weed removal in field pea in 1999 and 2000 at Swift Current, SK Plant 1000 Broadleaf Grassy Plant 1000 seed Broadleaf Grassy Yield density seed weight weeds weeds Yield density weight weeds weeds (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) Semi-leafless 5178a 90a 38a 87a 4844a 79a 13a 17b Leafed 4609b 91a 1.9a 72a 3902b 81a 11a 27a Weed removal Semi- Semileafless Leafed leafless Leafed 1 wk 5206a 84a 224a 234b 2b 74a 4577a 82a 203b 187a 12a 14b 2 wk 4945a 93a 230a 235b 3b 74a 4335ab 74a 200b 190a 12a 23a 3 wk 4350b 94a 227a 240b 1b 89a 3894b 80a 218a 191a 12a 26a Weedy check 5071a 91a 224a 260a a 4688a 85a 212a 187a 12a 27a Linear ** * NS ** NS NS * NS ** NS NS ** Quadratic NS NS NS NS * NS NS * ** NS NS NS CV Table 4. Time of weed removal in field pea in 1999 and 2000 at Outlook, SK Plant 1000 Broadleaf Grassy Plant 1000 seed Broadleaf Grassy Yield density seed weight weeds Weeds Yield density weight weeds Weeds (kg ha 1 ) (# m 2 ) (g) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (g) (# m 2 ) (# m 2 ) Semi-leafless 2729b 56a 212a 56a 21a 86a 115a 15a Leafed 2986a 45b 215a 59a 22a 84a 107a 17a Weed removal Semi- Semileafless Leafed leafless Leafed 1 wk 3523a 49a 239a 57a 16b 5178a 3631a 85a 228b 203a 144a 12a 2 wk 3325a 50a 212b 47a 30a 5481a 4279a 84a 238ab 205a 90a 7a 3 wk 2352b 50a 206b 67a 22ab 4634a 3294a 86a 248a 203a 103a 28a Weedy check 2230b 54a 197b 59a 19b 2752b 3490a 85a 214c 206a 110a 15a Linear ** NS ** NS NS NS NS NS ** NS NS NS Quadratic * NS NS 0.08 ** NS NS NS NS NS NS NS CV

5 MAY ET AL. EARLY WEED REMOVAL IN FIELD PEA 427 by the significant quadratic response. Delaying the herbicide application until 3 wk after emergence resulted in yields that were similar to the weedy check. The potential yield benefits from the herbicide were completely negated by the delay in application. In 2000, a FPT TWR interaction was observed for yield. With the normal-leafed FPT, there was no effect due to herbicide application unlike the semi-leafless FPT, where an improvement in yield was observed at all three times of weed removal. The taller normal-leafed FPT may be providing more competition than the shorter semi-leafless type. The benefits of early TWR were observed in both years. Scott Monthly temperatures during the growing season were similar to the long-term average but precipitation was higher than normal in both years (Table 2). Crop densities were similar in 1999 and lower for the semi-leafless FPT in 2000 (Table 5). Seed weight was lower in the normal-leafed FPT in Grassy weed densities were higher than the broadleaf weed densities in both years. Grain yield decreased with later TWR in both years, but with some important differences. A linear decrease was noted in 2000 and a quadratic response in In 1999, there was a yield benefit to delaying the herbicide application till 2 wk after crop emergence. The beneficial effects of herbicide application were apparent in both years. Similar weed densities were observed between Scott and Swift Current in 1999 and yet the effects of early TWR on crop yields were more pronounced at Scott. This could be due to greater weed biomass at Scott due to the earlier emergence of the weeds when compared to Swift Current. Indian Head Above-average precipitation was experienced during the growing season in both years (Table 2) resulting in less than ideal soil conditions for field pea growth due to some intermittent waterlogging conditions during the growing season. Monthly temperatures were below normal in both years. Crop densities were the same in 1999 for both FPT and slightly lower for the semi-leafless FPT in 2000 (Table 6). Grassy and broadleaved weed densities were similar for both FPT in both years. Grain yield was higher for the normal-leafed than the semi-leafless FPT in 2000 yields, but similar between FTP in A FPT by TWR interaction for seed weight was observed in both years. There was no effect of TWR on seed weight in the leafed FPT in 1999 whereas an increase in seed weight with delayed TWR was observed for the semi-leafless FPT. In 2000, seed weight declined with delayed TWR in the normal-leafed FPT while the opposite result occurred for the semi-leafless FPT. The latter observation was also noted in The lack of a yield response to the herbicide application, regardless of the timing, was likely due to the low weed densities. Redvers Much higher than normal growing season precipitation resulted in less than ideal growing conditions for field pea due to increased disease pressure (Table 2). Monthly temperatures would appear to be higher than normal, assuming that the normals for that site would be similar to those reported for Indian Head. Crop densities for both FPT were the same in both years, albeit lower in 1999 than 2000 (Table 7). The weed densities were the same between FTP in both years but overall, higher weed densities were recorded in 1999 than Seed weights were greater for the semi-leafless FPT in Time of weed removal in 1999 had no significant effect on grain yield and seed weight while in 2000, a delay in TWR resulted in a decrease in grain yield even though the weed densities were lower than those reported in In 1999, although there was a large increase in weed numbers from the 1st to the 2nd week after emergence, the later emerging weeds would have little effect on final grain yield explaining in part why the lower densities in 2000 had a larger effect on reducing crop yield. In 2000, the weeds would have emerged almost as the same time as the crop. Melfort Growing season conditions would be considered average for the 2 yr of the study (Table 2). Plant densities were greater for the semi-leafless FPT in both years while seed weight was greater in 1999 and lower in 2000 than the normal-leaf FPT (Table 8). The Densities of weeds were similar between both FPT except in 1999 when slightly lower grassy weed densities occurred in the semi-leafless pea. TWR had no significant effect on seed weight. Later TWR resulted in a linear decrease in grain yield in 1999 but not 2000 even though the density of broadleaf weeds was much higher in In 1999, grain yield was similar in the weedy check and TWR3 treatments indicating that a delay in the application time of the herbicide resulted in a loss of the economic advantage of the herbicide application. Canora The growing season conditions were near-normal for the 2 yr of the study (Table 2). Crop densities achieved in 1999 were higher for the semi-leafless FPT versus the normal leafed FTP and they were lower in 2000 (Table 9). Problems with seeder calibrations resulted in very low plant populations in Weed densities were similar between the FPT for both broadleaved and grassy weeds but overall higher in 1999 than Grain yields were lower for the semi-leafless FPT in 2000 but similar yields were noted in In 1999, there was a linear reduction in grain yield with TWR. In 2000, there was also a linear decrease in grain yield with TWR for the semi-leafless but not for the normal-leafed FPT. The greater yield benefits of herbicide application in 2000 versus 1999 may have been a reflection of the low crop densities in 2000 versus DISCUSSION The benefits of early TWR were observed in 7 of 14 siteyears. For 2 other site-years, a decrease in yield was observed with delays in herbicide application even though the check yields were the same as those on the first date of application. This suggests some loss in crop tolerance with

6 428 CANADIAN JOURNAL OF PLANT SCIENCE Table 5. Time of weed removal in field pea in 1999 and 2000 at Scott, SK Plant 1000-seed Broadleaf Grassy Plant Broadleaf Grassy Yield density weight weeds weeds Yield density weeds weeds (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (weeds m 2 ) (weeds m 2 ) Semi-leafless 4119a 73a 211a 16a 82a 2039b 71b 67a 180a Leafed 3338b 74a 193b 11a 78a 2908a 78a 59a 166a Weed removal 1 wk 3993a 70a 199b 6b 78a 3398a 75ab 53a 142a 2 wk 4246a 79a 200b 10b 87a 2903b 83a 63a 191a 3 wk 3788a 71a 197b 29a 77a 2381c 76a 62a 172a Weedy check 2886b 75a 212a 10b 79a 1213d 65b 74a 187a Linear NS NS NS ** NS ** NS NS NS Quadratic * * NS 0.08 NS NS NS NS NS CV *,** Significant at P < 0.05 and P < 0.01, respectively. Table 6. Time of weed removal in field pea in 1999 and 2000 at Indian Head, SK Plant 1000-seed Broadleaf Grassy Plant 1000 seed Broadleaf Grassy Yield density weight weeds weeds Yield density weight weeds weeds (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) Semi-leafless 2539a 70a 64a 28a 2539b 72b 9a 4a Leafed 2571a 79a 57a 25a 3119a 78a 8a 4a Weed removal Semi- Semileafless Leafed leafless Leafed 1 wk 2394a 82a 175b 190a 35b 28a 2730a 75a 219ab 191ab 7a 3a 2 wk 2516a 70a 194a 18a 49ab 26ab 2974a 76a 215ab 190ab 7a 3a 3 wk 2610a 71a 188a 188a 82a 34a 3005a 77a 226a 184b 11a 5a Weedy check 2700a 74a 188a 190a 77a 17b 2608a 73a 209b 199a 9a 5a Linear NS 0.08 * NS * NS NS NS NS NS NS NS Quadratic NS NS ** NS NS NS NS NS NS NS NS NS CV

7 MAY ET AL. EARLY WEED REMOVAL IN FIELD PEA 429 Table 7. Time of weed removal in field pea in 1999 and 2000 at Redvers, SK Plant 1000-seed Broadleaf Grassy Plant Broadleaf Grassy Yield density weight weeds weeds Yield density weeds weeds (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (weeds m 2 ) (weeds m 2 ) Semi-leafless 1955a 32a 188a 239a 65a 3148a 51a 97a 51a Leafed 1660b 32a 177b 208a 85a 2777b 48a 82a 33b Weed removal 1 wk 1747a 32a 181a 170b 57a 3202a 48a 63b 35a 2 wk 1714a 34a 185a 311a 70a 3189a 50a 89ab 48a 3 wk 1918a 31a 188a 223b 78a 2885ab 51a 107a 40a Weedy check 1851a 29a 177a 188b 94a 2575b 50a 99a 46a Linear NS NS NS NS NS NS NS ** NS Quadratic NS NS NS ** NS NS NS NS NS CV Table 8. Time of weed removal in field pea in 1999 and 2000 at Melfort, SK Plant 1000 Broadleaf Grassy Plant 1000 seed Broadleaf Grassy Yield density seed weight weeds weeds Yield density weight weeds weeds (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (g) (weeds m 2 ) (weeds m 2 ) Semi-leafless 3432a 55a 247a 249a 10b 2947a 73a 191b 469a 15a Leafed 2377b 39b 190b 253a 17a 2429b 63b 205a 351a 15a Weed removal 1 wk 3090a 46a 219a 133c 8a 2866a 68a 200a 245a 6b 2 wk 3113a 46a 226a 246b 17a 2446a 70a 199a 448a 19a 3 wk 2767b 48a 216a 260b 12a 2699a 69a 195a 445a 18a Weedy check 2741b 48a 218a 365a 16a 2741a 65a 199a 501a 17a Linear * NS NS ** NS NS NS NS * ** Quadratic NS NS NS NS 0.06 NS NS NS NS NS CV

8 430 CANADIAN JOURNAL OF PLANT SCIENCE Table 9. Time of weed removal in field pea in 1999 and 2000 at Canora, SK Plant Broadleaf Grassy Plant Broadleaf Grassy Yield density weeds weeds Yield density weeds weeds (kg ha 1 ) (plants m 2 ) (weeds m 2 ) (weeds m 2 ) (kg ha 1 ) (plants m 2 ) (weeds m 2 ) (weeds m 2 ) Semi-leafless 4397a 50a 74a 121a 13b z 18a 33a Leafed 4164a 41b 71a 109a 29a 15a 30a Weed removal Semileafless Leafed 1 wk 5379a 47a 30c 117a 2486a 2149a 22a 11c 34a 2 wk 5042a 45a 57b 118a 1316b 2088a 21a 13bc 32a 3 wk 4426a 45a 105a 113a 1638b 2219a 22a 22a 31a Weedy check 2275b 46a 97a 111a 1384b 2116a 20a 20ab 30a Linear NS ** NS ** NS NS ** NS Quadratic NS NS NS NS ** NS NS NS NS CV z Differences in plant density due to calibration error not differences between cultivars. the later herbicide applications rather than from the effects of weeds. For the remaining 5 site-years, there was no yield benefit in applying the herbicide and no yield penalty from applying the herbicides early. For three non-responsive site years, broadleaf and grassy weed densities averaged 165 m 2 (range 6 500) and 37 m 2 (range 5 81), respectively. In the responsive sites broadleaf and grassy weed densities averaged 60 m 2 (range ) and 77 m 2 (range ), respectively. These represent the weed densities in the check plots, 3 wk after crop emergence. Given that some form of weed control was used either just before or immediately after seeding, we conclude that in the sites nonresponsive to herbicides, the weeds probably emerged much later than the crop supporting the concept that later emerging weeds have very little effect on the yield of field pea as has been found in wheat and barley (Peters 1984; O Donovan et al. 1985). Field pea yield losses from weeds at the sites responsive to early TWR ranged from 20 to 64%, which is similar to previously reported results (Blackshaw and O Donovan 1993; Harker 2001). This study also compared a normal-leaf and a semi-leafless cultivar and their responses to early TWR. With the semi-leafless cultivar, the leaflets are replaced with tendrils, the end result being that there is less leaf area but better standability. Wall and Townley-Smith (1996) found that leaf type had little effect on field pea competitiveness while plant height (vine length) was an important determinant of competitiveness in field pea. In only 2 site-years, Outlook and Canora in 2000, was a FPT TWR interaction observed for grain yield. In both cases, a yield benefit of early herbicide application was observed for the semi-leafless cultivar but not for the normal-leafed cultivar. This observation suggests that early TWR is most important for less competitive cultivars. It should be noted that at the Canora site, seed calibration problems were encountered resulting in very low crop populations for the semi-leafless cultivar thereby affecting the results. Consequently, the concept of early TWR can be applied to both field pea types. This would also support the observation of Townley-Smith and Wright (1994) that FPT had little effect on overall competitiveness against weeds. CONCLUSIONS The results of this study support the merits of applying herbicides early after field pea emergence. There were no yield penalties experienced from early applications of this herbicide, but some yield penalties were observed with the later applications for certain site-years for reasons of weed competition and at others for reasons of loss in crop tolerance. This study also shows that a single application is sufficient to obtain the benefits of early weed removal as reported by Harker et al. (2001). The benefits of early weed removal were similar between normal-leafed and semi-leafless cultivar. Although spraying herbicides early should be recommended, producers are encouraged to use crop management techniques that minimize weed pressures as much as possible. Spraying herbicides early reduces the problems associated with delayed applications including adverse weather conditions and a decrease in crop tolerance. Some producers have expressed the concern that by delaying the herbicide application, they can control more weeds. However it is the early weed flushes that cause the greatest crop yield losses and control of later weed flushes only improves field appearance and seldom the grain yields. ACKNOWLEDGEMENT This study was made possible with funding from the Canada-Saskatchewan Agri-Food Innovation Fund, Agriculture and Agri-Food Canada and BASF. Special mention for the technical support of Roger Geremia, Orla Willoughby and Chris Holzapfel at the Indian Head Research Farm, Wendy Schatz and Rachel Osytryk at Canora, David Larsen at Redvers, Bryan Nybo, Darren Steinley and Brock Minogue at Swift Current, Herb Schell

9 MAY ET AL. EARLY WEED REMOVAL IN FIELD PEA 431 at Scott, Mark Pederson and Allan MacDonald at Outlook and Bruce Johnson at Melfort. Blackshaw, R. E Postemergence weed control in pea (Pisum sativum) with imazamox. Weed Technol. 12: Blackshaw, R. E. and O Donovan, J. T Higher crop seeding rates can aid weed management. Proc. Brighton Crop Prot. Conf. 3: Fu, Gaoming Nitrogen dynamics in a chickpea-wheat rotation in a hummocky field. Ph.D. Dissertation, University of Saskatchewan, Saskatoon, SK. 200 pp. Harker, K. N Survey of yield losses due to weeds in Central Alberta. Can. J. Plant Sci. 81: Harker, K. N., Blackshaw, R. E. and Clayton, G. W Timing weed removal in field peas (Pisum sativum). Weed Technol. 15: Lafond, G. P., Loeppky, H. and Derksen, D. A The effects of tillage systems and crop rotations on soil water conservation, seedling establishment and crop yield. Can. J. Plant Sci. 72: Littel, R. C., Milliken, G. A., Stroup, W. W. and Wolfinger, R. D SAS system for mixed models. SAS Institute, Inc., Cary, NC. 656 pp. Miller, P. R., McConeky, B. G., Clayton, G. W. Brandt, S. A., Staricka, Johnston, A. M., Lafond, G. P., Schatz, B. G., Baltensperger, D. D. and Neill, K. E Pulse crop adaptation in the northern Great Plains. Agron. J. 94: O Donovan, J. T., de St. Remy, E. A., O Sullivan, P. A., Dew, D. A. and Sharma, A. K Influence of the relative time of wild oat (Avena fatua) on yield loss of barley (Hordeum vulgare) and wheat (Triticum aestivum). Weed Sci. 33: Peters, N. C. B Time of onset of competition and effects of various fractions of an Avena fatua L. population on spring barley. Weed Res. 24: Statistics Canada Agriculture 2001 census. Farm data: Initial Release. [Online] Available: (May 2002). Stevenson, F. C. and van Kessel, C The nitrogen and nonnitrogen rotation benefits of pea to succeeding crops. Can. J. Plant Sci. 76: Townley-Smith, L. and Wright, A. T Field pea cultivar and weed response to crop seed rate in western Canada. Can. J. Plant Sci. 74: Wall, D. A. and Townley-Smith, L Wild mustard (Sinapsis arvensis) response to field pea (Pisum sativum) cultivar and seeding rate. Can. J. Plant Sci. 76: Zentner, R. P., Lafond, G. P., Derksen, D. A. and Campbell, C. A Tillage method and crop diversification: effect on economic returns and riskiness of cropping systems in a Thin Black Chernozem of the Canadian Prairies. Soil Tillage Res. 1695: 1 13.

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11 This article has been cited by: 1. R.J. French. Field Pea: Agronomy. [Crossref] 2. R.J. French. Field Pea: Agronomy [Crossref] 3. William E. May Niger Tolerance to Flucarbazone. Weed Technology 29:03, [Crossref] 4.. References [Crossref] 5. K Neil Harker, George W. Clayton, Robert E. Blackshaw Comparison of Leafy and Semileafless Pea for Integrated Weed Management. Weed Technology 22:1, [Crossref] 6. K. Neil Harker, Robert E. Blackshaw, George W. Clayton Wild Oat (Avena fatua) vs. Redstem Filaree (Erodium cicutarium) Interference in Dry Pea. Weed Technology 21:01, [Crossref]