Impact of Wild Blueberry Harvesters on Weed Seed Dispersal within ...

Weed Science 2009 57:541–546

Impact of Wild Blueberry Harvesters on Weed Seed Dispersal within and between Fields Nathan S. Boyd and Scott White* Agricultural equipment can disperse weed seeds over large distances. Efforts to minimize or prevent equipment-mediated dispersal should be a key component in any integrated weed management plan. Several experiments were initiated in commercial wild blueberry fields to examine the potential impact of harvesting equipment on weed seed dispersal within and between blueberry fields. Seed loads were examined on harvesting equipment between fields and results suggest that harvesting equipment is a major vector of seed dispersal. Seed loads were 397,000 in 2006 and 194,000 in 2007. Of all seeds located on the harvester, 66 to 79% were located on the belts or affiliated components. In 2006, a second experiment was established to examine within-field seed dispersal. A sampling grid was established over multiple distinct poverty oatgrass patches with seed heads at 44% of all sampling points. Following harvest, seeds were located at 67% of all sampling points. In 2006 and 2007, short-distance secondary dispersal of poverty oatgrass by harvesting equipment was measured. The relationship between distance from patch perimeter and seeds per unit area on the side approached by harvesting equipment and the far side of the patch was adequately modeled with an exponential decay model. Secondary dispersal within blueberry fields by harvesting equipment is inevitable. Dispersal may be reduced by avoiding dense weed patches, or altering harvest timing. Periodic cleaning of harvesting equipment between fields will help prevent the spread of weed seed. Nomenclature: Poverty oatgrass, Danthonia spicata (L.) Beauv. ex Roemer & J. A. Schultes DANSP; wild blueberry, Vaccinium angustifolium Ait. and Vaccinium myrtilloides Michx. Key words: Preventive weed control, patch dynamics, grasses, horticulture.

Lowbush blueberry plants grow within early succession communities where perennial plant species naturally predominate. Unlike most other horticultural crops, wild blueberries are not planted. Commercial fields are developed on abandoned farmland or deforested areas by encouraging the clonal expansion of existing native stands. The longestablished clones form a relatively continuous mat over a field that precludes management practices such as cultivation and crop rotation (McIsaac 1997). Competing perennial vegetation has typically been the major yield-limiting factor in wild blueberry production and over the past 20 yr, the selective PRE herbicide, hexazinone, has been relied upon for control of unwanted vegetation ( Jensen and Yarborough 2004; McCully et al. 1991). Repeated use of the same chemical for extended periods of time has resulted in reports of probable resistance to recommended rates of hexazinone as well as a twofold increase in the number of weed species in blueberry fields in every category, including eight new grass species ( Jensen and Yarborough 2004). Many of the newly invasive species are annual or perennial weeds that reproduce predominately via seed. Increased soil fertility due to increased fertilizer inputs as well as the creation of large bare areas in blueberry fields due to overuse of PRE products has increased field susceptibility to invasion by grasses and annual broadleaf weeds (Yarborough and Bhowmik 1989). The increase in weed diversity in blueberry fields is not surprising, given that both the relative and absolute abundance of major agricultural weeds across most crops has increased steadily over the last century despite control measures (Ghersa and Roush 1993). The issue of greatest concern to the wild blueberry industry is the relatively rapid spread of several predominant grass species within fields, between fields, and across regions. This rapid spread suggests DOI: 10.1614/WS-08-156.1 * Department of Environmental Sciences, Nova Scotia Agricultural College, Truro, Nova Scotia, B2N 5E3, Canada. Corresponding author’s E-mail: [email protected]

that effective seed dispersal vectors exist and that the range of the dispersal vectors is relatively large. This is of particular concern because only a limited number of grass herbicides are registered for use in wild blueberry. If resistance to currently registered herbicides were to occur, the probability of widespread dispersal is high. Primary seed dispersal, which is defined as dispersal by natural mechanisms, typically occurs over a relatively small area. Dispersal distance depends upon a range of factors including parent plant height, seed weight, wind speed, and seed shape. Most published reports have found that a large proportion of seeds tend to fall within 1 m of the parent plant. For example, Nadeau and King (1991) found that the majority of yellow toadflax (Linaria vulgaris P. Mill.) seeds fell within a radius of 0.5 m from the parent plant. Barroso et al. (2006) reported that natural dispersal of wild oat (Avena fatua L.) and sterile oat (Avena sterilis L.) rarely exceeded 1.5 m. Dispersal over longer distances typical relies upon wind, animals, or human activity. When seed maturation coincides with mowing, harvesting, or other management operations within agricultural fields, human activity may become the primary vector of dispersal (Humston et al. 2005). Secondary dispersal, which is defined as dispersal by human-mediated activities such as combining, may in some cases contribute very little to short-distance dispersal but contribute significantly to long-distance dispersal (Barroso et al. 2006). The dispersal distance varies not only with crop type and weed type but is also largely dependent on the type of equipment used, conditions during use, and travel speed (Ballare´ et al. 1987). For example, Blanco-Moreno et al. (2004) found that the primary dispersal of rigid rye grass (Lolium rigidum Gaudin) occurred in a very limited space around the parent plant but that dispersal by combines exceeded 18 m. McCanny and Cavers (1988) found that proso millet (Panicum miliaceum L.) seeds were deposited in a relatively even strip behind a combine harvester up to 45 m beyond the original patch with few seeds dispersed beyond 50 m. Howard et al. (1991) reported that Bromus seeds were

Boyd and White: Seed dispersal by wild blueberry harvesting equipment

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moved by combine harvesting 1.9 m behind the point of introduction and as much as 20 m ahead of the point of introduction. Also, Shirtliffe and Entz (2005) found that without chaff collection, wild oat seeds were dispersed as far as 145 m beyond the patch boundary. In a simulated experiment, Woolcock and Cousens (2000) demonstrated that seed dispersal by combines can increase the rate of spread up to 16 times that of natural dispersal. The result is widespread dispersal of weed species mature at the time of harvest within and between fields. Current weed management practices typically emphasize control of emerged seedlings with cultivation or herbicides. In a review article, Ghersa and Roush (1993) argue that a more permanent solution to weed problems can be found by preventing the dispersal of propagules. Weed populations will continue to grow even in fields with a dense and highly competitive crop stand if dispersal rates are high (Ghersa and Roush 1993). A few studies have documented significant drops in weed populations when human-mediated dispersal was eliminated (Salisbury 1961). Unfortunately, limited research has been conducted on weed seed dispersal and its role in plant population dynamics because of the complexity involved. Preventing seed dispersal may be especially important in a perennial crop such as wild blueberries where many of the common cultural practices used for annual crops are not feasible and herbicides are the predominant method of weed control (McIsaac 1997). In such cases, effective longterm management must include measures that prevent largescale seed dispersal. The objectives of this study were to (1) estimate the number of weed seeds carried by blueberry harvesters between fields, (2) identify plant species most apt to be spread by harvesting equipment, (3) examine seed dispersal of grasses within blueberry fields, and (4) identify potential management techniques that could be adopted to reduce weed seed dispersal. Materials and Methods

Weed Seeds on Harvesting Equipment. Eleven wild blueberry harvesters1 were sampled for weed seed contamination during the summer of 2006. Harvesters were all sampled before entering fields in Colchester County, Nova Scotia. Two of the 11 harvesters were from Maine and had been pressure-washed prior to being sent to Nova Scotia. The remaining nine harvesters were not washed prior to sampling and had been used at various locations throughout the Maritimes. In 2007, eight harvesters were sampled prior to entering fields in Colchester County. Two of the eight harvesters were from Maine and had been pressure-washed before sampling. All harvesters were visually inspected prior to sampling, and a total of 12 harvester components were identified as potential points where material containing weed seeds could accumulate. Material was collected from each component on each harvester using a metal paint scraper or brush. All material or a known percentage of material on each component was collected and samples were brought back to the Nova Scotia Agricultural College for processing. Samples were air-dried in paper bags, weighed, and sieved. Sieve sizes were 12.5 mm, 9.5 mm, 6.3 mm, 3.35 mm, 2 mm, and 1 mm. Material left in sieve sizes 12.5 to 3.35 mm was checked for presence of seeds and then discarded. Seed counts 542

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were made from material remaining. The total number of grass and broadleaf weed seeds was estimated by counting all seeds within five 0.4-g subsamples of the air-dried and sieved material. Counts from the five subsamples were averaged and used to estimate total harvester seed load. Seed counts were also used to identify components most prone to seed accumulation and to determine the contribution of each component to total seed load per harvester. Seed counts were conducted on relatively small subsamples due to the high number of very small seeds in each sample and the labor intensity of this type of research. Seed Numbers in Debris Piles. Large amounts of plant debris accumulate on equipment during the harvest operation. In weedy fields, debris accumulates on the rotating rake head that is used to remove the berries from the plant. Accumulated debris is removed periodically by equipment operators and typically dropped to form debris piles in the field at the site of removal. Seed content of eight debris piles was estimated in this study by (1) collecting a known portion of each pile, (2) air-drying and sieving the debris as described previously, and (3) counting the number of seeds in five 0.4-g subsamples. Secondary Dispersal of Poverty Oatgrass and Tickle Grass [Agrostis hyemalis (Walter) B.S.P.]. Long-distance dispersal by harvesting equipment is very difficult to estimate. In 2006, we attempted to address this issue with the establishment of a sampling over multiple distinct poverty oatgrass patches prior to harvest. Some of the seeds had already been dispersed when the sampling grid was put in place but a large proportion remained on the plants. Average seed number per seed head at the time of harvest was 4.6 seeds. A grid of petri dishes was established during the production season of a wild blueberry field. All petri dishes were placed in a small hole deep enough to ensure that the rim of the dish was even with the soil surface. The grid was composed of 16 transects of 84 petri dishes placed 50 cm apart. Transects were spaced 1 m apart. The entire grid was put in place 8 d prior to harvest. The number of seed heads at each sampling point in the grid was determined by counting all seed heads within a 25 by 25 cm quadrat centered directly above each petri dish. All petri dishes were collected shortly following harvest. When dishes were broken by harvesting equipment, material was collected as accurately as possible by collecting all dish pieces and any material on top of the pieces. Dishes were placed in paper bags and brought back to the Nova Scotia Agricultural College, where total seed number was counted. A similar but smaller grid was also established in 2006 at Mount Thom, Nova Scotia, within a tickle grass weed patch. Five transects were established with dishes placed 50 cm apart within the weed patch (the first 6 m of each transect) to allow greater sampling frequency within the patch. Dishes outside the patch were placed 1 m apart for a distance of 23 m, creating a total row length of 29 m with 35 dishes. Dishes were collected following harvest and the total number of seeds within each dish counted. In 2006 and 2007, three transects spaced 1 m apart were placed over four and five weed patches, respectively. Patches examined in both years had a diameter of 3 to 5 m and were composed predominately of poverty oatgrass. Patches evaluated in 2006 were within the grid described previously and the transects were part of the overall grid. In 2007, a 58-cm2 (the

Table 1. Summary of blueberry harvester components sampled for weed seeds in 2006 and 2007. Component

Sampling points

Sample location

Tractor grill Head frame

1 5

Head belts Loading belt Berry leveler Blower Loading deck

2 3 1 1 5

Grill located on the front of the tractor Five areas of the frame of the harvesting head including the front wheel support bar, areas around hydraulic pumps, top cover, and support bars behind the head cleaner and behind the roller Belt and shield located inside the harvester head used to carry berries from the harvester head to the main loading belt Belt, belt guard, and frame used to carry the berries from the harvester head to the boxes at the back of the tractor Rotating device located just above the berry boxes used to level berries Fan located at the end of the loading belt used to remove debris from berries falling to boxes All areas located at back of tractor where loaded boxes are stored including platform and hydraulics

same surface area as a petri dish) sample was taken with a shop vacuum every 50 cm within the patch and every 50 cm outside of the patch with transect length extended to 2 m on either side of the patch. The hose of the vacuum was lightly brushed over the surface to collect all surface residues. Previous observations by the author suggests that the method accurately estimates seed dispersal within blueberry fields because (1) residue tends to lie on the surface of an organic matter mat that is intertwined with rhizomes and covers the soil, (2) dispersed seeds lie on the surface and can easily be retrieved without retrieving excessive amount of additional debris or soil, and (3) very few seeds appear to remain on the surface from previous years (N. S. Boyd, unpublished data). Seeds were collected using a shop vacuum to eliminate the problem of the high frequency of broken petri dishes following harvest that was experienced in 2006. Seed production per seed head was estimated each year by counting the number of seeds on 12 seed heads randomly selected from plants located within the study area. Statistical Analysis. The mean seed number on harvester components and the standard error of the mean are reported. Seed number on harvesters was analyzed separately for each year. Harvesters that were cleaned prior to sampling were also analyzed separately. Seed count comparisons were made between washed and unwashed harvesters using Proc t test in SAS with the Cochran option.2 Maps of poverty oatgrass seed heads before harvest and seeds after harvest was created using the contour plot function in Sigma Plot.3 No additional smoothing algorithm was utilized. Frequency was calculated by dividing the number of sampling points

where seed heads or seeds occurred by the total number of sampling points. Each poverty oatgrass patch included in the analysis of short-distance secondary dispersal was considered a block. Year and dispersal direction were analyzed separately. Dispersal direction was a measure of dispersal on the side of the patch approached by harvesting equipment labeled as ‘‘before’’ throughout the remainder of this paper vs. dispersal on the far side of the patch after the harvesting equipment had passed through the weed patch labeled as ‘‘after’’ throughout the remainder of the paper. Transects were considered replicates and the average number of seeds at each specified distance from the patch perimeter was calculated. The relationship between distance from patch perimeter (x) and seeds per unit area (Y ) for each year and location (before or after) was adequately modeled by a nonlinear exponential decay regression model: Y ~ae {bx ze

½1

where the error term (e) is assumed to be independent and normally distributed with constant variance.

Results and Discussion

Weed Seeds on Harvesting Equipment. Harvester components were grouped into seven categories based on location on the harvester. Each category had varying numbers of sampling points that are described in Table 1. Average total seed count per harvester exceeded 397,000 seeds in 2006 and was just slightly less then 194,000 seeds in 2007 (Table 2). We

Table 2. Weed seeds located on blueberry harvesters that had previously been used to harvest multiple fields in Nova Scotia, Canada. Nine and six harvesters were sampled in 2006 and 2007, respectively. Year 2006

2007

a

Component Tractor grill Head frame Head belts Loading belt Berry leveler Blower Loading deck Total belts Overall total Tractor grill Head frame Head belts Loading belt Berry leveler Blower Loading deck Total belts Overall total

Broadleaf 28 22,600 103,000 46,000 5,470 911 5,210 149,000 183,000 4 27,800 27,900 49,300 79 826 16,900 77,200 123,000

a

(20) (5,380) (35,900) (14,900) (3,690) (723) (1,600) (50,000) (48,900) (3) (11,000) (9,170) (16,700) (50) (823) (10,700) (24,500) (38,800)

Grass 137 37,900 125,000 40,900 3,960 2,290 4,150 166,000 214,000 116 12,200 20,300 30,200 128 675 7,340 50,500 70,900

(106) (6,330) (46,200) (10,200) (2,120) (2,020) (1,690) (54,500) (54,200) (115) (4,610) (9,970) (18,070) (82) (675) (4,590) (27,900) (31,000)

Total 165 60,500 228,000 86,900 9,430 3,200 9,370 315,000 398,000 120 40,000 48,200 79,500 207 1,500 24,000 128,000 194,000

(126) (9,560) (73,900) (23,800) (5,740) (2,740) (2,890) (95,000) (90,500) (115) (15,400) (17,500) (34,000) (131) (1,500) (15,000) (50,400) (65,800)

Values within parentheses are the standard error of the mean.


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Table 3. Weed seeds located on blueberry harvesters that had been washed with a pressure washer using cold water prior to sampling. Two harvesters were sampled in 2006 and in 2007. Year 2006

2007

Component Tractor grill Head frame Head belts Loading belt Berry leveler Blower Loading deck Total belts Overall total Tractor grill Head frame Head belts Loading belt Berry leveler Blower Loading deck Total belts Overall total

Broadleaf a

0 6,080 22,700 632 14 0 7,130 23,300 36,600 0 1,500 (1,500)b 6,880 (6,880) 456 (262) 0 0 0 7,340 (6,620) 8,840 (8,120)

Grass 0 634 303 617 4 0 31 921 1,590 0 236 (230) 756 (756) 127 (11) 0 0 0 883 (745) 1,120 (974)

Total 0 6,710 23,000 1,250 18 0 7,160 24,300 38,100 0 1,740 7,600 583 0 0 0 8,180 9,920

(1,728) (7,600) (273)

(7,360) (9,090)

a Standard errors were not included in 2006 because only two harvesters were sampled and weed seeds were only found on one of the two harvesters. b Values within parenthesis in 2007 are the standard error of the mean.

therefore conclude that harvesters carry a significant weed seed load between fields and have high potential to act as a significant weed seed dispersal vector. Seed accumulation was greatest on conveyor belts or components of those belts, accounting for 79 and 66% of the total seed load on each harvester in 2006 and 2007, respectively (Table 2). This pattern of accumulation facilitates cleaning of equipment, given that a large proportion of the seeds were located within localized areas. Grasses made up 46 and 63% of the total seed load in 2006 and 2007, respectively. A wide range of species were present depending on weed pressures in harvested fields prior to sampling. Common weed species found on multiple harvesters included poverty oatgrass, tickle grass, fescue (Festuca spp.), goldenrod (Solidago spp.), cow-wheat (Melampyrum lineare Desr.), slender rush (Juncus tenuis Willd.), and red sorrel (Rumex acetosella L.). Very few seeds were found on the tractor with most seeds located on the belts and harvesting head. Harvesters that had been pressure-washed had fewer seeds in every sampled location on the equipment (Table 3). When comparing overall harvester seed loads there were significantly fewer grass (P 5 0.0013), broadleaf (P 5 0.0094), and total (P 5 0.0017) seeds on the washed harvesters than the unwashed harvesters. Average total seed load on harvesters that were pressure-washed was approximately 38,000 in 2006 and 10,000 in 2007. The effect of washing is confounded with the origin of the harvesting equipment, with all washed harvesters sampled following use in fields in the state of Maine and unwashed harvesters sampled following use in the province of Nova Scotia. The difference in seed counts could be attributed to a lack of weeds in blueberry fields in Maine. This is highly unlikely, given that management techniques, growing conditions, and weed populations are very similar between the two regions. Therefore, we conclude that pressure-washing will significantly reduce weed seed loads. All seeds located on a harvester will not be dispersed within a single field, but the probability of dispersal between fields is extremely high, given the number of seeds carried by harvesters between fields and the location of the majority of those 544

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seeds on moving components. Cleaning equipment with pressure washers between fields is a relatively low-cost technique that would reduce dispersal potential and facilitate weed management efforts in the future. Weed Seed Dispersal within Fields. The distance seeds are dispersed by harvesting equipment within a field is complicated by the type of equipment used. Seeds can be knocked forward by the harvesting equipment as it travels through the field or seeds can be collected in the rotating rakes and moved up the belts that carry the berries. Dispersal behind the equipment occurs when the berries are dropped from the belt to the storage boxes. During this drop, a fan blows air through the falling berries to remove residue that may include weed seed. Direction and distance of weed seed dispersal by the fan is affected by seed size, wind speed, wind direction, and the presence or absence of a down shoot designed to direct blown residue towards the ground. Weed residue also tends to collect on the harvesting equipment and is removed periodically by the harvesting crew. As a result, direction and distance of dispersal is expected to be more variable than typically observed with combines (McCanny and Cavers 1988). Secondary Dispersal of Poverty Oatgrass. In 2006 and 2007, we examined the movement of poverty oatgrass seeds beyond the perimeter of small (3 to 5 m diameter), isolated patches. In 2006, the relationship between distance from patch edge and seed number both before (F 5 215.3, P , 0.0001) and after (F 5 8.9, P , 0.03) was adequately explained with a nonlinear exponential decay model. Seed retrieval behind and in front of the patch relative to the direction of harvester movement was similar with slightly more seeds retrieved in the forward direction (Figure 1). The data suggest that a significant proportion of the seeds were not moved beyond 2 m from the patch perimeter. In 2007, seed dispersal both before (F 5 273.7, P , 0.0001) and after (F 5 199.5, P , 0.0001) the weed patch was also adequately explained with a nonlinear exponential decay model (Figure 1). In 2007, the regression curves suggest that more seeds were moved farther beyond the patch boundaries in the opposite direction of the traveling harvester than were moved in the direction of the harvester. This can be attributed to the use of a fan on the harvesting equipment to remove debris from the berries. The number of seeds dispersed drops exponentially with distance and the regression curve suggests that it is likely that most seeds were dispersed within 2 m (Figure 1). This is contrary to most studies in agronomic crops where seeds are typically only carried in the direction of movement of the harvesting equipment (Howard et al. 1991). In wild blueberry fields, seeds appear to be moved in both directions, which was also noticed in work conducted by Howard et al. (1991). Combined, these results suggest that harvesting equipment effectively disperses poverty oatgrass seeds beyond current patch boundaries. The analysis of small distinct patches adequately measures the seed movement by harvesters over short distances but long-distance dispersal by harvesting equipment is likely to occur and is very difficult to estimate. We conducted a 1-yr experiment in 2006 to observe poverty oatgrass dispersal within and between multiple weed patches. Widespread dispersal was observed with seeds often spread in small patches in the direction of harvester movement (Figure 2). Before

Figure 2. (a) The location and density of poverty oatgrass seed heads before harvest and (b) the location and density of poverty oatgrass seeds after harvest at one site in 2006.

Figure 1. Poverty oatgrass seed movement as affected by harvest equipment in 2006 (2006 before: Y 5 2.79e(20.8x), R2adjusted 5 0.97; 2006 after: Y 5 4.48e(20.37x), R2adjusted 5 0.57) and 2007 (2007 before: Y 5 3.96e(20.78x), R2adjusted 5 0.98; 2007 after: Y 5 1.7e(20.64x), R2adjusted 5 0.97). Regression curves represent seeds dispersed before and after the weed patch in reference to travel direction of equipment. Negative distance values refer to distances within the weed patch. Error bars represent standard error of the mean.

harvest, seeds heads were located at 44% of sampling points. Following harvest, seeds were located at 67% of all sampling points. It is important to note that the area sampled for seed content was smaller than the area sampled for seed heads. Therefore, detection may have been substantially higher if a larger area had been sampled. Although the data were only collected in a single season, the results suggest that harvesting equipment moves seeds substantially within blueberry fields if the seeds are mature and remain on the plant at harvest. This is not surprising, given that harvesting equipment has been identified as a major vector of seed dispersal in several crops (Ballare´ et al. 1987, Blanco-Moreno et al. 2004). In some cases, a significant proportion of the seeds remained near the original patch. In other cases, a large proportion of the seeds were removed entirely. This is likely to occur when the harvesting equipment removes entire seed heads. The author has observed harvesting equipment remove a substantial proportion of the weed biomass including virtually every seed head from a small patch. This material is then dropped at a later date or removed by equipment operators.

Secondary Dispersal of Tickle Grass. In 2006, seed dispersal around a single large and dense tickle grass patch was measured. Results suggest that the number of seeds dispersed in front of the patch declined rapidly with distance (Figure 3). Stem density tended to decline near the edge of the patch but seed numbers remained higher than seed numbers beyond the patch boundary. Weed Residue Piles. Plant debris on harvesting rakes clogs equipment, slows harvest, and reduces berry quality. To solve this problem, equipment operators stop the equipment to remove debris. Piles of weed residue are frequently deposited throughout the field and are often deposited significant distances from their origin. Eight piles composed predominately of fescue (Festuca tenuifolia) debris from one field in 1 yr were analyzed for seed content. As expected, seed numbers within piles varied widely depending on pile size and material within the pile. On average, there were 236,000 seeds in each pile, with 136,000 grass seeds and 100,000 broadleaf seeds. Despite the anticipated high degree of variability in seed numbers between patches, the probability of a new weed patch being initiated at the location of each pile is very high, given the range of seed numbers found. To eliminate this problem we recommend that weed debris be removed from the field after harvest or that the equipment operators be encouraged to clean their rakes at the field edge. Harvesting equipment is a significant vector of seed dispersal within and between fields when the harvest coincides with seed maturation. A significant number of weed seeds are spread at least 2 m beyond the boundary of a weed patch in all directions during harvest. Results obtained in 2006 suggest that additional numbers of seeds are


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Sources of Materials 1

Blueberry harvester, Doug Bragg Enterprises Ltd., Collingwood, Nova Scotia, Canada. 2 SAS, SAS Institute Inc., 100 SAS Campus Drive, Cary, NC 27513-8617. 3 Sigma Plot, SYSTAT Software Inc., 225 W. Washington St., Chicago, IL 60606.

Acknowledgments The authors would like to thank Angela Hughes for her excellent technical assistance. I would also like to acknowledge the assistance of all the summer students involved in this project. This research was funded by the Nova Scotia Technology Development 2000 program, Wild Blueberry Producer’s Association of Nova Scotia, and Oxford Frozen Foods. Figure 3. Tickle grass secondary seed dispersal beyond the weed patch as affected by harvest equipment at one site in 2006.

dispersed randomly throughout the field. Cleaning debris from harvest rakes and leaving it in the field also contributes to dense patches of seeds spread randomly across the field. Significant seed loads were also observed on harvesting equipment before the equipment entered a new field. This is of great concern because harvesting equipment is used over a very large area in many cases and could function as an effective dispersal vector and could introduce new weed species or spread herbicide-resistant populations over large areas. Probability of spread between fields could be addressed in part by periodic cleaning of equipment with steam cleaners or pressure washers. Seed dispersal patterns observed suggest that site-specific application of herbicides based on weed maps developed in previous seasons may not be appropriate. The largest blueberry grower in the region is interested in adopting site-specific approaches to weed management. Our results suggest that they should focus on the development of realtime imaging for POST products rather than on the use of mapping techniques. This conclusion was drawn based upon the widespread seed movement observed. If herbicide applications are based upon constructed weed maps, spraying should cover an area at least 2 m beyond the patch boundary in every direction. Several potential options are available to reduce seed dispersal within a field, including (1) avoidance of dense weed patches where berry yields tend to be low, (2) alteration of harvest time to avoid peak seed maturity times, (3) management of weed populations to minimize seed production, and (4) clipping the inflorescences of weeds in large patches prior to harvest.

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Literature Cited Ballare´, C. L., A. L. Scopel, C. M. Ghersa, and R. A. Snchez. 1987. The demography of Datura ferox (L.) in soybean crops. Weed Res. 27:91–102. Barroso, J., L. Navarrete, M. J. Sanchez del Arco, C. Fernandez-Quintanilla, P.J.W. Lutman, N. H. Perry, and R. I. Hull. 2006. Dispersal of Avena fatua and Avena sterilis patches by natural dissemination, soil tillage and combine harvesters. Weed Res. 46:118–128. Blanco-Moreno, J. M., L. Chamorro, R. M. Masalles, J. Recasens, and F. X. Sans. 2004. Spatial distribution of Lolium rigidum seedlings following seed dispersal by combine harvesters. Weed Res. 44:375–387. Ghersa, C. M. and M. L. Roush. 1993. Searching for solutions to weed problems. Do we study competition or dispersion? BioScience 43:104–109. Howard, C. L., A. M. Mortimer, P. Gould, and P. D. Putwain. 1991. The dispersal of weeds: seed movement in arable agriculture. Pages 821–828 in Proceedings of the 1991 Brighton Crop Protection Conference—Weeds. Brighton, UK: British Crop Protection Council. Humston, R., D. A. Mortensen, and O. N. Bjørnstad. 2005. Anthropogenic forcing on the spatial dynamics of an agricultural weed: the case of the common sunflower. 2005. J. Appl. Ecol. 42:863–872. Jensen, K.I.N. and D. E. Yarborough. 2004. An overview of weed management in the wild lowbush blueberry—past and present. Small Fruits Rev. 3:229–255. McCanny, S. J. and P. B. Cavers. 1988. Spread of proso millet (Panicum miliaceum L.) in Ontario, Canada. II. Dispersal by combines. Weed Res. 28:67–72. McCully, K. V., M. G. Sampson, and A. K. Watson. 1991. Weed survey of Nova Scotia lowbush blueberry (Vaccinium angustifolium) fields. Weed Sci. 39:180–185. McIsaac, D. 1997. Growing Wild Lowbush Blueberries in Nova Scotia. http:// www.nsac.ca/wildblue/facts/grow.asp. Accessed: January 15, 2009. Nadeau, L. B. and J. R. King. 1991. Seed dispersal and seedling establishment of Linaria vulgaris Mill. Can. J. Plant Sci. 71:771–782. Salisbury, R. 1961. Weeds and Aliens. London: Collins. 384 p. Shirtliffe, S. J. and M. H. Entz. 2005. Chaff collection reduces seed dispersal of wild oat (Avena fatua) by a combine harvester. Weed Sci. 53:465–470. Woolcock, J. L. and R. Cousens. 2000. A mathematical analysis of factors affecting the rate of spread of patches of annual weeds in an arable field. Weed Sci. 48:27–34. Yarborough, D. E. and P. C. Bhowmik. 1989. Effect of hexazinone on weed populations and on low bush blueberries in Maine. Acta Hort. 241:344–349.

Received October 1, 2008, and approved April 26, 2009.