Forage Legume Seed Crops: Pest Control, Pollination and Harvest Practices Thomas G Chastain CROP 460/560 Seed Production
Weed Management in Alfalfa Major weed problems include dodder, field bindweed, pigweed, and wild mustard. These are difficult to remove from alfalfa seed lots in the seed cleaning plant. Other problem weeds include Canada thistle, Russian knapweed, and sweet clover (no selective herbicide available, must be hand hoed). Grass weeds and volunteer cereals can be controlled with selective herbicides. Seedlings stands can be cultivated. Sweet clover in flower (top OSU Extension photo), cultivator in alfalfa seed field (bottom TG Chastain photo)
Weed Management in Red Clover Annual grass weeds and broadleaf weeds are major problems. Preplant herbicides such as Eptam are incorporated prior to planting. Herbicide applications are delayed until the 2-4 leaf stage of the seedling crop is reached. Wild mustard, wild radish, and several grass weeds are problems in red clover. Shallow cultivation is sometimes used. The crop can be mowed to eliminate weeds, but not after the first bloom as seed yield will be reduced. Weed control in white clover is similar to red clover. Few herbicides are available in the annual clovers. Wild radish (OSU extension photo)
Weed Management in Red Clover Clover broomrape (Orobanche minor) is a parasitic annual weed. It was first detected in 1998 and spread to several red clover seed fields. No good control measures are known, but some work on trap crops and herbicides have shown some efficacy in reducing infestations. Clover broomrape (Photo - Rebecca Norris, USDA APHIS PPQ)
Insect Pests Alfalfa - alfalfa seed chalcid, lygus bugs, alfalfa weevil, aphids, mites. Red clover - clover seed chalcid, clover aphid, clover seed midge, nitidulid beetle. White clover - clover seed weevil, lygus bug, nitidulid beetle. Alfalfa seed chalcid (Ken Gray photo) Alfalfa seed damaged by seed chalcid (Ken Gray photo) Clover seed weevil (Ken Gray photo)
Insect Pests Control measures - Use IPM methods. Involves knowledge of pest biology, sampling, knowledge of economic thresholds, selective insecticides, and biological control. Damsel bug is a natural biological control agent used to lygus bugs. Parasitic wasps can also be used as biological control agents of lygus bugs. Lygus bug (top), damsel bug (bottom) Ken Gray photos
The presence of resident insect pollinators in the vicinity of the field or importation of quality pollinators is a key to successful forage legume seed production. Four types of bees are the primary pollinators of forage legumes seed crops. Bee pollinating red clover seed field (TG Chastain photo)
Honey bee - is a social bee which is polylectic (bee forages on many plant species). Red clover needs 2 hives/acre. White clover and alfalfa needs 1-2 hives/acre. Two hives/acre are used for tripping in crimson clover. Honey bees are needed for tripping only since crimson clover is self-pollinated. White clover seed field and hives (TG Chastain photo) Honey bee (Ken Gray photo)
Bumble bee - is a social bee and is a good pollinator, but is hard to manage. The bumble bee is sometimes used in alfalfa and red clover seed crops especially when populations of these native bees can be found nearby. Bumble bee (Bombus californicus) (Ken Gray photo)
Leaf cutter bee - An oligolectic bee (feeds only on alfalfa) and nests in special boards. This is the predominant bee in the Treasure Valley. Bees are stocked at 8000-10000 female bees/acre. Chalk brood is a major problem in this species. This bee nests in holes usually in wood or foam bee boards. Nest is filled with leaves cut from alfalfa plants and eggs are laid in the nest. Alfalfa leaf cutting bee (top - Ken Gray photo), leaf cutting bee nest and bees (bottom TG Chastain photo).
Leaf cutter bee - Bee boards are housed in domiciles in alfalfa seed fields. Foam bee board (top right), close up of domiciles (bottom right), domiciles in alfalfa seed field (below). TG Chastain photos.
Alkali bee These bees are native to the Pacific Northwest. This bee nests in the ground and is not very portable, but is used in parts of the region for alfalfa seed crops. Alkali bee (top- Ken Gray photo), alfalfa seed production area near Walla Walla Washington (bottom TG Chastain photo)
Forage legume flowers have a fused pistil and stamens. This structure is known as the staminal column. The staminal column is released by the foraging bee in a process known as tripping. Tripping is an essential part of the pollination process in forage legume seed production. Honey bee tripping crimson clover flowers (TG Chastain photo)
The bumble bee and leaf cutter bee are most efficient in pollination as they trip many flowers quickly. The alkali bee is less efficient than bumble bees and leaf cutter bees. The honey bee is the least effective in tripping forage legume flowers such as alfalfa. Effectiveness of alfalfa flower tripping by four species of bee pollinators Species Flowers tripped/minute Bumble bee 17 Leaf cutter bee 15 Alkali bee 10 Honey bee 7
Harvest Windrow curing - fields are swathed when most seed pods turn brown. The crop is threshed by using a combine with a pickup header attachment. Spray curing - The crop is sprayed with a chemical desiccant to dry the crop prior to direct combining. Alfalfa seed harvest after spray curing in Washington (top Washington State University photo), combine harvest in California (bottom - University of California photo)