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Vegetable Seed Production PDF Print E-mail
Contributed by gunew   
Feb 28, 2007 at 11:32 PM

In most cases, seed crop growing in vegetable seed production requires environmental conditions that are very different from those for the harvest of the market-use vegetable crop. For example, ambient temperature, photoperiod, length of growing season, and precipitation amount and distribution may vary for the optimum production of market-use and seed crops. The following represents general requirements and considerations utilized in vegetable seed production.

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Planting

The planting of vegetable seeds for seed production is generally done by direct drilling or transplanting of green house grown seedlings. Biennial vegetable seed production, however, can be achieved using two planting techniques: "seed-to-seed" method vs "root-to-seed" method.

Vegetable seed crops must be planted so that sufficient vegetative development occurs to support optimum fruit and seed development. In addition, the increasing emphasis on hybrid seed production necessitates planting of male and female lines at times that assure successful pollination, a process known as nicking. This may require that genetically different male and female lines be planted at differing times so that synchronous flowering occurs.

Vegetable seed production also is unique from most agronomic crops because vegetables such as beets, carrots, and cabbage are biennials and must develop sufficient vegetative growth prior to cool temperature exposure in order that vernalization successfully induces flower formation the following season. Planting such crops too early causes winter kill or late season pest infestations. Planting too late results in a lack of vernalization which limits flowering and reduces seed yield.

Vegetable seeds also vary greatly in their tolerance to soil temperatures at planting. Pea, radish, and spinach require cool soil temperatures for optimum seedling emergence. Beets, cabbage, carrot, and onion are tolerant of cool soil temperatures although they do better in warmer soils. Squash and melons require warm soil temperatures for optimum seedling emergence. In regions with high summer temperatures, vegetables such as tomato, pepper, eggplant, and cucurbits are planted in the early spring to optimize flowering and seed set in early summer. Later plantings of these seed crops are possible by transplanting young plants previously started in glass houses. This avoids the delayed establishment of plants from direct seedings due to cooler night temperatures and wetter soils encountered in the early spring.

Row spacings and planting densities of vegetable seed crops differ from those for fresh market production. Sufficient space for flower development, air movement to reduce pathogens, unrestricted access to inflorescences by pollinators, mechanical cultivation, and harvest operations are necessary for seed crops. The method of pollination (wind or insect) affects row spacings and planting densities. Row spacings and planting densities are also important considerations in the production of vegetable seed hybrids. In order to maximize the amount of hybrid seed produced, the optimum ratio of female to male rows must be determined.

Fertilization

Vegetable seed crops have similar nutrient requirements as fresh market crops. Where possible, nutrients that stimulate reproductive development can be supplemented with normal fertility regimes. Boron, for example, increases the number of fruiting sites and seed produced per pod in legumes such as pea and bean. Molybdenum also is essential for optimum nitrogen fixation of legume seed crops.

Another approach to stimulating reproductive development in vegetable seed crops is to split the application of fertilizers at planting and before flowering. This avoids luxury consumption of the elements at the time of application, improves crop uptake efficiency, and increases the formation of reproductive structures. Generally, lower nitrogen and phosphorus levels are used in seed production because these encourage continued vegetative growth. Because most vegetable seed crops are grown in rows, fertilizers are typically applied either in bands in the planting row or as a side-dressing beside the row after establishment.

Weed and Pest Control

The management of weeds and pests in vegetable seed production poses additional problems due to their longer growing season not encountered for vegetables grown for market use. This extended growing period necessitates an increase in residual pesticide activity or additional pesticide applications. In some cases, governmental regulations do not permit the use of the same pesticide for a vegetable seed crop that is used for the market use crop. This is because the approval process is directed at the more economically important value crop. Some pesticides which control insects also detrimentally affect insect pollinators essential for cross pollination or hybrid seed production and cannot be used in a seed production field. Such restrictions on the use of pesticides in vegetable seed production imply that the safe use of pesticides will be more difficult in the future and greater reliance on crop rotations and biological control will be necessary.

Harvesting, Threshing, and Drying

Vegetable seed crops are divided into two categories based on the type of fruit harvested. Dry-seeded fruits include the brassicas, legumes, and onion and wet-seeded fruits include the cucurbits, melons, and tomatoes. The methods of harvest and extraction of seeds vary dependent on the type of fruit harvested.

Of great concern in dry-seeded fruit vegetable seed production is the potential for loss of yield due to seed shattering when harvesting occurs after physiological maturity. Farmers visually monitor the development of the dry-seeded fruit and harvest the crop before shattering occurs. In indeterminate crops such as carrot, seeds mature in sequential order with those on the primary umbel maturing first followed by those on secondary and tertiary umbels. Seeds produced from the first lettuce flowers also tend to be larger and of higher quality than seeds produced from later flowers.

Seed harvest from wet-seeded fruits is often determined by fruit color. Seed quality in pepper as determined by percentage germination and rate of germination is higher in seeds harvested from red fruits compared to those harvested from green fruits. In tomatoes, maximum seed germination is attained when the fruits are at a mature green stage compared to those which are at the red or fully-ripe stages.

After the fruits are determined to be at the correct stage for maximum seed quality, they are gathered and the seed extracted. In tomato, cucumber, and some melons, a mucilaginous layer around the seed must be removed prior to cleaning. This can be done by two methods. The first, and most common, is the retention of seeds in the crushed fruit pulp for a few hours to more than a day so that fermentation occurs. This process is closely monitored because the fermentation creates heat and mechanical injury to the seeds if continued too long. The second approach is using of sodium carbonate or hydrochloric acid.

Cleaning and Storage

The physical quality of the seed lot following harvesting, threshing, and drying must be evaluated to determine whether precleaning is necessary. If the seed lot possesses large amounts of inert material such as stones or plant debris, it should be passed over a scalping machine. If the seed possesses undesired appendages which cause the seeds to clump together such as carrot, then it must be passed through a debearder to ensure free-flow of individual seed units. Impact debearders are also used in multigerm beet seeds to split the fruit into single seed units.

After precleaning, vegetable seed crops are conditioned in the same way as other seed crops using the same principles and equipment. Some additional improvements can be made. For example, greater seed density improves the seed quality of lettuce, tomato, and onion crops. This can be accomplished by placing the seed mass into a solution of specific osmotic concentration. The more dense seed fractions are retained and the more buoyant seeds removed. After cleaning is completed, vegetable seeds are stored using the principles of successful seed storage.


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Last Updated ( Mar 28, 2007 at 09:34 AM )
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