and local customs, but specialist pea seed producers tend to use a N:P:K ratio of 3:1:2 which is a higher nitrogen level than that normally used for the market crop. Work by Browning and George (1981a) has indicated that increased seed yield can be obtained with relatively high levels of N and P. In addition, seeds produced from the higher N and P mother plant regimes were found on analy- sis to contain higher levels of N and P. However, the seeds from the higher nitrogen regimes in the same experiments were found to be less vigorous when subjected to the conductivity test (PGRO, 1978). This indicated that nutrient regimes for seed yield differ from those required to produce high quality seed.
Peas are very susceptible to manganese deficiency especially on wet soils with relatively high organic levels. The symptoms of the deficiency are brownish hol- low centres to the cotyledon seen when unripe seeds are split open. The condition is commonly called ‘marsh spot’. Manganese sulfate is included in base dressings at a rate of 40–100 kg/ha where manganese deficiency is known to occur.
Alternatively, manganese sulfate is applied as a foliar spray at a rate of 10 kg/ha in 200–1000 l of water but this must be applied as soon as the symptoms are diagnosed, otherwise it will be too late to improve seed quality and yield.
Another indication of low vigour in pea seeds is the incidence of ‘hollow heart’ and ‘bleaching’. The former is characterized by a cavity of dead tissue on the adaxial surface of the cotyledons (Fig. 11.1) and the later by a yellowing of the green seeds as a result of chlorophyll bleaching (Maguire et al., 1973). It is generally accepted that both these conditions are caused by high temperatures during maturity or seed drying (Perry and Harrison, 1973). During the course of the investigations on mother plant nutrition by Browning and George (1981b) it was shown that the low nitrogen and high phosphorus regime pre- disposed developing seeds to hollow heart whereas the high nitrogen regimes predisposed them to bleaching.
Irrigation
Salter and Goode (1967) reviewed the literature related to the irrigation of peas and found that most workers have reported a moisture-sensitive stage from the
Fig. 11.1. Pea seeds with hollow heart compared with normal seeds.
start of anthesis until petal fall. This moisture-sensitive stage is very evident regardless of the amount of available water before or after flowering. Moisture deficits prior to anthesis only affect the weight of haulm produced but shortage of water during pod growth can also affect yield.
The general practice in temperate regions is, therefore, to ensure that adequate irrigation is available during and after flowering, whereas in arid areas and dry seasons in the tropics sufficient water should also be given to ensure satisfactory plant growth before flowering commences.
Sowing
Pea seeds are directly drilled into the field relatively early in the spring at the rate of 150 kg/ha in rows 40 cm apart, although sowing rate and plant density depend on the seed size and growth habit of the cultivar. Some North American seed producers use a sowing rate of up to 250 kg/ha and a much shorter dis- tance between rows for the final multiplication stage. The higher plant densities reduce the time span of pod maturity but increase the difficulty of roguing.
Flowering
This species is a quantitative long-day plant. The earlier-cropping cultivars gen- erally produce flowers on lower nodes than the taller and later types.
Pollination and isolation
Pea flowers are almost totally self-pollinated. This occurs in the late bud stage before the flower is completely open.
The seed purity requirements are generally higher for cultivars produced for vegetable crop production than for stock feed or other agricultural purposes.
Recommended isolation distances for peas are relatively short in many countries and aim mainly to prevent admixture during harvesting. It is impor- tant that adjacent cultivars should be at least 20 m apart with the distance increased to at least 100 m for basic seed production.
Roguing stages
1. After emergence (when plants are approximately 15 cm high): remove the taller off-types. For basic seed production particular attention is given to check- ing that foliage, including stipules, is typical of the cultivar.
2. Flowering: remove early flowering plants from late flowering cultivars, check flower colour and remove any plants with flower colour which is not true to type. Check flower number per node.
3. Pods formed: check that pod shape, size and colour are typical of the culti- var; remove late flowering plants; remove non- or low-yielding plants.
Harvesting
The harvesting operation normally commences when the majority of pods have become parchment-like. By this stage the maturity of individual seeds is sufficiently advanced for them not to be adversely affected by subsequent dry- ing. Biddle and King (1977) showed that the seed quality is reduced if seeds are harvested when their moisture content is above 30–36%. However, mature seed with a moisture content of 12% or less is subject to mechanical damage.
Combining
In areas where the seeds dry sufficiently on the plants the crop is harvested by direct combining. A relatively low drum speed is used and care taken to avoid mechanical damage to the seeds.
Desiccants are used by some pea seed producers in areas where the pre- harvest drying-off of the plant material is relatively slow. They are applied when the crop starts to senesce and the lowest pods have turned pale brown and parchment-like. The usual rate of application, depending on product is 3 l in 100 l of water sprayed on at the stage when random samples of seeds have a moisture content of 40%.
Crops which have been treated with a desiccant are either combined directly when the moisture content of seeds is approximately 28%, or cut into windrows 4–5 days following application of the desiccant.
If combining is not possible due to unavailability of machines or if the pro- duction scale is too small, an alternative is to cut the crop when the earliest peas have dried to the parchment stage and the foliage is starting to dry off, characterized by a reduction in the intensity of green in the leaves and haulm.
Sample pods should contain fully developed seeds which are firm, taste starchy and are readily detachable from their pods.
Windrowed crops are usually turned, although each time the windrows are turned some seed is likely to be lost by shattering.
An alternative to windrows is to cut and stack the crop on to tripods. This increases the rate of drying and reduces the time in which the seeds can dete- riorate as a result of pathogen activity but the method has a relatively high labour content.
Material from windrows or tripods is threshed when the seeds’ moisture content is approximately 30% using a thresher with rubber-covered tines.
Seed yield and 1000 grain weight
The average yield of a pea seed crop is approximately 2000 kg/ha.
The 1000 grain weight varies from the small to the larger seeded types. But a guideline is 330 g for the larger seeded types to 150 g for the smaller ones.
Pathogens
The main seed-borne pathogens of Pisum sativum are listed in Table 11.1.