Several factors limit successful hand emasculation and pollination in grain legumes:

Cowpeas are easier to cross than other grain legumes. Flowers are large and easy to manipulate, the keel is straight, beaked and not twisted. Cowpea flowers have few floral nodes per raceme, and tend to have a lower rate of abortion than many other species. Usually 8-12 seeds are produced per cross. Figure 1 and Figure 2 show an inflorescence and parts of a cowpea flower.

Conventional crossing methods have several disadvantages:

Premature flower drops and bud abortion are greatest when the seed plant nears maturation, when the two gametes are incompatible, and when temperatures are high and humidities low.

IITA developed a rapid and effective method of hand emasculating and crossing cowpeas. It consists of removing the upper half of the petals starting with a partial cut opposite the stylar and staminal section. Following pollination with a freshly opened flower, leave the crossed bud uncovered.

The process of emasculation and pollination can be accomplished at the rate of one to two a minute with an average of only 10-20% success. This means a minimum of 2-10 minutes per cross (5-10 individual flowers crossed) to assure success. However, synchronizing flowering under low temperatures and high humidity increases the success of hand crossing to 50%.

Normally, at least 8-10 seeds are produced per developed fruit. Attempts on male sterile (MS2 MS2) plants produce a much higher percentage of success (70-80% on an average) and they require only pollination - no emasculation.