Cowpea forage is a source of protein and is quite digestible for ruminants (OM digestibility more than 60%) (Anele et al., 2011a; Cook et al., 2005). It is suitable for growing, fattening and lactating animals, including dairy cows (Mullen, 1999). Dual-purpose varieties, although lower in protein than forage-type varieties, require little or no input, and provide sufficient biomass in marginal lands, without additional fertilizer, to provide a livestock feed supplement during the dry season (Anele et al., 2011a). Maize-cowpea intercrops may have considerable potential as forage: intercrops have a higher DM digestibility than maize or cowpea grown alone, they are richer in protein than maize alone and are higher in water-soluble carbohydrates than cowpea alone. The optimum forage quality occurs at the milky stage (Dahmardeh et al., 2009a). Because animals tend to consume selectively the leafy parts, intake decreases as leaf availability declines, which highlights the importance of the leaf component for yield, quality and animal production (Mullen, 1999).
Cowpea haulms can provide adequate protein and energy to sustain ruminant production during an extended dry season (Anele et al., 2011b). They are often used for sheep as a supplement for poor quality basal diets (Anele et al., 2010). DM digestibility is about 65-70% (Karachi et al., 2004; Savadogo et al., 2000b), and differs greatly between leaves (60-75%) and stems (50-60%). Because of this difference, the proportions of leaves and stems in the haulm directly affect its nutritional value (Mullen, 1999; Singh et al., 2010).
Most studies on cowpea haulms have been done with sheep given the haulms to supplement roughage-based diets. Intake of cowpea haulms by sheep can reach 86 g OM/kg BW0.75/d, and the selective consumption of leaves results in higher intakes of protein and digestible OM than expected from the offered haulms (Savadogo et al., 2000a). Rams ate up to 60 g OM/kg BW0.75/d of cowpea haulms as a supplement to sorghum stover. Although supplementation decreased total DM intake, this was compensated by an increase in stover digestibility (Savadogo et al., 2000b). In sheep fed 200-400 g/d of cowpea haulms as a supplement to a basal diet of sorghum stover, the resulting average live-weight gain (80 g/d) was twice that obtained with sorghum fodder alone (Singh et al., 2003). In male Ethiopian Highland sheep, supplementation of maize stover with cowpea haulms (150 or 300 g DM/d) improved DM and protein intake, OM digestibility, average daily gain, final live weight, carcass cold weight and dressing percentage. Because the N retention, as a percentage of N intake, was higher when cowpeas were offered at a low level, it may increase efficiency to offer smaller quantities over an increased period, especially where resources are limited (Koralagama et al., 2008). Cowpea haulms were used as a supplement for West African dwarf sheep fed a basal diet of Pennisetum purpureum (Anele et al., 2010).
Whole cowpea plant
Cowpea can be used as whole plant. Its digestibility appears to vary little with the crop maturity or environmental changes (Mullen, 1999).
In Australia, cowpea was intensively grazed by steers without any adverse effect on live-weight gain during late summer to early autumn (1200 kg/ha/d) (Holzknecht et al., 2000). However, in India, cowpea did not regrow adequately to provide late autumn grazing (Singh et al., 2010). In the South-East USA, cowpea was incorporated in a subtropical grass pasture for grazing cows and calves, but did not persist in July and August (Vendramini et al., 2012).
Intercropping of maize and cowpea at a seed ratio of 70:30 increased fodder production and produced silage of high digestibility (higher than maize silage alone supplemented with urea) when harvested at the heading stage, i.e. about 35% DM (Azim et al., 2000).
In Ethiopia, in crossbred growing steers, cowpea hay was given at 1.5 kg (30% diet) to supplement a hay diet, and resulted in live-weight gain of nearly 250 g/d (Varvikko et al., 1992). Fed to steers at 1% of body weight in cereal-legume cropping systems, cowpea hay led to live-weight gains of 280 to 373 g/d, depending on the cropping system (Umunna et al., 1997). In calves fed teff straw, cowpea hay supplemented at up to 1.5% BW was found as efficient as lablab hay (Lablab purpureus) to improve DM intake, rumen ammonia concentration and teff straw degradability (Abule et al., 1995). In India, cowpea hay was fed ad libitum to lambs supplemented with barley grain (Singh et al., 2010). In Zimbabwe, it was used as a supplement (at 30% of the diet) to improve ME intake and microbial protein supply when the lambs consume low-quality forages such as maize stover (Chakeredza et al., 2002). In South Africa, cowpea hay was given as a supplement (50, 100, 150 and 200 g/day) to Pedi goats fed ad libitum buffalo grass hay (Paspalum conjugatum). Some cultivars had high amounts of condensed tannins, but these did not exert negative effects on intake and digestibility (Ravhuhali et al., 2011).