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The seed can either be broadcast or drill seeded in 18 cm or 40 cm
rows. Pre-planting cultivation is an adequate control measure for
hay and green manure crops, as cowpeas are vigorous twining
legumes which smother and choke out weeds if the recommended
sowing rates are used, resulting in well-established crops.
Since phosphorus is an important nutrient for leguminous crops,
South African soils are often phosphorus deficient, and superphos-
phate applications of 100 kg/ha - 250 kg/ha are recommended while
potash should be fertilised at 50 kg/ha - 150 kg/ha prior to sowing.
Utilising cowpeas as hay crop is becoming more common by the
day. The perfect time to make hay from cowpeas is at peak flowering
stage, which generally occurs 70 to 100 days after sowing.
Most cowpea crops will mature at the same time whether sown
early or late. Those crops sown earlier have the opportunity to
produce higher hay yields because of a longer growing season.
Studies have shown that cowpea hay yields are generally between
2 500 kg/ha - 6 000 kg/ha, depending on the growing and manage-
ment conditions.
When using cowpeas as a cover/green manure crop, it can be in-
corporated/rolled at any time when sufficient green material is
available, but it is best done at the time of peak flowering when the
material is of high quality and will decompose quickly in the soil.
Soil conservation and health benefits
Research regarding the contribution of cowpeas to soil improve-
ment has shown that cowpeas can produce about 4,5 tons of dry
matter per hectare, each ton containing approximately 30 kg of
nitrogen. The nitrogen content of cowpeas is about 1,5% - 3,5%.
Producers can take advantage of the natural ability of cowpeas
and other legumes to fix atmospheric nitrogen. For good legume
growth and nitrogen fixation, the soil should have adequate phos-
phorus, a good balance of micronutrients (iron, sulphur and mo-
lybdenum), appropriate pH, and good aeration (no compaction or
waterlogging). Cowpeas are sensitive to zinc deficiency. Cowpeas’
N contribution to a cropping system when incorporated is approxi-
mately 120 kg/ha.
Additional benefits are obtained from the organic matter content of
the incorporated plant material. This organic matter helps improve
soil structure, water infiltration, and soil water holding capacity.
Organic matter will also increase the ability of the soil to hold plant
nutrients by improving the soil cation exchange capacity.
Organic matter provided by legumes is quickly decomposed by
soil microorganisms and does not persist for long in the soil. To
build up the humus content of the soil, select slowly decomposing,
fibrous grasses as green manure, or use grass-legume mixtures.
Possible grasses for mixture with cowpeas include sorghum sudan-
grass hybrids. Long-term incorporation of annual legumes such as
cowpeas will increase the soil humus content, but at a slower rate
than would grass cover crops (Valenzuela and Smith, 2002).
Management challenges
Studies have shown that diseases such as powdery mildew (during
wet winter months and under humid conditions), leaf spot, rust and
mosaic virus have been recorded in cowpeas. Control measures are
normally not necessary or practicable. Guinea fowl are also known
to have a liking for cowpea seedlings once emerged and can be very
destructive.
Animal production aspects
Harvesting cowpeas later in the season will ensure higher forage
yields, but will also result in a lower quality. Nevertheless, cowpeas
have a higher forage quality than some subtropical annual grass-
es such as millet and sudan grass. Many studies have shown that
cowpeas can have a high crude protein concentration, varying be-
tween 19% and 24%, depending on the stage of harvest and the
management applied.
Cowpea forage has been successfully fed to beef cattle all over
the world and often by small scale producers in Africa. To date,
no known reports exist of poor palatability or anti-quality problems.
The most appropriate method to preserve the quality of this crop
is to make haylage or silage. Cowpea hay tends to be very similar
to soybean hay, being very stemmy. This characteristic limits the
fast curing of the plant material, taking longer to dry.
The following results were obtained from a trial in the Dundee dis-
trict (KwaZulu-Natal). The total rainfall during the summer season
of 2003/2004 was 473,6 mm – well below the long term average of
745 mm per annum that falls mainly during summer.
Grazing commenced on 5 April 2004 and ended on 1 June 2004,
when all the available grazing material was utilised. Mutton Merino
ewes (with their lambs), as well as dry sheep, were used to graze
the cowpeas during the mentioned period of 57 days. During the
grazing period pods were already formed.
The nutritional value of the whole plant was as follows: Acid deter-
gent fibre (ADF) 29,07%, neutral detergent fibre (NDF) 43,5 and pro-
tein 15,46%.
The calculated carrying capacity for a two month (60 days) grazing
period was 15,1 small stock units (SSU)/ha/60 days. The following
assumptions were made in the calculations: One ewe, with an aver-
age mass of 65,3 kg, was taken as the equivalent of 1,28 SSU, one
lamb (average 19,3 kg) was taken as 0,51 SSU and one dry sheep
(average 47 kg) was taken as 1 SSU.
The lambs grew on average 278,9 g/day and they gain on aver-
age 15,9 kg/lamb for the grazing period. This is highly acceptable
for lambs on this type of dryland pasture. Although in a lactating
phase, the ewes also managed to gain weight (38,6 g/day) and that
will contribute to their body condition in the next breeding season.
Conclusion
Cowpeas as an annual legume crop, offer many opportunities for
CA based cropping systems. This crop makes a significant contribu-
tion as a leguminous crop through N fixation. With cowpeas com-
monly known as a drought tolerant crop, much attention is given to
using this crop to produce high quality forage in a drier environment.
Therefore, cowpeas have many benefits such as providing high
quality green manure to improve soil quality, it is a strong competi-
tor with weeds and finally provides high quality forage for the live-
stock production component of an integrated crop and pasture
based livestock production system.
Lastly, seeding cowpeas with a subtropical grain crop such as maize
can be extremely valuable in the long term for such a crop rotation
system.
For more information, contact Dr Wayne Truter at wayne.
Prof Chris Dannhauser at
Dr Hendrik Smith at or Mr Gerrie
Trytsman at
References
Van Zyl, EA and Dannhauser, CS (2004). Cowpeas as a fodder crop. SA Co-op
Vol 22 (2), November 2004.
Valenzuela, H. and Smith, J. (2002). Cowpea. Green Manure Crops: Cowpea CTAHR.
2SA-GM-6
CONSERVATION AGRICULTURE
ON FARM LEVEL Conservation agriculture
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January 2016