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t
his article is the 22
nd
of a series of articles highlighting a spe-
cific pasture crop species that can play an imperative role
in conservation agriculture (CA)-based crop-pasture rota-
tions. Besides improving the physical, chemical, hydrological
and biological properties of the soil, such species, including
annual or perennial cover crops, can successfully be used as
animal feed.
Livestock production systems are in many ways dependent on the
utilisation of pasture species, in this case as a pasture ley crop, and
can therefore become an integral component of CA-based crop-
pasture rotations. It is imperative to identify a plant species fulfilling
the requirements of a dual purpose crop, i.e. for livestock fodder
and/or soil restoration.
This article focuses on a legume cover crop with pasture crop po-
tential used to improve soil conditions and to provide a high quality
forage for ruminants.
Vigna unguiculata
, cowpea or
akkerboon
Cowpeas are a summer annual legume and a fast grower, reach-
ing heights of 60 cm when grown in favourable conditions. There
are many cultivars and they vary greatly in growth habit. Some are
short, upright bush types and others are tall and vine-like.
In times of drought a cowpea taproot can grow down to 2,5 m (de-
pending on soil type) to reach moisture deeper in the soil profile. The
vining type is preferred for forage or cover crop use, while the bush
type is better suited for direct mechanical harvesting.
Agro ecological distribution
Although cowpeas will tolerate recurrent waterlogging, they will
not grow well if the soil is waterlogged for extended periods. Cow-
peas grow best during summer and the suitable temperature for
germination is in the order of 8°C and for leaf growth 20°C. The opti-
mum temperature for growth and development is around 30°C.
The time of flowering of photosensitive varieties is dependent on
time and location of sowing and may be more than 100 days. Re-
search has also shown that in early flowering varieties, the flowering
period can be extended by warm and moist conditions. Cowpeas
grow well in heat and is more drought-tolerant than many other
crops. It can grow under rainfall ranging from 400 mm to 800 mm
per annum. Well-distributed rainfall is important for the growth and
development of cowpeas.
In certain areas of South Africa, the frequency of rain is very high,
resulting in serious waterlogging while in other areas it is so unreli-
able that moisture conservation remains vitally important for crop
production.
Cowpeas react to severe moisture stress by limiting growth and
reducing leaf area by changing leaf orientation and closing the
stomata. Therefore, ample rainfall is important during the flower-
ing or pod forming stage. Flower and pod abscission during severe
moisture stress also serves as a growth-restricting mechanism.
Cowpea varieties will grow on a wide range of soil types; how-
ever the legume shows a preference for sandy soils, which enable
better root growth. This adaptation to lighter soils is coupled with
drought tolerance through reduced leaf growth, less water loss
through stomata, and leaf movement to reduce light and heat load
under stress.
This crop is more tolerant to infertile and acid soils than many
other crops, requiring a soil pH of 5 to 6. It is also extremely im-
portant to remember that even though cowpeas can survive water-
logged conditions, they prosper in well-drained soil and therefore
struggle on heavier soils.
Management and utilisation
The optimum sowing times are October to December. Early-sown
crops are less erect, more vegetative and have a lower yield than
those sown at the optimum time. It is also imperative that cow-
peas are inoculated with a specific nodular bacteria i.e.
Bradyrhizo-
bium
spp. to ensure effective nodulation for nitrogen fixation, but
this is not necessary if cowpeas or other legumes (mung beans, pea-
nuts and stylos) have previously been grown in the same area.
Land preparation prior to sowing is similar to that of other field
crops. Alternatively, minimum tillage is recommended and sowing
directly into a grass, legume or crop stubble is beneficial. Cowpeas
have large vigorously growing seedlings and are therefore better
suited to minimum tillage practices. A seeding rate of 20 kg/ha -
30 kg/ha should be used, which is needed to obtain optimal dry
matter and grain production yields.
WAYNE TRUTER,
University of Pretoria,
CHRIS DANNHAUSER,
Grass SA,
HENDRIK SMITH,
Grain SA and
GERRIE TRYTSMAN,
ARC-Animal Production Institute
Integrated crop and pasture-based
livestock production systems
– Part 22
AGRICULTURE
Conservat ion
Series
ON FARM LEVEL
Conservation agriculture
Januarie 2016
22