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Benefits of winter cover crops
in a mixed farming system
W
ith a total of only 14% land suitable for arable cropping,
South Africa is not well endowed with resources for
dryland (rainfed) farming. Soil degradation often occurs
due to monoculture cereal production, intensive tillage
and limited crop rotation.
In conservation agriculture (CA) systems, crop rotation that includes
soybean and sunflower produces limited amount of crop residues
with associated problems. These crops are mostly grown on sandy
soils and erosion is a common phenomenon, especially when crop
residues leave insufficient soil cover during winter. Without suffi-
cient cover these sandy soils often develop crusts after rain events.
The impermeability of these crusts leads to runoff, soil erosion and
eutrophication of water resources. When these conditions prevail
during and after a winter fallow period, arable soil is vulnerable
to degradation, water loss and weed infestation. Winter or cool
season cover crops are seen as an ideal CA practice to combat
these problems.
Cool season cover crops
The most commonly used cover crops fall into one of three broad
groups or families. General information on key aspects of these
three groups is mentioned below.
Small grains (grasses)
This functional group includes grasses like rye, triticale, oats and
black oats (
Photo 1a
and
Photo 1b
). Winter cereals are the most
widely used winter cover crops as part of the crop rotation system.
These crops have a fibrous root system that will benefit the forma-
tion of soil organic matter deep in the soil profile. Oats is preferred
for grazing. Biological production varies between 3 t/ha/year to
5 t/ha/year for the different crops. All crops in this particular func-
tional group will form associations with arbuscular mycorrhiza
fungi (AMF).
Rye, when left to mature, has a high C:N ratio and will form a better
mulch that will cover the soil for long periods. Planting into rye after
termination might lead to the immobilisation of N by microbes in
the process of breaking down the mulch.
Winter cereals are typically planted in late summer through early fall
(February to March). Planting winter cereals as part of a mixture, af-
ter early rain in September in the North Eastern Free State, was also
successful and supplied valuable grazing during November.
Grasses respond well to N fertiliser and increased biomass produc-
tion and root development is common after application. It is sug-
gested that 40 kg - 60 kg N is applied at planting.
Brassicas
Brassica cover crops have grown in popularity recently due to their
ability to provide many of the same benefits as grasses, but with
residues that break down more rapidly after termination. Radish is
by far the most productive cool season crop with between 8 t/ha to
10 t/ha dry matter (DM).
In the water table sandy soils of the North West Free State, it raised
the soil carbon at a depth of 80 cm by the value of 0,3% during a sin-
gle season. Brassicas often used in mixtures include turnips, radish
(fodder and tillage) and kale (
Photo 2
).
63
April 2018
ON FARM LEVEL
Conservation agriculture
Cover crops / Integration of livestock
GERRIE TRYTSMAN,
ARC-Animal Production
1a
1b
2
1a: Winter small grains (grasses) sole stand.
1b: Winter small grains (grasses) delayed intercropping in maize.
2: Brassicas (radish).