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ON FARM LEVEL
Conservation agriculture
Certain brassicas are also becoming well known for their ability to
produce a large taproot that is effective in breaking soil compaction.
As brassicas decompose, they produce chemical substances that
can fumigate the soil and kill plant parasitic nematodes.
Brassicas is a non-mycorrhizal plant family and evolved on disturbed
sites with ample available P. When livestock graze on brassica pas-
tures, bloat might occur, but in a mixture, this does not seem to
happen. In a pure stand, infestation of bagrada bugs is a common
occurrence and can destroy the crop.
Legumes
Legumes in association with nitrogen-fixing bacteria have the ability
to ‘fix’ nitrogen from the atmosphere. Inoculation is thus essential.
Some of that ‘fixed’ nitrogen will be available to provide low-cost
nitrogen for the following summer crop, a very important feature
that makes planting of legume cover crops practical and economical.
Legumes are valued as cover crops in integrated systems for their
high feeding value.
Common legumes used as winter cover crops in cropping systems
include vetch (hairy and grazing vetch), sweet clover and crimson
clover. To survive the winter, legumes should be planted at such a
time that it is well developed before first frost.
In a North West Province trial (Ottosdal) vetch was planted as pure
and mixed stands and with a C:N ratio of 15 it contributed to N
mineralisation (
Photo 3a
and
Photo 3b
). The amount of N added
by legumes varies among species, but it is directly proportional to
the amount of biomass produced.
The residues of cool season legumes break down fast after termina-
tion. Legume plants also have the ability to form associations with
mycorrhizas.
Crop mixtures: Why?
Single species leave resources partially unexploited, leaving room
for other species to continue or complete the resource exploitation.
More diverse systems are hence more productive due to increased
niche complementarity or resource use efficiency.
Grass-legume mixtures can be particularly advantageous because
they combine the benefits of both and quickly lead to a good
canopy cover (
Photo 4
). The presence of N-rich legume residues
can also help break down grass residues more quickly. Disadvan-
tages of mixtures can include increased seed cost and more compli-
cated management.
Potential benefits of cool season
cover crops
The potential benefits of cool season cover crops are explained in
Table 1
.
Practical suggestions implementing cool
season cover crops
Planting of cool season cover crops
Depending on the producer’s objectives, three opportunities exist
where cool season cover crops can be established:
Intercropping into standing maize can be an effective establish-
ment method for certain cover crops in short-season environ-
ments. In general, legumes and brassicas with extended seedling
growth are candidates for in-season seeding (typically from De-
cember to January in the summer rainfall area) (
Photo 5
). Plant-
ing cereals (grasses) during this period is not recommended, as
these species have a higher initial light requirement. Spreading
seed with fertiliser spreader or using tailor made drills can be
used when maize is in four to eight leave stage.
Seeding at physiological maturity of grain crops (i.e. during Feb-
ruary or March in maize or soybeans) can provide additional time
for establishing grasses and grass-legume mixtures as relay or
delayed intercropping (
Photo 6
). As the grain crop dries down,
sunlight breaks through the canopy and improves conditions for
germination and early cover crop growth. Seeding cover crops
into mature maize requires high-clearance or aerial seeding
equipment.
Seeding cool season cover crops after soybeans are harvested
can simplify the planting process. In the Vrede area, a mixture of
cool season cover crops was planted after soybeans were har-
vested, followed with 40 mm of rain. Livestock was introduced
later in winter (
Photo 7
).
Putting seed mixtures together
This outcome will depend largely on the producer’s objective. In
the drier North West Province, for example, creating water saving
mulches should be a priority. Rye and black oats can produce resi-
dues that decompose slowly. During the first two months after plant-
ing cash crops, evaporation from the soil surface should be curbed
with soil cover. In areas where rainfall is more reliable, legumes can
reduce input cost by adding N.
A typical mixture for the Vrede and Reitz areas as proposed by
Barenbrug, is as follows:
10 kg/ha black oats
5 kg/ha stooling rye
3 kg/ha vetch
5 kg/ha fodder peas
Benefits of winter cover crops
3a
3b
4
3a: Winter legumes (grazing vetch), sole stand.
3b: Winter legume (vetch) delayed intercropping in maize.
4: Winter cover crop mixture with grasses, brassicas and legumes.