FEATURE
Integrating sheep into crop
rotations can suppress weeds,
save money, and protect the
environment
Chloe Maclaren
Current Address: PhD Student, Centre for Agroecology, Water and Resilience –
Coventry University, Department of Agronomy – Stellenbosch University.
E-mail Address: [email protected]
I
ntegrating sheep into diverse crop
rotations is an effective way to man-
age weeds in conservation agriculture
systems, according to recent findings
from a long-term trial at Langgewens
Research Farm. The trial compared the
weed seedbank in eight different crop
rotation systems over twelve years, with
four systems including legume forage
crops grazed by sheep, and four sys-
tems containing only cash crops and/
or cover crops. The systems containing
sheep not only had fewer weed seeds
present in the soil, but also used sub-
stantially less herbicide and fertiliser
than crop-only systems. This indicates
that integrating sheep into cropping
systems can reduce farm expenses and
contribute to environmental protection.
Crop diversity in these rotation systems
was shown to further improve perfor-
mance: a higher number of crop types
further reduced weed numbers and also
promoted higher wheat yields.
Weed management can be a challenge
under conservation agriculture due to
the loss of tillage as an option to control
weeds. With few alternatives available,
farmers often become reliant on her-
bicides. Within a few years, weeds that
are tolerant of or resistant to herbicides
tend to flourish. In winter cereal crops of
the Western Cape, the most problem-
atic weed in this regard is ryegrass. Even
the use of multiple herbicides does not
always prevent the spread of ryegrass,
as ryegrass is well adapted to tolerate
herbicides in general. It has a waxy leaf
surface that reduces the amount of her-
bicide that penetrates into the plant,
and it also has a variable germination
time that allows it to avoid pre-plant
herbicide applications. Different spe-
cies of ryegrass also readily hybridise
05
with one another, and this genetic mix-
ing allows herbicide resistance to rap-
idly evolve and spread.
Ryegrass is an example that highlights
the need for a diversity of tools in weed
management that each affect weeds
in different ways. In the Langgewens
study, rotation systems with a combi-
nation of grazing, herbicides and high
crop diversity had the fewest weeds.
These systems also had the highest di-
versity of weeds, which can be impor-
tant to support farmland wildlife (and
to add a scatter of colourful flowers to a
farm!). A low number of weeds and high
weed diversity indicates that the pres-
sures faced by weeds differ each year.
Differing impacts from herbicides, crop
rotation, fertilisers and livestock reduce
the chances that any single weed spe-
cies encounters favourable conditions
to increase its population each year,
and so weed numbers are limited. In
addition, different impacts also reduce
the chance that any species will be
knocked back enough each year to be-
come extinct within the farm. Therefore,
using different weed management tac-
tics means a diversity of weeds can be
maintained at low populations: benefi-
cial to wildlife and with minimal impact
on crops.
It is important to note however that
crop and management diversity be-
tween years is more important than
diversity within years for weed manage-
ment. For example, many studies have
shown that using tillage and herbicides
together is often not more effective
than using herbicides alone. Ryegrass
has shown us that using mixtures of dif-
ferent herbicides can delay resistance
for a while, but not forever: it is harder
to resist two active ingredients at once,
but consistent selection to do so will
eventually promote this ability. In con-
trast, studies where management varies
between years have shown much better
weed control – such as in the Langgew-
ens study, in systems where high herbi-
cide cash crop years alternate with low
herbicide use and grazing. This effect is
not limited to conservation agriculture
nor to field crop systems. Similar results
would be expected for organic farm-
ers alternating tillage and grazing, and
alternating vineyard and orchard floor
management practices should also lead
to better weed management in those
systems.
In the Langgewens trial, ryegrass as
well as knotweed increased in cash crop
phases and reached very high numbers
in the seedbanks of crop-only systems.
However, both of these species are pal-
atable to sheep and were reduced in the
grazing systems. To fully embrace this
effect, farmers could view ryegrass and
knotweed as forage species promoted
by their cash crops, to be exploited
during forage phases. Meanwhile, the
forage phases are an important tool to
manage weeds, whilst also offering eco-
nomic benefits through reducing herbi-
cide and fertiliser requirements, and
diversifying farm products. The lower
use of herbicides and fertilisers in the
grazed systems also reduces pollution
and reduces health risks to farmers and
their staff.
Overall, integrating sheep into field
cropping systems seems to be a win-
win solution for farmers and the envi-
ronment. However, not everyone can
or wants to farm livestock. In that case,
crop diversity can still improve weed
Grassroots
Vol 18
No 4
December 2018