SA Graan Januarie 2015

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Januarie 2015
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ON FARM LEVEL Conservation agriculture
The utilisation of forage sorghum is often challenging, as it has an
exponential growth rate and to optimally utilise the quality thereof,
it is required to graze it at a height of 30 cm - 60 cm and maintain it
at that height to be able to control the growth.
The latter reiterates the recommendation to plant smaller areas at
intervals over a period to avoid having all the pasture ready for graz-
ing at the same time. The opportunity also exists to make silage
with the excess growth that the animals are unable to graze in time.
To ensure good regrowth, the pasture should never be grazed or
harvested shorter than 15 cm in height. A combination of rotational
grazing and mechanical harvesting should be used to optimally uti-
lise this pasture.
Management challenges
The most significant challenge that is faced with forage sorghums
is the high levels of prussic acid or hydrocyanic acid (HCN) they
contain. A high risk exist for livestock to experience some form of
prussic (cyanide) poisoning if the animals ingest either new growth
or regrowth of plants soon after harvesting or when the plant
growth is stunted by frost or drought.
The levels of prussic acid vary in different sorghum species and
varieties. One of the main objectives of breeding new cultivars is to
reduce the HCN content of these plants.
Soil conservation and health benefits
A seasonal rotation cover crop, such as forage sorghum, can play
an imperative role in improving soil structure, water infiltration, and
root penetration. Additional benefits include reduced soil crusting,
soil erosion, runoff, and nutrient leaching.
This species can also provide an affordable on-site management op-
tion to enhance the inherent soil organic matter through an extreme-
ly vigorous root system, and secondly a suitable option to dry out
very wet soils that are regularly irrigated either in winter or summer.
Animal production aspects
This high yielding, fast growing, annual pasture species can produce
up to 13 tons of dry matter (DM) per hectare. Research has shown
that the grazing capacity for a well-managed (fertilised and grazed)
forage sorghum pasture can be between 5 LSU/ ha - 10 LSU/ ha for
approximately 100 days with an average daily gain of 0,73 kg/day
- 1,05 kg/day. The crude protein yield of the pasture can vary be-
tween 90 g/kg - 150 g/kg DM, all depending on the level of fertilisa-
tion and rainfall.
Increased crop diversification with
forage sorghum
Forage sorghum is well-suited to be used as an annual cover crop
in a crop rotation system due to its strong root system and high
biomass production. There are many ways to insert forage sorghum
as a cover crop into a crop rotation to increase the diversity.
It can be established in a pure stand or in a mixture with other (cover)
crops or as a full season crop to maximise its beneficial properties.
Forage sorghum can be used for a short period of time in the spring
before the cash crop or later in the fall after a main crop has been
harvested. In South Africa where crop rotations are mostly very
tight, it is often easier to insert a fall cover crop after the harvest of
a short season, cash crop or silage, provided there is enough time,
nutrients and soil water for it to establish before early fall frosts.
In certain situations an annual cold season legume (or mixture) could
be planted directly after termination of the cover crop, such as graz-
ing vetch (Vicia villosa), oats and radish, followed by a cash crop.
After killing the cover crop, weeds in the fallow must be controlled
to conserve soil water to maximise the benefit of the cover crop.
A suitable no-till planter should be used to plant the grain crops
directly into the residues of the cover crop, without any other cultiva-
tion practice disturbing the soil.
Normal integrated fertiliser (based on soil fertility levels and yield
targets), weed and pest control practices should be followed. Dur-
ing the transformation phase after newly established CA grain fields
(e.g. during the first few years), a 30% increase in N fertilisers is
recommended on maize, due to the immobilisation of N in the soil.
Thereafter the availability of N to the plant should be assessed
through an appropriate analysis, and fertiliser application should be
adjusted accordingly.
Conclusion
Integrated livestock production systems can benefit enormously
from such a high yielding, fast growing pasture crop. Its low input
requirement and ability to adapt to a wide range of conditions,
makes this species highly attractive.
Forage sorghum is a good forage crop to use in rotation with other
grain crops. If it is not used intensively for silage production for ex-
ample, but rather extensively as an interim crop for grazing, soil nu-
trient replenishment will take place.
It is always important to recognise that a pasture ley crop’s eco-
nomic value, irrespective if it is a perennial or annual species, is not
visual in monetary terms, as is the case with a cash crop. Its value
is rather seen in a saving, i.e. lower requirement for weed control,
an inexpensive source of good quality roughage to be converted to
animal protein, and subsequently a recycled source of nutrients at a
very low cost.
Lastly, this crop with its high growth rate and yield is indicative of
a significant amount of organic matter returned back to the soil
through root biomass.
For more information, contact Dr Wayne Truter at wayne.truter@
up.ac.za, Prof Chris Dannhauser at Dr Hen-
drik Smith at or Mr Gerrie Trytsman at
References
Donaldson, C.H. 2001. A practical guide to planted pastures. Kalbas Publishers.
ISBN:0-620-27198-1
Dickinson, E.B. 2010. Pasture handbook. Kejafa Knowledge Works. ISBN: 978-
0981-41358-7
CONSERVATION AGRICULTURE
“
Integrated livestock
production systems can
benefit enormously
from such a high yield-
ing, fast growing pas-
ture crop.
“