A strategy for bush encroachment control

When sweet thorn competes with grazing plants for light and water, it starts to suppress grass production. It is therefore important to implement control measures to manage this encroachment. Dr Louis du Pisani, an independent agricultural consultant, discusses some of the factors that should inform a farmer’s bush encroachment management strategy.

Dr Louis du Pisani

In a previous article on the encroachment of sweet thorn (Vachellia karroo) in the Eastern Cape (see FW, 31 July 2020), it was stated that the encroachment of sweet thorn and other bush species is adversely affecting the production potential of large tracts of rangelands in the Eastern Cape, reducing the profitability and sustainability of livestock production. The article concluded that if bush encroachment continued unchecked, the damage might become irreversible.

Sweet thorn competes with grazing plants for light and water and starts to suppress grass production at a density above 300 tree equivalents (TE) per hectare.

One TE is equal to a tree with a height of 1,5m. The availability of grass for grazers such as cattle and sheep is therefore negatively affected at bush densities above 300 TE/ha.

According to research conducted in the 1980s, when browsers such as goats are incorporated into the livestock production system, the combined grass and shrub grazing capacity starts decreasing at bush densities above 1 320 TE/ha.

TREE DENSITY AND RAINFALL

In a later study, it was determined that the ratio between tree density of sweet thorn and rainfall can be used as a threshold factor to determine when to reduce trees. At a ratio of over 8,23 TE/ha/ mm annual rainfall, grass production is affected, and a reduction of tree numbers should be considered to maintain optimal grass production levels.

Control of sweet thorn is most beneficial in terms of increased grass production in areas of the Eastern Cape with a mean annual rainfall of less than 600mm, while it is less beneficial in the sub-humid areas with a mean annual rainfall in excess of 600mm. However, sweet thorn can become so dense and impenetrable in sub-humid areas that control becomes necessary to allow grazing animals access to these thickets.

Research shows that there is a positive relationship between sweet thorn and grass’s crude protein content, with the presence of sweet thorn enhancing the grazing quality of the grass sward. The total removal of sweet thorns from the system is thus expected to have a negative impact on animal production. Three basic control methods are available: fire, mechanical control with rollers and earthmoving equipment, and chemical control with herbicides, also known as aboricides.