Revell Science | Independent Science Consultant
Water use by shrubs can create ‘reservoirs’ below a belt.
After 3-4 years, the reservoir is probably larger than any single inundation (rain) event.
This means shrub belts can help limit surface water flows during and after rain.
But the 'reservoir effect' does not extend very far away from the shrub belt - most of the effects last only 2-5 metres from the edge of the belt.
If you want to build a bigger reservoir below ground, reduce the inter-row spaces or plant more rows of shrubs in each belt.
… but it’s a balancing act to avoid reducing space allocation for pasture.
If groundwater is hostile to pasture growth (e.g. saline), a small lowering of the water table by shrubs can allow annual pasture species to germinate and establish.
Tree or shrub belts can compete with crops and reduce grain yields for up to 5-10 m from the edge of the belt.
But in terms of forage production, forage shrubs can be considered as an addition, not a substitute.
Think of shrubs as the supplement to the pasture.
Annual grass pasture is reduced when grown alongside shrubs, but the
reduction is less than the forage biomass from the shrubs – so total feed on offer is higher.
The shrub forage plus pasture forage is the ‘systems’ that can work so well for animals. It’s not shrubs or pasture; it’s shrubs with pasture.
The micro-climate can have a large effect on the efficiency of production of both pasture and livestock.
With wind breaks, the’ quiet zone’ extends up to 10 times the height of the wind break.
Shrubs as an under-story can limit the’ wind tunnel’ effect that can occur with tree belts.
Shelter belts reduce wind speed, thereby reducing soil moisture loss by evaporation.
This means that, in dry times, pasture growth can be higher; contrary to the oft-held view that shelter belts will compete for water during dry times.
‘Enrich’ research found shrub belts reduced daily maximum temperatures by 1-1.5°C and increased the minimum daily temp by about 0.5°C; and wind speed reduced by about 20% (at animal height).
If animals are cold they will eat more, but more of the food energy is used to maintain body temp. If animals are hot, they’ll eat less and use energy to sweat or pant, i.e. they will be less efficient producers.
Forage shrubs can provide green feed in summer and autumn when most conventional pastures have low nutritive value.
This means supplementary feed costs are reduced.
Shrubs typically provide crude protein and minerals, but the pasture is the main energy source.
Research with grazing a shrub-pasture mix in autumn, without hand feeding, has achieved sheep growth rates of up to 200 g/head/day.
Grazing shrubs around the break-of-season and into early winter allows you to defer the grazing of annual pastures, which means the pasture is more productive.
A diverse feed base helps livestock meet their nutritional requirements under variable conditions.
Animals must be managed so they have repeated, positive experiences of novel forages to ensure they include them in their diet. There are practical ways this can be achieved.