Stubble material and soil erosion
Sheep eat large amounts of stubble materials, especially when little seed is left. Many of the smaller fragments, such as shattered leaves, are trodden into the soil and wasted. Thus, 3-5 tonnes (t) per hectare (ha) of stubble present at the start of grazing is typically reduced to 1-2t/ha when sheep are removed.
According to soil experts, about 1.5t of material per hectare (150 grams per square metre) is needed to protect soils and minimise the risk of soil erosion. This equates to lupin stubble covering about 40% of the ground. Those with stock need to consider how much feed is available and, with their current stock numbers, whether it will last over the autumn feed gap without reducing groundcover to less than the critical 50%.
Reducing ground cover below this exponentially increases the risk of erosion. Selling stock early can reduce grazing pressure and therefore reduce the likelihood of overgrazing and subsequent soil erosion. If groundcover is getting low and selling is not an option, remaining stock should be held in paddocks with gravelly-surfaced soils, since these are less prone to erosion than trampled and loose sandy or clayey soil.
Loosened soil is only protected from the wind by plant cover that is still attached to the ground. When cover in a paddock is reduced below 50% and the paddock is exposed to winds of 30 kilometres per hour or more, loosened soil starts to move and become sorted. The fine fraction, made up of clay, organic material and nutrients, are lifted and transported out of the paddock as dust and the coarse sand is left behind in the paddock or on fence lines. A vast majority of soil nutrients are stored in the top few centimetres of soil. If the surface 10 millimetres (mm) of soil is sorted by wind, the nutrient-rich dust will be removed, leaving the coarse material behind. Experiments have shown that this can result in a 25% yield reduction for the following crop. If cover is marginal now, then despite removing stock, there remains the risk that further decomposition will result in insufficient ground cover to prevent erosion in autumn next year.
Keep in mind that plants begin decomposing naturally once they die and that sunlight and rain both increase the rate of decomposition.
It is essential to have a supply of good quality water that can meet the demand of sheep grazing on stubble paddocks. Depending on the weather, the feed type and the salt content of the water, weaners can drink up to seven litres per day and adults more than this. Water troughs in stubble paddocks need to be cleaned regularly to encourage sheep to drink enough water so that their performances aren’t affected.
Mineral and trace element supplements
Cereal grains are low in sodium and calcium so sheep grazed on these stubbles for long periods may have an inadequate intake of these essential minerals. However, there is not a lot of evidence that sheep on cereal stubbles suffer any illness or reduced performance specifically because of sodium or calcium deficiencies.
Many farmers provide loose mineral mixes or blocks in case sheep require some mineral or trace element. Provision of salt may assist in the prevention of urinary calculi and ice plant poisoning.
Various trace elements are included in most commercial mineral mixes but there are no proven benefits of using them. The levels of inclusion of the minerals are low enough to ensure there is no risk of poisoning but if sheep are truly deficient in one of the trace elements, the inclusion rate and intake may be inadequate to correct the deficiency. If trace element deficiency is suspected, seek professional advice on its diagnosis and the most effective means of treatment.
Urea supplementation, via block licks, loose mineral mixes or molasses based licks, has not been shown to be commercially helpful when sheep have access to adequate roughage and grain but inadequate protein. The response to urea supplementation is variable and unpredictable. Supplementation with lupins, or other grain legumes, is a more reliable way to raise dietary protein levels.
The risk of lupinosis occurring in sheep grazing lupin stubbles is increased:
- with increasing the time after harvest before sheep are put onto the stubble
- as the amount of lupin seed available declines (particularly below about 50kg/ha)
- if the sheep have no prior experience of eating lupin seed
- following significant rainfall (about 10mm)
- with varieties that are more susceptible to the fungus responsible for the disease (such as Danja). No lupin variety is totally resistant to the growth of the fungus that produces the toxin responsible for lupinosis.
Waterbelly (urinary calculi) in rams and wethers has a number of causes. It occurs most commonly in summer and autumn when many sheep are grazing cereal stubbles. Continuous provision of salt, as a loose mix or in blocks, may aid in the prevention of waterbelly by promoting increased water intake, resulting in more dilute urine.
Health problems in sheep grazing canola stubble in Western Australia (WA) have been rare but farmers should be aware of the following potential problems:
- A small number of WA farmers have reported that sheep on green canola stubble produced a profuse, foul-smelling scour.
- Two WA farms experienced sheep deaths on canola stubble, which subsequently were found to be due to lesser loosestrife poisoning. This plant prefers wet to waterlogged areas.
- Brassicas in general, including canola, can cause nitrate poisoning, pulmonary emphysema and haemolytic anaemia in stock. These conditions have occurred occasionally in sheep in eastern Australia, but not in WA.
Ice plant poisoning generally occurs within a few days after sheep are put into a new stubble paddock, particularly cereal stubbles. The sheep seek out and avidly eat the dead dry plant, perhaps because of its salty taste. Continuous provision of plain salt, in a loose mix or blocks, may satisfy any craving for salt and reduce the likelihood that sheep will eat ice plant.
Grain poisoning, or lactic acidosis, may occur on any stubbles other than canola, though it is least common on lupin and oat stubbles. The risk of acidosis is increased:
- if the sheep have not previously been eating the grain
- if there are heaps of spilt grain or considerable amounts of unharvested grain
- if the sheep are hungry on entry to the paddock.
Other diseases that may occur in sheep that are grazing stubbles include polioencephalomalacia (polio), enterotoxemia (pulpy kidney), annual ryegrass toxicity, scabby mouth, pink eye and caltrop poisoning. These diseases do not have a particular association with any type of stubble and may also occur in sheep in other situations.