The spilt grain in cereal stubbles contains starch that can cause acidosis if rapidly consumed. Acidosis is caused by the lowering of the pH in the rumen of sheep, leading to part of the microbial population being killed off and a lessened ability by the rumen to process fibrous feed. To minimise the chances of developing acidosis, sheep should be acclimatised to the grain before being put onto ungrazed cereal stubbles.
Spilt cereal grain will not provide enough protein for growing lambs and it is important to feed out some lupin grain as well. This will also help the sheep to better utilise crop residues.
Young sheep on cereal stubbles are unlikely to achieve weight gains of more than 80 grams per head per day but this can be improved by supplementation with 100-150 grams of lupins per head per day. This supplement can be given once a week and can be spread across the paddock using a spreader, or it can be put on a hard surface to avoid contamination. Barley is the most useful cereal stubble and farmers often comment that sheep do better on barley stubbles than on other cereal stubbles. Pastures after barley crops are often much better than those following wheat or oat crops and baled barley stubble has proven to be a good feed source for adult sheep.
The nutritional quality of cereal straw is generally very poor, mainly because of its low digestibility and low nitrogen content and straw alone is seldom able to provide a maintenance diet for sheep. Its main advantage is that it is readily available, cheap roughage that can be used together with other feeds to provide a source of roughage for feeding sheep during summer and autumn.
The digestibility of a feed is the main factor which determines the amount of energy provided by that feed. Concentrates such as barley or lupins are 80-90% digestible and provide a high energy diet, whereas roughages such as straw or hay are generally of lower digestibility (35-55%) and provide less energy. Protein is also an essential component of any diet and different classes of stock have varying nutritional demands for protein. Demands for protein are high during late pregnancy and during lactation and high wool growth rates can only be achieved with a high protein diet. Lupin seeds are about 30% protein and are, therefore, a particularly important source of supplementary protein. Straw, however, contains less than 4-5% protein and an additional source of protein is generally needed with straw diets.
The variations in digestibility between different parts of wheat straw show that the leaf blade and sheath in the stubble have a digestibility of about 59%, whereas the stem material is only 29% digestible. The proportion of leaf material compared with stem is low and sheep preferentially graze the leaf material. Rain during summer and autumn can significantly reduce the digestibility of the stubble, mainly through leaching out the soluble or digestible components of the straw. The nitrogen content of grazed stubbles is also too low to sustain adequate microbial growth in the rumen. This may restrict digestion of dietary fibre and with it the sheep's ability to digest the roughage diet efficiently. An alternative to leaving stubbles in the paddock is to conserve the straw in bales for use later in summer or autumn, when grazing feed supplies are low in quality and quantity.
Stubbles grown in wet areas will be of lower quality than those from drier areas, particularly late in the season when rain may have reduced digestibility further. Farmers should monitor the performance of sheep throughout summer and autumn, either by weighing them or by condition scoring, to provide feeding strategies that will prevent unwanted losses of bodyweight or condition.
Between 150-250 kilograms (kg) of seed per hectare usually remains on the ground after harvest. In fact, rates of 300-400kg per hectare are common. Lupin seed makes excellent sheep feed and this fallen seed may be enough for between one and three months grazing (depending on the stocking rate, development of lupinosis, risk of wind erosion and rainfall).
Typically, weaner sheep show rapid weight gain and wool growth in the first few weeks of grazing stubbles, then reduced gain and weight loss as the seed is used. High seed intakes and rapid growth generally occur during early grazing regardless of stocking rate. However, heavy stocking means that the seed is used up fast, the rapid growth period is short and the weaners lose weight sooner than if stubbles are grazed at lower rates.
Lupin stubbles can be grazed with good gains in weight by about 10 weaners per hectare for up to two months if there are good levels of grain available. Stocking can be higher than 10 weaners per hectare where farmers want to use the lupin stubble early in summer to prevent storms ruining its feed value. Therefore, stocking rates of up to 20 weaners per hectare are more common where summer rain is likely. Good weight gains can be achieved at these higher stocking rates, but the grazing time will be shorter.
As a rough guide, sheep tend to gain weight when there is more than 50kg of lupin seed per hectare (ha) in the stubble and lose weight below this. Generally, four Gungurru seeds in 0.1m2 is equivalent to about 50kg/ha. When seed falls below 50kg/ha, early signs of lupinosis often develop. (Lupinosis is a disease which affects livestock that eat dead lupin stems colonised by the fungus Phomopsis leptostromiformis. The fungus produces toxins, called phomopsins, in warm moist conditions. When consumed, the phomopsins damage the liver which can result in the animal becoming jaundiced.)
Weaner sheep on lupin stubbles, and probably other high protein stubbles, will not travel much more than 600-800 metres from the water supply to graze. This may result in parts of large paddocks being overgrazed and developing the potential to erode. There is also the possibility of stock developing lupinosis. The distant parts of the paddock may have feed that is barely used so multiple or movable water troughs will allow large paddocks to be grazed more evenly.
If canola stubbles contain green leaf and stem material, they will provide high quality feed that is most beneficial to young stock and ewes being flushed for mating. It is common for ewes to be joined on canola stubbles.
Canola stubbles should be grazed before the green material wilts and the plant dies. Because of their high palatability, green canola stubbles will be grazed out quickly and rates of weight gain will decline once the green material has been eaten. Weight gain of stock on dead, brown canola stubble will be no better than that on cereal stubbles, that is, less than 100 grams per head per day.