KEY POINTS
- Sheep can only consume a small proportion of the dry matter available in stubbles.
- Spilt grain is the most nutritious component of the diet of sheep grazing cereal stubbles.
- Adult dry sheep can only maintain liveweight for a short period of time when grazing dry stubbles alone.
- Young sheep will only maintain liveweight when spilt grain and green material are also available.
- To minimise weight loss of adult sheep and maintain liveweight of young sheep, supplementation of stubbles with cereal grain or lupin seed will be necessary.
Introduction
Stubbles are a major feed source for sheep during the summer-autumn period in Western Australia.
While there is often up to three tonnes of dry plant material per hectare available, sheep grazing stubbles eat only about six per cent of it. Leaf material (59 per cent digestible) makes up about a quarter of the stubble material, while the stem material (29 per cent digestible) accounts for just under half. The large amount of indigestible stem material reduces the overall value of stubbles.
When grazing cereal stubbles, the aim is to maintain the liveweight of young sheep, to minimise the weight loss of adults and to maximise utilisation of nutrients available from stubbles. There are large losses of edible material caused by shattering during harvest and trampling as sheep selectively graze the stubble paddocks. Microbial breakdown and wind also contribute to the loss of the high quality components of stubbles.
The energy content of the dry plant material determines how much the stock can eat. About a quarter of fresh wheat stubble has a digestibility of more than 55 per cent, one third has a digestibility between 50 to 55 per cent and the rest is indigestible stem material.
Young sheep (25 to 30 kilograms) will start to lose weight once they are eating stubble material below 55 per cent digestibility which is the threshold for maintenance of liveweight.
Composition of stubbles
Leaf material and spilt grain provide the most nutritious components of the diet of sheep grazing cereal and legume stubbles. The average nutritive values of stubbles sampled in Western Australia are recorded in Table 1. The availability of small amounts of green material (weeds or crop regrowth) following summer rains will also increase the productivity of animals that graze stubbles. The sheep’s diet will consist of more than 80 per cent green material when there is as little as 40 kilograms per hectare of green material in the stubble.
Mature sheep grazed on stubbles may initially increase in liveweight because of intensive selection for these more digestible parts of fresh stubbles (grain, weeds and leaf material). However, lighter or younger sheep are unlikely to maintain liveweight for as long, unless considerable grain and/or ‘green pick’ are available because they simply cannot eat enough of the nutrients they need for growth. Adult dry sheep (55 kilograms) grazed on crop residues will only maintain their liveweight for 60 to 100 sheep grazing days per hectare. Young sheep (25 to 30 kilograms) will not maintain liveweight on dry plant material alone and will require supplementation within 7 to 14 days to avoid weight loss.
Table 1. The dry matter, energy, protein and fibre content (dry matter basis) of straws and stubbles fed to sheep. The average across the range of values is shown in brackets.
Supplementation is needed to maintain the weight of young sheep on stubbles as well as to improve the efficiency with which nutrients within the stubble material are converted to liveweight. The aim of supplementation is to stimulate the sheep to eat more of the stubble by providing a protein-rich feed such as lupins. The protein and soluble carbohydrate in the lupins stimulate the microbial population in the rumen which can then process the incoming stubble more quickly. It is important to use the dry feed available in stubbles before it is lost through summer rain and trampling.
Young sheep (25 to 30 kilograms) should be fed 50 to 75 grams of lupins per day from the first or second week on stubbles. This should help to increase their intake of higher quality stubble components such as spilt grain and leaf material. However, the intake of lupins needs to be monitored to ensure that the sheep are not substituting them for stubble. After week two, the lupin supplement will need to be increased to 150 grams per head per day because the more digestible stubble components will usually have been eaten by this time.
Cereal stubbles
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. If possible, lambs should be moved regularly to new stubbles so that they can have maximum access to spilt grain and leaf material. An appropriate mineral mix should be fed to overcome any possible deficiencies of sodium, calcium, and sulphur.
After the removal of weaners, the stubbles can usually be grazed for a while by older sheep.
Sheep on Canola stubbles.
Canola stubbles
Observations on the performances of sheep indicate that canola stubbles do have some value as a stock feed but information collected so far is limited. Any green pick arising after swathing or from late maturing plants will increase the value of the stubble, particularly for young sheep. Liveweights of young sheep on canola stubble increased by up to two kilograms over 12 days (the shortest period of weight gain) to 42 days (longest period of weight gain) on several farms located in the wheatbelt. Grazing time was extended with an increase in the proportion of green or partgreen canola stems in the stubble.
Also of importance was the availability of small green branches at the start of grazing. Where there has been good weed control, most canola stubbles only have dry, hard and brittle stems. Sheep eat some of the residual canola seed in the swathed rows but its influence on their performance is not known. Supplementation with grain is usually required to maintain the liveweights of young sheep grazed on canola stubbles.
Chaff Cart Heaps
Recent developments in cart collection systems have improved the availability of the higher quality components of cereal crop residues by separating harvested material into leafrich and stem-rich components. The average nutritive values of the material in chaff-cart residues in Western Australia are shown in Table 2.
The heaps from the chaff cart collection system provide an accessible source of feed for sheep at discrete locations within a paddock. The average energy content of the collected material is generally enough to maintain the liveweight of adult dry sheep.
Table 2. The dry matter, energy, protein and fibre content (dry matter basis) of chaffcart residues. The average across the range of various values is shown in brackets.
When cereal paddocks are harvested about half a tonne per hectare of chaff cart residues is available for stock to graze while on the stubbles. Swathing the crop increases both the amount of material collected by chaff carts and the capture of weed seeds.
There has been some monitoring of sheep given access to the harvested crop residue during summer on a limited number of farms throughout the wheatbelt. The number of grazing days in paddocks with cart heaps was extended by up to 31 per cent compared with those stubble paddocks without chaff cart residues.
In large stubble paddocks, providing strategically located chaff heaps improves the utilisation of the material available over the whole stubble paddock. Sheep are attracted to graze these chaff heaps particularly when they are located beyond the over-grazed areas surrounding a watering point.
Monitoring sheep performance on stubbles
There is a wide variation in the voluntary intake, ingestion of nutrients and performance of sheep when grazed on dry cereal and lupin stubbles. It is essential to weigh, or at least condition-score, a sample of 50 animals in a flock on monthly basis to determine when the value of the stubble has decreased and supplementation is necessary for the maintenance of liveweight.
Further Reading
Aitchinson, E. (1988). Cereal straw and stubble as sheep feed. Journal of Agriculture, Western Australia. 29: 96-101.
Croker, K.P. and Suiter, R.J. (1977). Stocking rates for weaner sheep on standing crops. Journal of Agriculture, Western Australia. 18: 21-23.
Jacob, R. (1984). Utilisation of cereal stubble for sheep feed. In ‘How much wheat do sheep grow’ (Western Australian Department of Agriculture and North Midland Branch of the Australian Society of Animal Production: Northam. WA).
Purser, D.B. (1983). The nutritional value of stubbles. In ‘Stubble Utilisation’ (Rural and Allied Industries: Perth. WA. pp 13-26).
Warren, B. (1991). Sheep performance on cereal stubbles. In ‘Stubble management in farming systems’ (Technical Report No 40, Western Australian Department of Agriculture. pp 44-52).
