Straws and related by-products

To convert 4- 5 toimes of fresh grass to 1 tonne of dried material requires about 300 1 of oil, and so although dried forages could be regarded as excellent feeds for ruminants, the high cost of preparing them restricts their use to speciality feeds for non-ruminants. In Britain, much of the dried forage is consumed by horses. Elsewhere, and especially in the USA, dried lucerne is used for poultry as a source of vitamins and also to provide xanthophyU as an egg-yolk colourant. 

wheat and rice crops are the main sources of world straw supplies, but in the UK barley provides much of the 15-20 per cent of total straw production that is used for animal feeding. When oats were grown for horses, oat straw was preferred for animal feeding, but the decline in the area of this crop has made its straw less important. 

Of the cereal straws, oat straw used to be popular in many areas of the UK as a bulky food for store cattle, along with roots and concentrates, and in limited quantities as a source of fibre for dairy cows. A dth the increasing use of barley grain as a major concentrate food for farm animals, especially in northern Europe, large quantities of barley straw are available and attention has concentrated in recent years on methods of trying to improve the nutritional value of this low-grade material. 

The digestibility of the organic matter of these straws rarely exceeds 0.5 and the metabolisable energy value is about 7 MJ/kg DM or, in the case of winter barley cultivars, less than this. Of the ash fractions, silica is the main component and straws generally are poor sources of essential mineral elements, as can be seen from the results of a comparison between hays and barley straws shown in Table 20.5. 

Apart from the low digestibility of these cereal straws, a major disadvantage is the low intake obtained when they are given to ruminant animals. Whereas a 650 kg cow will consume up to 12 kg of medium-quality hay, it will eat only about 9 kg of straw. Improvements in both digestibUity and intake can be obtained by the addition of nitrogen in the form of protein or urea. 

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Copper is widely distributed in foods, and imder normal conditions the diet of farm animals is likely to contain adequate amoimts.The copper content of crops is related to some extent to the soil copper level, but it is also affected by other factors such as drainage conditions and the herbage species. Seeds and seed by-products are usually rich in copper, but straws contain little. The normal copper content of pasture ranges from about 4 mg/kg to 8 mg/kg DM.The copper content of milk is low, and hence it is customary when dosing young animals, especially piglets, with an iron salt to include a trace of copper sulphate. 

It should be pointed out that the raw materials for VAM and its related polymers (i.e. ethylene and acetic acid) are produced from fossil resources, mainly crude oil. It is possible to completely substitute the feedstock for these raw materials and switch to ethanol, which can be produced from renewable resources like sugar cane, com, or preferably straw and other non-food parts of plants. Having that in mind, the whole production of PVAc, that nowadays is based on traditional fossil resources, could be switched to a renewable, sustainable and C02-neutral production process based on bioethanol, as shown in Fig. 3. If the vinyl acetate circle can be closed by the important steps of biodegradation or hydrolysis and biodegradation of vinyl ester-based polymers back to carbon dioxide, then a tmly sustainable material circle can be established. 

Figure 6.4.4 and Table 6.4.2 provide gas chromatographic and gas chromatographic-mass spectral information relating to the thioacidolysis products derived from an extractive-free wheat straw and the corresponding lignin isolated by alkali (Scalbert 1984). 

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