Alkali treatment

The principle behind the treatment of cellulosic substrates with alkali is that it hydrolyses ester bonds between the cell wall polysaccharides (cellulose and hemicellulose) and lignin, rendering the material more susceptible to rumen microbial degradation. Early techniques in the late 19th century were industrial processes requiring both heat and pressure. However, in the Beckmann process, the first on-farm methodology, cereal straw was soaked for up to 2 days in a dilute (1.5%) sodium hydroxide solution, then washed to remove any excess alkali. This technique improved degradability but considerable soluble (i.e. potentially degradable) material was lost during the washing process. The use of more concentrated solutions, either sprayed on to chopped or shredded straw, or applied by dipping baled straw into vats which was then allowed to ‘mature’ for up to a week prior to feeding, reduced these losses. The delay ensured that residual sodium hydroxide had reacted with carbon dioxide, to form sodium carbonate. Because alkali treatment raises the ash content, the apparent digestibility of organic matter improves less than that of dry matter.

The response to treatment varies inversely with the quality of the untreated straw. To realize the potential improvement in degradability, sufficient dietary nitrogen and sulphur must also be provided. Sodium hydroxide is the most commonly applied alkali, though potassium hydroxide (often as wood ash), calcium hydroxide, alkali hydrogen peroxide andcalcium oxide (lime) have all been used. A disadvantage of the technique is that water consumption is increased (a potential drawback in arid regions), leading to increased urine output, which generates a problem with quantity and disposal of bedding. The high urinary output of sodium may damage soil structure.

The technique has also been used to treat cereal grain. The action disrupts the integrity of the seed coat, increasing the accessibility of the starch to the rumen microorganisms without the requirement for physical processing. Conventionally harvested grain is blended with sodium hydroxide, water is then added and the material mixed. This reaction produces considerable heat, following which the grain should be remixed prior to storage. The amount of sodium hydroxide required for optimum digestibility varies with the fibre content of the grain husk. About 25 kg t1 is used with wheat and 40–45 kg t1 for oats. Treated grain can be fed direct or after mixing with water, which causes the seed coat to swell and rupture. The slower release of starch relative to that from ground or rolled grain interferes less with fibre degradation, allowing higher intakes of roughage to be maintained. Residual alkali helps to maintain rumen pH, reducing the incidence of acidosis when high levels of grain are offered. An additional benefit is that sodium hydroxide treatment has a preservative effect on highmoisture grain, reducing both bacterial and fungal growth. Offered to cattle, treated grain maintains a higher rumen pH, tends to increase the acetic:propionic acid ratio, and reduces the incidence of rumenitis in comparison with cattle fed conventionally processed material. Similarly, when high levels are offered to dairy cows, depressions in milk fat content are minimized and roughage intake is maintained.

The requirement for supplemental dietary nitrogen, and the observation that other alkalis also improved digestibility, led to the development of systems using either gaseous (NH3) or aqueous (NH4OH) ammonia. Ammonia is injected into straw stacks sealed with plastic sheeting or film, or into large bales, as either gas (straw must contain at least 10% moisture) or solution (100 l of 300 g NH3 l1). Under temperate summer temperatures the process is generally complete in 4–6 weeks and results in organic matter digestibility increasing from 45% to 55% and intake by anything up to 30%. Nitrogen content is also enhanced (1.4 vs. 0.8% in dry matter), thereby increasing rumen microbial activity and yield. It is recommended that, as nitrogen retention is directly proportional to the straw moisture content, treatment should occur as soon as possible after combining. Treatment with gas can also be undertaken in ‘ovens’. Oven treatment takes only 24 h and enables straw to be treated during periods of cold weather or under winter conditions. An added advantage is that ammonia treatment inhibits spoilage organisms, especially moulds, thereby increasing the storage properties of damp straw.

In tropical environments the high ambient temperatures mean that the treatment of rice, maize or sorghum straws is achieved in 2–3 weeks. Urea, or even urine, can be used as the ammonia source, as the higher temperatures speed the conversion of urea to ammonia by urease enzymes present in straw. Urease levels have been enhanced by the addition of jackbeans to the straw prior to treatment.

Toxic symptoms may arise if high quality forages (e.g. grass or lucerne hay) are ammoniated and offered to ruminants. This takes the form of a hyper-excitability, commonly referred to as ‘crazy cow syndrome’, which is totally unconnected with bovine spongiform encephalopathy (BSE). Roughages with a high carbohydrate content prior to ammoniation are particularly implicated, with the compound generally associated with this effect, 4- methylimidazole, being formed by the interaction of sugars with ammonia in the rumen.