The application of ammonia treatment techniques for forages still remains limited in tropical countries for the main reasons which are as follows:
anhydrous ammonia is rarely available locally and importation only for treatment is difficult to conceive,
as has been shown in the previous Chapter, this treatment technique requires costly equipment (special storage containers, transport means and plastic sheeting) and an infrastructure of rural roads to the farms which is often lacking,
the treatment requires a certain level of skill which the farmer is unlikely to have acquired. Thus relying upon assistance of an extension agent, the farmer will not have full independence when using the treatment,
and finally, the treatment is not without hazards: handling anhydrous ammonia, a toxic gas, is a delicate procedure requiring equipment in perfect state of repair.
However, this same ammonia can alternatively be generated from urea without any risk, this being commercially available as a fertilizer (46 N). This source of ammonia has distinct advantages over the latter being universally available, easy to transport, store and handle and also being less costly. The majority of countries in Africa and Asia use it as a fertilizer for food crops (maize, millet, sorghum, rice,…). It is thus readily available.
Treating, using urea as the source for generating ammonia, is a technique which can be easily mastered by farmers. This consists of spraying a solution of urea onto the dry mass of forage and covering with materials available locally so as to form a hermetic seal.
In the presence of water and the enzyme known as urease, and if the ambient temperature is sufficiently warm, the urea hydrolyses into gaseous ammonia and carbonic gas through reaction with the enzyme which may be described in a simplified manner as follows:
|CO (NH2)2 + H2O||------->||2 NH3 + CO2|
|Once hydrolysis is completed|
|one molecule of urea(i.e. 60 g) generates two molecules|
|of ammonia (i.e. 34 g).|
|5 kg of urea thus allows production of 2.83 kg of ammonia.|
The ammonia thus generated provokes the (alkaline) reaction which gradually spreads and treats the mass of forage. It acts in just the same way on the vegetal matter as if anhydrous ammonia is used:
dissolving the parietal carbohydrates (mainly the hemicelluloses),
swelling the vegetal matter in an aqueous environment, so easing access by the rumen's cellulolytic microorganisms,
reducing the physical strength of the cells, so easing mastication by the animal and digestion by the microbes,
enriching the forage in nitrogen, as is also the case if anhydrous ammonia is used.
Net effects on the forage are increase to its digestibility (by 8 to 12 points), to its nitrogen content (which will be more than doubled), to the intake (by 25 to 50 %), and thus to the nutritive value.
Treating with urea is based upon its transformation into ammonia. In order for the treatment to succeed, most of the urea must first be hydrolysed into ammonia and then this must diffuse correctly so fixing itself to the forage and modifying it chemically. One must therefore ensure favourable conditions for both good ureolysis and good ammonia treatment, in the knowledge that these two processes take place simultaneously within the forage matter.
Having already described the conditions for a successful alkaline reaction, the objective of this Chapter is to study in detail the conditions needed for successful ureolysis and noting how this can eventually influence the action of ammonia treatment.
Hydrolysis of urea is an enzyme reaction which can only take place if urease is present, an enzyme which “slices” the urea molecule. This reaction is very complex.
Treatment conditions should thus favour the development of ureolytic bacteria within the forage being treated: moisture, temperature, treatment duration, at the same time deterring any microorganisms which might cause mould or decomposition.
The only case (see § 423 below) where it might be necessary to artificially add urease is for urea treatment which is carried out with very little water at low or even fresh ambient temperatures.
Early straw treatment with urea formed the basis for numerous controversial discussions concerning optimum application rates, as the degree of ureolysis used to be underestimated and by that, the amount of ammonia produced - this being the only agent responsible for the alkaline treatment efficiency (see Chapter 3). It is now well established that optimum application rates lie between 4 and 6 kg of urea per 100 kg of straw matter, which corresponds to treating by ammonia in the range of 2.4 to 3.4 kg NH3 per 100 kg of straw (for example: straw with a DM content of 90 % would require an application rate of ammonia falling between 2.7 and 3.8 kg per 100 kg of straw DM).
Higher urea application rates do not bring about a significant increase to the nutritive value of the straw (SCHIERE and IBRAHIM, 1989). These authors suggested in Sri Lanka, application rates of 4 % for urea. The controversy certainly merits further study when one considers the following aspects:
treating with urea requires a moisture content higher than that for treating with anhydrous ammonia; for equivalent ammonia application rates, treatment is then more efficient and there is then a tendency to be able to reduce urea application rates;
the alkaline treatment through use of urea, although more efficient, is slower than that caused by ammonia as it takes place in the presence of intermediate compounds from the ammonia production process (carbamates). These slow down the rate of nitrogen fixation and hydrolysis of the cell walls (SAHANOUNE, 1990). Most research studies concerning urea have been undertaken in the warmer regions, thus with relatively short treatment periods (from 10 days to 3 weeks). Even if ambient temperature manages to speed up the treatment (see § 424) it is possible that it may not be fully accomplished. The “response” to the urea application might also not be fully achieved. The immediate conclusion is therefore not to treat with more than modest application rates.
as has been shown (SAHANOUNE, 1990) hydrolysis of the urea may stop or falter when large amounts of free ammonia (not yet fixed) build up within the forage matter. It is therefore quite likely that for high urea application rates and short treatment times, response to the urea will be limited, not all of it having had sufficient time for transformation.
and finally, response to the alkaline treatment depends upon the initial quality of the forage (see Chapter 3). It is highly likely for example, that urea concentrations considered optimal for treating certain kinds of rice straw will not prove suitable for straw from wheat or certain natural pastures.
Moreover, high application rates increase treatment costs to the farmer who has only limited financial resources.
Efforts have been made to reduce urea application rates to 2 or 3 % by mixing it with lime, Ca(OH)2 which favours the urea hydrolysis and above all, the alkaline reaction. Work concerning this aspect is still at an experimental stage and references referring to the response of the animals are still too few to allow extension of the technique (ZAMAN and OWEN, 1990; WANG and FENG, 1993; KAYOULI, 1994). A recent trial in Vietnam (BUI VAN CHINH et al., 1994) concerned rice straw treated with 2.5 kg of urea, 0.5 kg of lime and 0.5 kg of salt, yielding interesting results for crossbred cattle (§ 7332).
Hydrolysis of the urea can only occur if water is present. The amount of water to be added to the forage is thus a determining factor for the success of the treatment.
Urea hydrolysis works better the more water is available. There are however practical limitations to the amount of water because the reaction takes place in a complex environment within which the urea solution is incorporated. Moreover few publications have been presented concerning studies to understand ureolysis in a heterogenous environment (water plus forage) (but see WILLIAMS et al., 1984 a and b; SAHANOUNE et al., 1991 and 1992; YAMEOGO et al., 1993; amongst others).
This is why results of research which has attempted to define ideal water quantity requirements have sometimes been contradictory, both in temperate and tropical countries (WILLIAMS et al., 1984; WAISS et al., 1972; SUNDSTOL et al., 1978; SCHIERE and IBRAHIM, 1989; MANDELL et al., 1988; SAHNOUNE et al., 1989; REDDY et al., 1989; ALHASSAN and ALIYU, 1991; STIEFEL et al., 1991; MAO et al., 1991; NYARKO et al., 1993; CHENOST, 1993; KAYOULI, 1988, 1994 a and b).
The decisive practical element concerns less the quantity of water added than the highest degree of moisture content which can be achieved. Considering the wide scale results of research work, trials and observations. one may conclude that in order to achieve complete ureolysis in a heterogenous environment:
the final moisture content of treated forage must never be less than 30 % (or in other words, final dry matter content of the treated forage must never be more than 70 %).
The reasoning behind these moisture content limits include the following aspects:
it is practically impossible to apply too much water to the straw. Water losses simply due to runoff would be high and furthermore, the forage matter would become too saturated and soft,
the generated ammonia must correctly diffuse throughout the mass of forage. But ammonia is hygroscopic and so risks being “trapped” by the water before fixing itself to the cell walls,
a forage mass which is too wet will favour mould growth if hermetic sealing is not perfect,
excessive water application encourages leaching of the urea towards the bottom of the silo in the case of forages which are only slightly absorbent, provoking an over concentration of urea or even rotting of the forage situated towards the bottom of the silo, increasing the risk of poisoning the animals.
the moisture helps compaction of the forage which helps to drive out the air, thus increasing the concentration of ammonia as long as the treatment enclosure is well sealed. This aspect of texture/compaction will be referred to below in § 43.
The final moisture content should never be greater than 50 %.
Ideally, it should lie in the range of 30 to 50 %.
Table 3 shows a specific example of the minimum and maximum amounts of water which should be added to straw according to its dry matter content. Only slight variation in the DM causes considerable differences in the amount of water which is needed. One should be extremely vigilant to this matter in tropical regions of the Sahel where not only is the straw or natural pasture very dry (with a DM often above 92 %) but also the humidity level of the air is extremely low, favouring rapid and intense evaporation rates. One must also be vigilant, but in the opposite sense, in temperate regions with damp forages (with a DM of only 85 % or less).
The decision as to whether to add more water (for very dry straw in a dry climate) or less water (to straw which is damp) will depend upon due consideration of the factors specific to each situation:
don't restrict the amount of water used if this is not a limiting factor: firstly, compaction will be easier and secondly, maintaining a correct moisture level within the forage can only be of assistance when the climate is very dry and materials available for coverage are in short supply.
don't add too much water if the straw is damp and the local climate humid, so risking to further moisten the forages.
Ambient temperature plays a determining role concerning treatment duration because of its effect on a number of factors:
the development of ureolytic bacteria,
|Water to add (1/100 kg of straw)||Initial DM content of the straw (%)||Final Moisture Content (%)|
in the case of treating with 5 kg of urea per 100 kg of bulk straw
Example: Calculation of the amount of water to add to straw having a DM content of 90 %, treated with 5 kg of urea, in order to achieve a final moisture content of 30 %
|Moisture content||= 30% i.e. DM = 70%|
|DM %||= (90 + 5)/(100 + 5 + x) = 70/100|
|= dry weight (straw + urea)/total weight (straw + urea + added water)|
|x||= 30 litres|
the speed and the intensity of the ureolytic reaction (this speed doubles for a temperature rise of 10°C, conversely it slows to half speed when the temperature falls 10°C),
the efficiency of the alkaline treatment.
The alkaline treatment (see § 32) is correctly accomplished after a week when ambient temperature is 30°C or more; from one to four weeks are needed for temperatures between 15 and 30°C. Because of the remarks already made in § 32, these treatment periods are the same for urea treatment as long as in the meantime the ureolytic reaction proceeds normally.
The ideal ambient temperature for ureolysis is between 30 and 40°C (30°C is also the reference temperature for titrating urea through action by urease in the laboratory).
For temperatures above 25 or 30°C ureolysis is accomplished in the heterogeneous environment after a few days or even less if moisture content is not a limiting factor. It follows that for ambient temperatures from 30 to 40°C, maximum treatment efficiency is achieved after a week. Studies in India have shown that for rice straw treated with 4 to 5 kg of urea and 60 litres of water per 100 kg of straw, similar treatment efficiency was observed for treatment times of 8, 5 and even 4 days (STIEFEL et al., 1991).
At lower ambient temperatures activity of the ureolytic bacteria is slowed down as is the ureolysis. It follows that in more temperate zones or on high tropical plateaus such as in Madagascar or the Kilimanjaro region, despite high daytime temperatures, the nights can be very fresh (even with risk of frost) when treatment is undertaken and so longer periods are required. These are basically the same as those necessary for successful alkaline treatment. A treatment time of 3 weeks is generally found acceptable as one finds only slight traces of residual urea and acceptable rise in digestibility (see Table 4). For safety, five week periods have been recommended for these regions. In fact, as we have shown (Figure 12) for straw treated during the month of September in Auvergne (France), when nights are fresh, the treatment efficiency, as measured in terms of Digestibility in vivo, continues to improve over time. Observations in China have shown that the hydrolysis of urea was almost complete after three weeks at 25°C, whilst at 15°C it was necessary to await two months for correct treatment (MAO et al., 1991).
|DM (%)||Crude Protein content (% DM)||Estimated digestibility|
|TOGO||Maize stalks (4)*||Pit||5||50||nd||nd||90||nd||3.8||7.5||40.0||48.0||S|
|NIGER||Sorghum stalks(18)*||Banco &Seko||5||50||nd||78||90||nd||5.0**||10.0**||38.0**||47.0||S|
|NIGER||Rice straw (28)**||Banco &Seko||5||50||nd||92||90||nd||4.0||11.0||39.0||50.0||S|
|CAMBODIA||Rice straw (20)*||Trad. stack||5||50||nd||84||90||nd||3.1||9.0||38.0||48.0||S|
|MADAGASCAR||Rice straw (13)*||Pit||6||80||5||nd||92||61||4.8||8.0||28.4||42.7||C|
|NIGER||Bush straw (20)*||Banco &Seko||5||60||nd||86||90||nd||3.5||10.0||36.0||45.0||S|
|LAOS||Rice straw (7)*||bamboo||5||50||3||82||89||nd||3.9||9.8||40.6||50.4||S|
* = No. of samples
NT = Non treated
S = Digestibility in sacco
** = Standard deviation
T = Treted
C = Rexen Ce;;i;ase Digestibility (1977)
nd = not detrmined
Figure 12: Evolution over time of the organic matter digestibility in vivo (OMD) for a wheat straw treated only with urea (U), with a crude soybean supplement (US), with a supplement of molasses (UM) or a supplement of both soybean and molasses (USSM). Comparison with straw which has not been treated (NT) and with straw treated with aqueous ammonia (NH3) (Chenost and Besle, 1993).
(The quantities used for treating 100 kg of straw with a 90 % DM content were: urea: 6 kg, molasses: 14 kg, soybean 1.2 kg, NH3: 3.5 kg. The amounts of water were calculated so that a final moisture content for each treatment would reach 25 %).
Adding an additional source of urease such as soyabean flour or even better Canavalia ensiformis (Jack bean) which is rich in the enzyme, speeds up the reaction of ureolysis compensating for any urease deficiency and consequently reduces the overall treatment time required (SAHNOUNE et al., 1991; CHENOST and BESLE, 1992). No such additional urease has been shown to be necessary when treatment is carried out at temperatures above 25°C, particularly when moisture content is above 25 to 30 % (WILLIAMS et al., 1984 a and b; SAHAOUNE et al., 1991; IBRAHIM et al., 1984; CHERMITI, 1994).
In practice: ambient temperature in tropical countries is high and is not a limiting factor for treatment with urea, except in the particular case at high altitude. Adding supplementary urease, which can involve additional cost to the farmer, is unnecessary for the moisture contents achieved in practice. Tropical temperatures are ideal for ureolysis and the alkaline treatment if concentrations of 5 kg of urea are dissolved in 50 litres of water and incorporated into every 100 kg of dry forages,
for two weeks in dry tropical regions or on the humid plains,
for three to five weeks in mountainous tropical regions or in Mediterranean climates where night temperatures can fall considerably.
Treating with ammonia responds better the more the forage is of poor quality (§ 324). It follows that the same situation applies for urea treatment (CHENOST and DULPHY, 1987; TUAH et al., 1986; KIANGI et al., 1981; DIAS DA SILVA et al., 1990; BA, 1993; COLUCCI et al.,1992; SCHIERE and IBRAHIM, 1989).
As a general rule, the quality of barley straw is better than that from either durum or soft wheat. Thus in Morocco, certain specialists recommend not to treat barley straw but rather to put greater effort into treating wheat straw.
It is generally recommended to treat only forages which are “dead” (shrivelled and no longer green) and which are not damp so as to avoid possible mistakes of applying either too much water or urea. A number of trials have been carried out in Cameroon concerning conservation of damp hay (LHOSTE, personal communication) and in Burkina Faso for treating millet and sorghum straw just after harvest (DM 60 %) (ACHARD, personal communication); these showed success when only small amounts of urea solution were applied. One should however be very careful before generalising this practice.
The last factor for successful treatment concerns the degree of hermetic sealing of the treatment environment, both from the point of view of avoiding losses of the urea solution introduced or of the ammonia generated and also to ensure an anaerobic environment (serving as a guarantee against the development of mould within the damp treated forage mass). In effect, the ammonia which is lighter than air, diffuses throughout the forage and has a tendency to escape if the forage is not sufficiently compacted and the forage heap not sufficiently air tight. The pressure generated progressively by the ammonia from the urea is, however, much lower than when treatment is undertaken by injecting anhydrous ammonia gas.
The objective of this section is to recall the salient points which should be considered when putting this technique into practice. These should be adapted to local conditions, be as simple and efficient as possible and be in accordance with the basic principals described above.
The combined experimental results and experience gained at field level under various agro-climatic and socio-economic conditions, particularly in Africa, Asia and Madagascar, clearly show that:
there is no standard, universal rule which may be applied, rather reasoned methods designed according to specific conditions of each situation.
These methods have been described in various publications and reports, from which a few examples will now be drawn and described (SCHIERE and IBRAHIM, 1989, DOLBERG et al., 1981 a et b; KAYOULI, 1988, 1924 a and b; CHENOST, 1993, 1995).
The strategy and type of treatment to adopt will depend upon three main principals:
the physical condition of the straw or forage to be treated:
in bulk: chopped or in long stalks,
in bunches: made either manually or by machine,
the amount of forage to be treated which will depend upon the number of animals to be fed and for how long the feed will be given,
the material and financial resources available to the livestock farmer (which concerns the crop calendar).
The treated forage can be conserved for several months as long as it is well sealed up again after each time it is opened to take out some feed. Although it is not always feasible, it might be possible to treat all the forage in a single session, sufficient for feed requirements throughout the entire dry season.
There is thus likely to be need for different types of treatment and storage methods. Accordingly, various alternative methods will be described first, followed by the practical details.
The pit or trench
These are the cheapest and simplest enclosures but also the ones for which serious mistakes can be made if they are not well thought out and put into practice.
Construction should only be planned in heavy, firm soils so that the cut is clean and the walls will not crumble; they should be excavated on higher ground where there is low risk of water entering by runoff (avoid depressions) or by underground infiltration (avoid proximity to rice fields or the water table level). Light or sandy soils should be avoided as the walls crumble and collapse and for cases where the treated forage will be fed during the rainy season (with consequent risks of infiltration and mould development).
Pits of this type have proved their worth in certain countries (Tanzania, Madagascar, …) where the ferrolytic soils are coherent and in which farmers are very familiar with digging techniques (particularly in Madagascar where they use the “angady”, a long straight spade). The holes can eventually be covered with banana leaves (see Photo 7) or with layers of plastic sheeting (Photo 8).
The pits should not be made too deep (giving problems for retrieving the material) nor too large (presenting problems for adequate cover and risk of exposure to the eventual rains). Dimensions of 2 m × 1 m with a 1 m depth are ideal.
This is thus a method to use when only small amounts of forage are to be treated (from 200 to 300 kg).
Pits or trenches are not generally recommended by the authors due to the amount of work needed to dig them and to the risk of inundation during the rainy season which will cause a loss of forage and/or deterioration of the pit.
This is still a hole (without any constructed walls) but it is dug into a mound or sloping ground. The two advantages as compared to a pit or trench are direct access and much lower risks of water contamination.
Again, firm clay soils are required. This type of trench is well adapted to the high plateau region of Madagascar where local topography is perfectly appropriate.
Compared with either holes or trenches, a corridor implies elevation of the sides (true walls). Silo costs depend upon their size and significant on the material used for constructing the walls. They may be constructed form cured bricks or breeze blocks.
These silos are efficient but involve certain investment costs (bricks, cement) which the farmer might not always be able to bear.
A local technique of making silos has been developed in Niger and is now practised on a wide scale (KAYOULI, 1988); the method has now been taken up in other African countries (including Mauritania and Madagascar). It consists of constructing the silos walls from banco (see Photo 9). Banco is a mud/straw mix (or mud mixed with another fibrous material) which is made in rural areas and often used for rural buildings (houses, grain stores, compound enclosures, etc, …). The silo can either be built alone or alongside an existing compound wall (see Photo 10), in this latter case it would only require two lateral walls. If the silo is large, a small access door can be arranged along one of the walls to facilitate the treatment operations and for retrieving the forage. Silos made in banco are very efficient. They are well adapted to the Sahel region and sudano climatic zones of Africa where they are in current use.
Silos made from stalks of millet, sorghum or maize
Farmers in the Sahel region often gather some of the stalks from cereal crops for making fences and for domestic constructions. These principles have been developed in certain areas of Niger so as to use traditional woven fencing materials such as millet stalks, to construct silos with walls similar to those of their compounds. These fences are reinforced by the farmers, with wood stakes and covered on the inside with mats of plaited Andropogon gayanus (known locally as “séko”) (Photo 11) so as to obtain an air tight seal. This method is particularly well adapted to the pastoral zones of the Sahel.
Bamboo (from the Malaysian word “bambu”) is a very common tree in many Asian countries (and elsewhere). It is often used for rural constructions, enclosures, etc.… Large bamboo containers or cylinders (called “panniers”) for which the side walls are pretreated with dung from cattle or buffalo to improve hermetic sealing, have been successfully used for straw treatment in Bangladesh (DOLBERG et al., 1981a and b).
Silos made from timber or bamboo
Most small farmers in Thailand, Laos, Vietnam and Cambodia store their rice straw in enclosures made either from timber or bamboo poles. These enclosures, covered with a slanting roof, are often built up on stilts or on wooden platforms to avoid penetration by runoff water and accessibility to the animals. This type of storage system is used with success in Laos for straw treated with urea. Hermetic sealing of the walls is ensured by covering them with locally available materials such as banana leaves, coconut palm branches, used fertilizer sacking, plaited bamboo matting (traditional fences) or even the straw itself (see Photos 13 – 16).
Existing but unused buildings: houses, stores, …
Any solid construction made from banco or cereal stalks can of course be used for urea treatment, assuring that the walls are air tight.
A storage method for straw gathered in bulk was recently successfully developed in Niger (see PEYRE de FABREGUES and DALIBARD, 1990). It consists of stacks which are consolidated with a wire netting protection of the “Ursus” type, which remain uncovered but rounded at the top so as to avoid rain infiltration. This type of stack can be used for urea treatment as long as hermetic sealing is provided by coating the exterior with mud, a dung mixture or banco.
As has been shown in demonstration trials in Syria and Jordan (Photo 17), small amounts of forage can be treated in buty1 sacks which, although certainly involving an initial investment, can at least be reused, are durable and are easy to transport. Typical sack dimensions of from 1 to 1.5 m in diameter and 1.5 m tall (giving a potential storage volume of between 1.5 and 2.25 m3) allow treatment of between 150 and 200 kg of chopped straw when the sacks are filled to a depth of about 1 m (which leaves about 50 cm of free sacking for tying them up).
Traditional stacks of straw made from bunches
In some Asian countries and in Madagascar, farmers make small bunches of rice straw weighing between 200 and 300 g. They pile them up in crossed layers, building rectangular or rounded stacks with inclined roofing tops shaped from the bunches (Photo 18).
Such stacks are sufficiently compacted and air tight to allow treatment of each layer by spraying whilst the stack is built. The authors have successfully practised this method in Cambodia under the FAO Project TCP/Cambodia/2254(E).
In Madagascar, where weather conditions are very unpredictable before the dry season commences, the authors successfully used treatment techniques for bunches placed in pits or trenches.
Stacks made from bales
Forage balers are still rarely used in Africa or Asia, except for special cases such as the FAO projects targeted at “Mowing/Baling” in Mauritania and “Milk Production” in Tanzania. In contrast, baling machinery is already well distributed in North Africa and the Near East.
In all of these countries and regions, urea treatment is accomplished by spraying the stack, layer by layer, whilst it is constructed. Stack height should remain of reasonable size (from four to five layers) so as to facilitate unloading after treatment. This straw treatment technique has now already been successfully introduced into Morocco (Photo 20), Jordan and to other Mediterranean countries. It has also been tried out on a large scale in Tanzania (Photo 21) in the region of Kilimanjaro/Arusha (CHENOST et al., 1993) as well as in Portugal (see DIAS DA SILVA et al., 1988).
Covering the stacks with mud
Plastic sheeting is costly and cannot be always reused; various alternative solutions have already been considered and continue to be a focus of interest.
Farmers in certain regions of the Middle East and North Africa have adopted the practice of storing their straw in the field in traditional stacks, covering the top (shaped as a roof sloping in two directions) and the sides with mud. It seems logical to think that this type of covering could be efficient for treatment with urea where the release of ammonia would be slow and would build up little pressure. First trials have already been carried, showing promising results in Tunisia (Ben Salem et al., in press).
In Mauritania where plastic sheeting is not available and where the very low air humidity imposes the need for good hermetic sealing, treatment methods for mown and baled forage have tended to make use of banco, a mixture of mud and straw; this is either applied fresh in a manner similar to that practised in North Africa, or as bricks which are used to make corridor silos.
It should be noted that mud can also be used for stacks of straw which has not been baled.
The special case of round bales
This is only recalled briefly as the technique has only been used in France for conservation of spaths (CHENOST et al., 1986) and maize stalks (CHENOST et al., 1991) and on an experimental basis, for the mechanised field treatment of straw with very low moisture content (CHENOST and BESLE, 1992).
Urea is gradually added to the round bales as they are made with an applicator positioned above the pick-up mechanism of the baler (Photo 22); it is applied as a solid to the spaths, stalks and damp forages (DM close to 40 %), or as a solution of water, urea and crude soybean flour (see § 424) sprayed onto straw or dry forages through a series of nozzles. The round bales of stalks, spaths or straw are then placed individually inside large plastic bags.
These are described below in Appendix 1.
A - Treating small quantities within walled enclosures
This is the most common procedure. Because it is difficult to treat chopped straw in a heap or in large stacks (cohesion of the forage mass) one aims to only treat the minimum amount needed by the animals over a predetermined period of time. This should be at least equal to the time required to complete treatment of a second batch which can then be opened up once the first batch has been used up. This method then continues for additional batches and it is seen that a “battery” of two enclosures are needed (Photos 9 and 10). The system has been introduced in Egypt where it is know as the “three wall system” and the turn-round time for each treatment is generally 3 weeks (see § 424).
In order to determine the overall volume and dimensions needed for the enclosure, one needs to know the amount of forage required and its density once it is placed in the enclosure.
Cattle feeding on treated forage, voluntarily consume about 2.0 kg of dry matter (DM) per 100 kg liveweight and per day. Thus a 300 kg cow needs 6 kg per day, including any losses between the enclosure and the feed trough.
For example: a farmer has to feed his two cows, each with a liveweight of 300 kg. How much (dry) forage must he treat to last 3 weeks?
which for 2 cows over 21 days is:
6 kg x 2 cows x 21 days = approximately 250 kg
- volume requirements (density):
The authors' experience shows that, depending upon how much effort is put into compacting the forage in the pit, trench or corridor, and according to the amount of moisture added, straw or natural bulk forage will reach a density of between 80 and 120 kg/m3 when it is dry at the start. For finely chopped straw the density can easily reach over 100 kg/m3. This is known as “tibin”in the Near East where it is obtained after the grain has been removed by stationary threshers which chop the straw.
In the above example one would therefore need to plan a silo or a hole for the straw, having a volume of between 2.5 and 3.0 m3.
The shape should afford an access width as narrow as possible compared to the overall length so that the forage can easily be sealed tight again after each occasion treated forage is taken out, so avoiding entry of too much air. Thus the length of the enclosure will be proportional to the total amount of forage to be treated.
B - Treatment of large amounts of forage in stacks or heaps
Treatment of large quantities is normally done in a stack, rather similar to the method with anhydrous ammonia. The liquid solution is applied to the bales, layer by layer and the stack covered with a hermetic cover. This treatment method will be described later in the text. The size of the stack will depend upon the following factors:
the size and the density of the bales. Classic bales measuring 35 x 50 cm and 80 cm long, baled to medium density (100 kg/m3) generally weigh about 10 to 15 kg each.
the size of available plastic sheeting. This aspect has already been mentioned above (§ 32) in the context of treating stacks with anhydrous ammonia.
Treatment is best undertaken at the start of the dry season, just after harvest. This is because water and forage supplies are still available at this time and the farmer has both time available and ready cash for purchasing the urea. It is also possible to only handle the straw on a single occasion, the treatment operation being carried out whilst the traditional stack is being constructed.
In addition, the weather is still not too hot and the physical work involved should be easier to bear. In most cases, a family of 4 can treat about a ton of straw in 4 hours. Obviously the work can be much better organised if it is well planned out several days beforehand.
Urine can be used as a source of urea. In effect, although it consists of 90 to 95 % water, urea is the most important of the solid constituents and makes up more than 70 % of the nitrogen content in the urine. However the make-up of urine tends to be very variable. Its composition depends upon the amount of water ingested, the amount and quality of the proteins ingested and upon the concentration of energy in the ration (which affects the efficiency of utilisation of the proteins). It also depends upon the animal species and their general physiological condition. For domestic mammals, the urea content of urine ranges from 2 to 25 g/litre (DIAS DA SILVA, 1993).
First trials concerning the use of urine as a urea source for straw treatment were carried out in South East Asia in the early 1980's. According to a review undertaken by DIAS DA SILVA (1993), results concerning treatment with urine obtained by different authors have been variable. Application was made at a straw/urine rate (by weight) of between 1:1 and 1:3. This means that the straw moisture content is sometimes raised quite high. Measurements made either on the animal or in the laboratory concerning improvement to the forage digestibility gave variable results although they could reach levels similar to those obtained by classic urea treatment methods. Despite this, the amount of urea given through such treatments is less than that with the classic technique. It seems that moisture content plays a favourable role concerning the treatment efficiency but acceptability of the treated material is not always improved. The reviewer concludes that further detailed studies are required before urine treatment techniques can be put into practice (DIAS DA SILVA, 1993).
Finally, one of the reasons why this treatment method has not really been developed stems from the purely practical difficulties involved in collection and storage of the urine.
It is clear that urea treatment constitutes a simple and efficient technique which is not onerous. It is flexible and can be adapted to various situations, each quite different to the other. Urea treatment techniques have been sufficiently well tried amongst rural communities as to have had any mystery removed. Essentially, one should consider all the various factors which might influence success and which have been detailed above, comparing them with any constraints which may have to be overcome.
One point which will not be further discussed by the authors in this text, despite being controversial, concerns the amount of urea which is needed. This should never drop below 5 kg of urea per 100 kg of dry forage, particularly when hermetic sealing is provided through the use of locally available materials.
Particular attention should be made to aspects concerning the treatment duration, the amount of water added to the forage being treated and the standard of hermetic sealing which can be achieved in the treatment environment. The relative importance of these aspects depend in practice, upon the climate, the amount and condition of the forage to be treated and the length of storage time which is envisaged.
In a typical tropical climate, experience shows that treatment can be complete after three or only two weeks. In contrast, it is advisable to extend this period in tropical regions at altitude. Five weeks are judged necessary in high plateau regions where there is high risk of overnight frosts. It is preferable to continue treatment beyond true limits, rather than for periods which are too short as the ammonia which is released assures conservation of the forage. The only major constraint may lie within the cropping calendar and concern other agricultural tasks; one should always try, wherever feasible, to carry out treatment during periods of slack labour requirements.
Certain misunderstandings might exist relating to the degree of hermetic sealing or economies which may be achieved through the use of locally available materials rather than plastic. The hermetic seal is certainly less important in the case of treatment with urea than for that with anhydrous ammonia where the high gas pressure can allow it to escape before it is fixed. In fact, air-tightness becomes a correspondingly more important factor as the amounts of forage under treatment become smaller. What happens in practice is that the peripheral areas of each treatment which are in contact with the air, inevitably become damaged or suffer moulding and are thus unsuitable for feeding to the animals.
When large amounts of forage are treated (in big stacks) one can afford to be less vigilant and only provide summary coverage because the peripheral straw plays a role of self sealing the mass of forage inside. The proportion of damaged matter will remain low when compared with the internal portion which remains intact; losses are of little importance when compared to the overall economies achieved by not using plastic sheeting on these large stacks.
This is much more difficult when treating small quantities of forage and in the limit, the damaged portion may become proportionally more than the part which remains intact, unless sufficient precautions are taken. Control test results concerning the use of locally available materials (see Tables 4 and 5). show that it is possible to achieve efficient treatment at village level even if plastic is not available or is too costly.
Much has been said and indeed, is still being discussed, concerning how much water should be added. Current recommendations lie in the range of between 40 and 100 litres per 100 kg of straw. There is no general rule and the final decision must rely upon good sense: don't restrict the amount of water applied if this is available in abundance; in contrast, and when water is scarce and expensive, amounts may be reduced within reasonable limits in accordance with the need to achieve adequate compaction of the forage and depending upon ambient humidity levels (be very cautious in hot dry climates with high rates of evaporation).
Urea treatment should not pose difficulties as long as the extension agents have been well trained and have fully understood the fundamental principles which should be considered when looking for practical solutions adapted to local conditions. It constitutes a technique which may be used with equal success at artisanal scale for small farmer communities, as on a larger scale by cooperatives or large farms situated in countries where supplies of industrial ammonia do not exist.
|Type of treatment||Urea (per 100 kg)||Digestibility (%)||N % 6.25 (% DM)||References|
SAADULLAH et al., (1981a)
|COCONUT AND BANANA LEAVES||5||nd||6.3|
|OPEN HEAP||nd||53||nd||IBRAHIM et al. (1984)|
|COVERED WITH PLASTIC||nd||60||nd|
|CONTROL||0||nd||2.8||TORO and MAJGAONKAR (1987)|
|NO CONVERING MATERIAL||4||nd||8.4|
|COVERED WITH BAMBOO MATING||5||52||10|
|COVERED WITH PLASTIC||5||54||13|