Livestock Research for Rural Development 12 (1) 2000

Citation of this paper

Review: Treatment and utilization of crop straw and stover in China

Gao Tengyun

Animal and Veterinary Science College, Henan Agricultural University,
Zhengzhou 450002, P.R.China

Abstract

China produces more than 500 million tonnes of crop straw and stover every year .By promoting feeding of herbivorous animals with crop straw and stover, the beef and mutton output could be increased markedly and a great amount of feed grain would be saved. This paper reviews the different techniques for the utilization of crop straw and stover, and highlights the achievements in the 1990's.The techniques that have been used include physical treatment, chemical treatment and microbial treatment methods. Data are given on the actual rate of use in China of these crop residues.

Key words: Straw, stover, ammoniation, ensiling, animals, feeding


Introduction

Surveys in recent years indicate that the agricultural areas in China have great potential to promote feeding of herbivorous animals. China produces more than 400 million tonnes of grain each year, which gives rise to more than 500 million tonnes of straw and stover -- about fifty times the grass harvested (cut) in the northern pastures! In addition, the agricultural areas also produce large quantities of  by-products, such as cottonseed meal, rape seed meal and cereal bran. The by-products can be used to replace concentrate in the diet of herbivorous animals.  By using these plentiful feed resources, the abundant human resources and the available le technologies, the crop-producing areas can become the major base for the feeding of  herbivorous animals (Chen Junsheng 1992). In fact, two thirds of the cattle and sheep (equivalent to three fourths of the beef and mutton) already come from the crop farming areas. However, the full potential for developing herbivorous animals in the crop-producing areas has still to be realized.

In China the aim is to increase to 10 million tonnes of meat by the end of 2000, by accelerating the program of implementation of livestock raising with straw and stover. Then the annual beef output will be 7 million tonnes and the annual mutton output will be 3 million tonnes, increases of  71% and 52%, respectively, compared with production in 1995 (Guo Tingshuang 1996). The percentage of straw used as feed will be raised from 25% now to 40% by the end of 2000 (about 240 million tonnes). The urea-ensiled straw will be 120 million tonnes, and the ammoniated straw will reach 60 million tonnes.  Total feed grain to be saved will be 40 million tonnes. The ammoniation techniques will be extended to 20 million farmers.

By enlarging the program of "Returning straw and stover to farmland through animal digestion" the soil fertility will be improved. By the end of 2000, the organic manure produced by cattle and  sheep will reach 1,910 million tonnes, an increase of 900 million tonnes compared with that of 1995.  This manure can fertilize 40 million hectares of land and increase by 8.14 million tonnes the production of grain (Chen Junsheng 1997).

Physical treatment of straw and stover

Rolling wheat straw together with green alfalfa

In the countryside of Shanxi and Shaanxi provinces, farmers have a tradition of rolling wheat straw together with green fresh cut alfalfa. They first put a layer of wheat straw on the threshing ground, then put green freshly-cut alfalfa and wheat straw layer by layer. Usually there are three layers in total, including a layer of green alfalfa inside. Then they begin to roll the wheat straw and green alfalfa with a stone roller drawn by cattle or a donkey. As a result of the rolling over and over again, the juice is pressed from the alfalfa and is absorbed by the "flattened" wheat straw. After one or half a day of drying in the sun, it can be stacked or preserved for feeding to cattle (Lei Tianfu and Zan Linsen 1994). This method has many advantages, maintaining the  alfalfa with high quality as well as increasing the palatability and nutritive value of the wheat straw. It is a good example of scientific cattle farming in our country created by our intelligent ancestors.

Straw knead cutter

The straw "knead" cutter is a new type of straw processing machine designed in China (Wu Renze 1989). The feature of the processing is between the hay chopper and the grinder. After processing with the "knead" cutter, the crops residues --  especially maize stover --  will be formed into threads with lengths of 8-12cm.  The feed intake rate of the stover can be increased from 50% to 95% by this method.  The straw "knead cutter"  is a machine which can improve the utilization rate of straw greatly, by simple mechanical processing. It is very suitable for maize stover which have a potentially high nutritive value.  Comparing with hay choppers, the degree of processing done by the straw "knead" cutter is much higher, so that its power consumption is nearly doubled. 

Hot steam spray feed processing 

The working principle of the "hot steam spray" feed processing includes two components: the thermal and the mechanical effect (He Jian 1987). The effect of the pressurised hot steam (170C) is partly to hydrolyse the Hemicellulose and partly to increase the surface area of the straw particles through the combination of heat, moisture and pressure.  The processing appears to increase both the digestibility and feed intake. This technique can also be used to detoxify cottonseed and rape seed cakes. It is also very useful for deodorizing and sterilizing dung which makes it safer when it is recycled as feed for animals.

Cubing technology for straw treated with urea and Ca(OH)2 

The first equipment for cubing treated straw in China (93KCT 1000 type roughage cubing complete equipment) was made by Huada Machinery Works of Guang Dong and Huhehaote Animal Husbandry Machinery Research Institute in 1991. It has the following main uses: (a) cubing low value roughage treated with compound chemicals; (b) making pellets and cubes composed of roughage, concentrates, small amounts of vitamins and trace element; (c) making pellets and cubes of alfalfa and high value hay (Yang Runde et al 1995).


Chemical treatment of straw and stover

Straw ammoniation by different sources of ammonia
Ammoniation by urea

The usual ratios among straw,  water and urea are 100:40:4.5.  But some researchers recommend using 5 kg urea and 80 kg water per 100kg air dry wheat straw (Zhang Weixian et al 1995). In practice, the extension agents recommend the ammoniated of straw with a lower level of urea, such that the ratios among straw, water and urea might change into 100:40:3. After the urea is dissolved in the water, the solution is uniformly sprayed on the wheat straw. Then the straw is put into a cement pit and covered with plastic film, and the edges sealed with mud. The ensiling period is 25 days under the temperature of 20 - 30C . Zou Xiaqing et al (1995) suggested that the ensiling time should be 4 weeks when the range of air temperature is from 10 to 30C , but that it should be 3 weeks when the air temperature range is from 21 to 36C.

Ammoniation  by ammonium bicarbonate

The ratios among straw, ammonium bicarbonate and water are 100:12:40. The wheat straw should be cut into small pieces (4 to 5cm in length), then dissolve the ammonium bicarbonate in water, and the solution thus obtained should be evenly spread on the straw. Finally it should be stacked, or put in a concrete-lined pit,  and covered with plastic film to make it air-tight. 

Another method can also be adopted, that is: first, straw is sprayed with water, then ammonium bicarbonate is spread upon every layer of straw, and finally it is sealed (Liu Jianxin 1998).

Ammoniation by urea plus lime

The ratios among straw, urea, lime and water are 100:2.5: 8: 40. First, urea is dissolved in limewater, then spray upon every layer of straw, and finally seal up the container in which the straw is filled. Another method which was practiced by Wang Jiaqi and Feng Yiang Lian (1993) is as follows: mature maize stalk and rice straw are chopped into 10mm lengths, mixed with 30% water, 3% urea and 4% calcium hydroxide, sealed in plastic bags and heated at 50C for 4 hours after which it can be used for feeding.

Anhydrous ammonia treatment

Usually the stack treatment method is used. A stack of straw bales  is made and sealed with plastic film. The delivery pipe from a small anhydrous ammonia cylinder is then inserted into the stack and NH3 added at the rate of 3 kg/100kg wheat straw (air dry basis). The treatment period lasts 25 days under the temperature of 20 to 30C .The stack is aerated for a day before the straw is fed to animals.

The effects of ammoniated straw with different ammonia sources 

Zhang Weixian et al (1995) carried out an experiment to compare the efficiency of different ammonia treatment methods and its economic feasibility. The chemical components of straw treated with ammonia/urea were analyzed.

Table 1: Results of chemical analysis (% of DM) of straw treated with anhydrous ammonia, urea or untreated (control)
Sample DM CP Fat NDF ADF CF LIG ASH
Control 89.0 3.40 0.57 80.0 54.3 44.2 12.0 7.2
AS 87.8 12.9 0.59 73.6 57.2 41.5   9.8 8.1
US 50.6 9.90 0.56 77.3 55.7 42.9 10.0 8.2
AS: anhydrous ammonia treated wheat straw;  US: urea-treated wheat straw.

Chinese Yellow cattle (young bulls; n=48) ) with initial liveweight 160 - 210 kg and  12 to 14 months of age were fed straw treated with anhydrous ammonia or urea, plus1.0, 1.5 or 2.0 kg/day of cottonseed cake. The liveweight gains during 60 days for the cattle given urea-treated wheat straw  were 602, 687 and 733 g/d,  for the 1, 1.5 and 2.0 kg/day of supplement, respectively; while for cattle given the anhydrous ammonia treated wheat straw the gains were 655, 728 and 786g/d respectively. There was more than 50g difference between the anhydrous ammonia and urea treated straw at the same level of supplementation (P<0.01). The economic analysis showed that the feeding benefit was increased by 20% for the treatment with anhydrous ammonia treated straw under the current local economic circumstances.  It was concluded that the nutritive value and feeding benefit of using anhydrous ammonia to treat wheat straw were obviously higher than when urea was used (P<0.01).

A similar trial was done by Zou Xiaqing et al (1995).  The data on the composition of the treated straws are in Table 2.

Table 2:  The chemical composition of straw treated with anhydrous ammonia or ammonium bicarbonate (% of DM)
Item DM CP CF Ca P
Straw ammoniated by urea (U) 67.7 7.90 30.0 -- --
Straw ammoniated by ammonium bicarbonate (AB) 61.3 8.00 26.5 -- 0.11
Straw treated with urea plus lime (UL) 81.2 6.08 26.5 2.5 0.12
Untreated straw (control) 86.4 3.52 30.7 0.39 0.1

The daily intake of cattle fed with the straw ammoniated by ammonium bicarbonate (AB) was 10.3, 14.3and 56.1% higher than for groups U, AB and control,  respectively. The palatability was the best when straw was ammoniated by ammonium bicarbonate .The details are in Table 3.

Table 3: Palatability of straw treated with different sources of ammonia
Item Feed intake speed (min/kg) Daily average feed intake (kg/day) Feed intake frequency (times/min)
Straw ammoniated by urea (U) 28.8 5.6 6
Straw treated with ammonium bicarbonate (AB) 24.6 6.4 8
Straw treated with urea plus lime (UL) 25.8 5.8 8
Untreated straw (control) 52.9 4.1 4

During the 30 days testing period, the average daily gains in weight of young Holstein cows (at the age of 16 to 18 months) in treatments U, AB, UL and control were 823, 837, 770 and 573 g, respectively. The feed conversion data are in Table 4.

Table 4:  Feed requirements for every kg of  liveweight gain (kg)
Group Straw (DM) Concentrate
Straw ammoniated by urea (U) 4.6 3.7
Straw treated with ammonium bicarbonate (A B) 4.7 3.6
Straw treated with urea plus lime (UL) 6.1 3.9
Untreated straw (control) 6.2 5.2

Sun Xuezhao and Yang Quanming (1995) compared ammonium bicarbonate with urea in the ammoniation of wheat straw. Three groups of sheep were fed with three different forms of ammoniated straw (70% of the diet) and concentrate (30%). The results indicated that ammonium bicarbonate could be used to ammoniate wheat straw if urea was not available.

Straw ammoniation with different installations
Double concrete pools for straw ammoniation

We usually refer to this kind of concrete pool as a double ammoniating pool. It is very suitable for households to ammoniate straw for cattle or sheep raising. The farmer should choose a high place in the yard near the beef cattle barn to build the double ammoniating pool. The concrete pool is rectangle in shape, 2 m long, 1.3 m wide and 1 m deep.  The concrete pool should be built with cement and bricks. After finishing to lay the bricks, cement whitewash of the inside should also be done carefully, in order to keep it airtight.

In general, the farmers usually build two of the above mentioned concrete pools side by side, with a brick wall in the middle. The reason for the two pools is that they can ammoniate different batches of the straw in turn. While the ammoniated straw of one pool is used, the straw in another pool is treated at the same time. It is better to build the pond with the upper half above ground. This simplifies putting in and taking out the straw and avoids the risk of water flowing in or an animal treading on the straw.

The unit weight of cut wheat straw in the pool is about 80 kg/m3 and of ground maize stover is about 110kg/m3. Thus one pool can be used to treat about 200 kg of wheat straw each time.  It is very practicable for farmers who raise 2 to 3 cattle to use the double ammoniating pools. The system is very popular in the cropping areas of central China, especially in Zhoukou Prefecture of Henan Province.

Heated ammoniation pool method

A traditional method in China involving a special bed  ("kang") used to cultivate sweet potato vines is applied in straw ammoniation.  It is called the heated ammoniation pool method. The specific operational steps are as follows: at first, a pool  8 -10 m long, 1.5 - 2.0 m deep and 2 m wide is built with bricks and cement. It can be on the ground or half in the ground. Then a stove is built at one end of the pool, and two covered channels are laid under the bottom of the pool which allow the warm air pass through. The two channels join at the other end of the pool to connect with the chimney. Straw is put in the pool and treated with urea or ammonium bicarbonate solution and sealed it by covering with plastic film. Then some straw (the weight is about 5 to 10% of that to be ammoniated) is burned in the stove for about 6 to 8 hours. The temperature in the center of the pool will reach about 30C. One week later, the straw will be ready. The quality of straw ammoniated and the time for treating depend on the temperature in the pool which can be controlled by the quantity of straw to be burned.

Using the chamber of flue-cured tobacco to ammoniate straw

Usually, the flue-cured tobacco chamber is idle in winter. It can be used to ammoniate straw. Straw with the ammonia source is loaded in the chamber which is then sealed. The suitable temperature is about 40-50C  and it can be controlled by the duration and degree of heating. The heat source is usually coal. After 3 to 4 days, the straw will be well ammoniated.  It is a good method for use in the tobacco growing area (Guo Peiyu et al 1993).

Straw ammoniating oven

In areas where the temperature is below 0C  in winter and the unit has more than 50 cattle and a planned supply of feed is needed, the straw ammoniating oven has proved to be a suitable facility. It is composed of a chamber and two straw trailers which can be moved in and out on rails. The ammonia source is usually anhydrous ammonia or ammonium bicarbonate. Anhydrous ammonia is easy to operate but needs high pressure containers. Ammonium bicarbonate can be put in the straw directly because the oven temperature is high enough  (about 90C) to decompose it into ammonia. Urea can not be used in the oven because the high temperature will restrain the biological reaction of urease ( Liang Tingxiang et al 1993). The heating sources have been electricity, steam and coal. The chamber can be constructed with metal sheet or with other suitable building materials .For the electric-heated oven, the oven temperature and the ammoniate time can be controlled by automatic control apparatus. 

A new type of ammoniating oven, which is heated by coal and simultaneously produces steam, has been developed in Shanxi Province of China. In Shanxi Province, to produce same quantity of heat, the cost of electricity is ten times as much as that of coal. But heating with coal needs one labourer to look after the stove. Therefore, this method is suitable to the area where coal and labour are readily available. The chamber of the oven is constructed with bricks and cement and some thermal insulation materials. When the insulation door is opened, the straw trailers can go in or go out along the rails. At the other end of the chamber, a pit is dug and a stove is built in it. Above the stove a water pot is located to produce steam. When heating, the warmed air from the stove passes through the main smoke channel, then turns back to the two branch smoke channels and finally gets out from the chimneys.

Ammoniation for different roughage
Wheat straw ammoniation

The wheat straw was chopped to 2 3cm, stacked layer by layer and sprayed with 5% urea solution (i.e. 5kg urea and 35 kg water) for spraying 100kg straw, then weighted down and sealed with a plastic film and left for 4 weeks before being opened. Before the feed was used, the stack was allowed to aerate for 5 6 days to allow for the escape of volatile ammonia and for drying.

In order to study the effect of ammoniated winter wheat straw on the growth of beef cattle, 10 Jinnan steers weighing 150 to 250kg were used in a comparative experiment. Concentrates in both groups were the same (2kg each day), composed of 74.6% maize, 23.0% cottonseed meal, 2.0% ground limestone and 0.4% salt. In the control group wheat straw was fed while in the experimental group ammoniated (5% urea) straw was given. In 92 days the daily gain of the experimental steers averaged 695g, 32.4% higher than for the control (P<0.05). In the first month it was 8.3% higher, in the second month 18.9% higher, and in the third month 90.7% higher. This indicated that benefits from ammoniated straw feeding increase with duration of feeding.  The cost of 1 kg weight gain of the experimental group was 2.91yuan compared with 3.48 yuan for the control. The DM intake per 100kg weight was 2.30 kg for the experimental group vs.2.08kg (P<0.05) for the control.

Maize stover ammoniation

100 kg maize stover was treated with 45 kg urea solution (urea 5kg, water 40kg), fully mixed and kept in a sealed bin for 20 days. Zhong Huilong et al (1995) carried out the experiment on the effects of finishing buffalo in a short time with ammoniated maize stover. The details are in Table 5.

Table 5: Liveweight gain and feed consumption of  buffaloes fed ammoniated maize stover

Group Number Daily weight gain (kg) Maize stover conversion
Experimental 4 1.1780.16 4.25
Control 4 0.7360.88 6.79

In another experiment, Tan Nailin et al (1993) fed Holstein bulls with ammoniated maize stover. In this experiment, ammoniation improved the crude protein content of maize stover from 4.5% to 8.5% and the dry matter disappearance at 48 hours from 48.9% to 67.0%. There were 10 bulls in each group with average weight at the beginning of 118kg. Details are in Table 6. 

Table 6: The roughage and concentrate of diets in the experiment (kg/d/head)
Group Roughage 4th month  5th month  6th month
1 Ammoniated maize stover 2.0 2.5 3.0
2 Ammoniated maize stover 1.5 2.0 2.5
3 Ammoniated maize stover 1.0 1.5 2.0
Control Untreated maize Stover 1.5 2.0 2.5

 

Table 7: The weight gain and the feed conversion efficiency of the cattle
Group Roughage Liveweight gain (Kg) Average daily gain (g/head) Concentrate consumption (kg) Concentrate conversion 
1 Ammoniated maize stover 140 760190 414 2.96
2 Ammoniated maize stover 122 670200 322 2.63
3 Ammoniated maize stover 103 560130 230 2.22
Control Untreated maize Stover 115 630160 322 2.80

As shown in Table 7, the 2nd group consumed 0.17kg concentrate less than the control group for every 1kg liveweight gain, although the concentrate intake was same for them. Because of the feeding of ammoniated stover, the average liveweight gain of the 2nd group was 40g/d/head higher than the control group, although both of the groups consumed the same amount of concentrate. It was concluded that the ammoniated stover had more nutritive value than the untreated stover. The results also proved that ammoniated straw could be used to replace part of the concentrate.

Rice straw ammoniation

Ammoniated rice straw is usually prepared by the "stack method". When ammonium bicarbonate is used, one tonne of rice straw is treated with 100kg ammonium bicarbonate and 250 kg water for 30 days at ambient temperature of 15 to 20C. In the experiment of Liu Jianxin et al (1995), 32 Holstein heifers were fed with ammonium bicarbonate treated rice straw (ABRS), supplemented with vetch silage. It was concluded that supplementation of ABRS with vetch silage improved the growth rate of the animals, saved concentrates and decreased feed cost of liveweight gain, and that the optimal level of supplementary vetch silage was of the order of 20% of the diet DM intake. The chemical composition of the diets is in Table 8:

Table 8 : Chemical composition of ammoniated rice straw and vetch silage

Roughage pH DM (%) CP (%DM) NDF (%DM)
Ammoniated rice straw -- 65.0 9.4 65.7
Vetch silage 4.0 12.0 20.2 56.1
Comparison of ammoniated wheat straw with ammoniated maize stover

Fresh wheat straw and maize stover were ammoniated and fed to cattle (Table 9).. 

Table 9: Liveweight gain, feed intake, feed conversion rate and feed cost of cattle fed ammoniated wheat straw or maize stover

Item Wheat straw Maize stover Ammoniated
 wheat straw
Ammoniated
maize stover
Experiment period (d) 80 80 80 80
Number of cattle 12 12 12 12
Initial liveweight (kg) 297.4 297.4 297.4 297.4
Daily gain (kg/d) 0.3480.13 0.5130.23 0.6440.21 0.7440.20
Cottonseed cake* (kg/d) 1.5 1.5 1.5 1.5
Straw intake per 100kg liveweight (kg) 2.14 2.70 2.48 2.83
Crude protein in the straw, % DM 3.03 5.89 11.63 16.47
Feed conversion ratio (kg feed/kg gain) 4.32:1 2.93:1 2.33:1 2.02:1
Price of roughage (yuan/kg)  0.06 0.06 0.12  0.12
Total cost of feed (yuan/kg gain) 2.80 2.12 1.74 1.64
*: Price (yuan/kg) of cottonseed cake: 0.56

The data in Table 9 (Li Ying et al 1993) showed that not only did ammoniation result in higher straw intake, faster and more efficient growth but that feed costs were reduced. The cattle on ammonia-treated maize stover, after completion of the trial, had a very healthy appearance. The feed conversion ratio as a result of straw treatment, particularly with ammonia treated maize stover, was improved. Thus, the farmers can in future be advised to feed their animals with ammonia treated maize stover and small amount of locally produced cottonseed cake (1.5kg) as supplement. 

Microbial treatment methods of crop straw and stover

Green yellow maize stover for ensiling

Studies were conducted by Zhang Lancao and Ji Yilun (1995) to compare the quality and nutritive value of maize silage made of maize plants at milk, dough and fully dented stages. All silages appeared to ferment properly. The pH values of silage at the milk, dough and fully dented stages were 3.55-3.65, 3.40-3.78 and 3.70-3.81, respectively. The lactic acid contents were 3.93 - 4.19, 77 - 4.28 and 2.01 - 2.56%, respectively. The 48hour  DM and OM rumen degradabilities of all silages showed that those at milk and dough stages were significantly higher than that at the fully dented stage (P<0.05). This means that the better stage for ensiling is dough stage or milk stage. But in the cropping areas in China, farmers plant maize mainly for harvesting ears, so only the green to yellow maize stover can be obtained for ensiling. How to use this kind of stalk to make silage will be a problem.

To make the silage, the appropriate water content of silage materials should be 68 to 75%. With the green maize stover for ensiling, the water content of the material usually is deficient so water should be added. The normal ratio of stover and water is 100:15 kg. The maize stover should be filled into the silos quickly, and should be compacted continuously during filling. 

The value of green yellow maize stover silage for feeding

It was reported by Xie Chongwen and Song Shicai (1993) that ensiling maize stover helped to preserve the nutrients, especially vitamin A.

Improvement of quality by adding little amount of urea and maize meal

Adding urea (0.3% urea) is a way to increase the content of nitrogen. To adjust and control carbohydrate fermentation, 0.4% maize meal can be added.

Ammoniation and ensiling of wheat straw and maize forage

Though ensiling has been widely spread, there remains the problem of nutrient loss in the effluent. A solution is to use straw to absorb the silage effluent and to have the ensiled maize absorb the ammonia, with general improvement in nutritive value. The method is: (a) put chopped wheat straw at the side of silo bin; (b) spray on the urea solution (c) chop maize plants; (d) put treated wheat straw and maize plants into bin, then mix; (e) press and seal; (f) cover up with soil and check; (g) open up for determination.

The making of ammoniated wheat straw maize silage (AWSMS) limits or prevents the loss of ammonia and silage effluent in ammoniating and ensiling, which improves the fodder's quality. AWSMS is also safe and palatable. The chemical composition of the fodder is shown in Table 10.

In the experiment of in Zhen Erying et al (1995), the standard milk production increased by 23% (P<0.05), by substituting the AWSMS  for the traditional wheat straw and maize silage. Moreover, the use of the silo bin reduced the expense for ammoniation.

Table 10: The chemical composition (% in DM)of ammoniated wheat straw maize silage (AWSMS) 
Fodder Name CP Either 
extract
Crude fiber NFE Crude ash
AWSMS 9.69 2.31 31.8\ 38.5 4.77
Wheat straw 3.81 1.00 36.8 43.4 5.92
Maize silage 5.03 3.11 25.3 50.2 3.89


Ration formulation based on straw and stover

The suitable level of cottonseed cake in the ration 

An experiment was conducted by Gu Chuanxue et al (1993) in which the different groups of  steers were given ammoniated wheat straw ad lib together with a daily allowance of 0.00, 0.25, 0.50, 1.50, 2.00, 2.50, 3.00, 4.00 kg/d of cottonseed cake. The results clearly demonstrated that the feeding of ammoniated straw alone was inadequate (99g/d liveweight gain) and that there was no economic advantage in giving more than 1.5 kg daily of cottonseed cake. These findings were supported by the results of a similar trial reported by Zhang Weixian et al (1993, 1994).

 

From the production responses obtained from these experiments, it can be assumed that it was the bypass protein supplementation from the cottonseed cake, which efficiently increased the daily gain of the animals in the ammonia-treated straw-based diet.


References

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Received 1 November 1999

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