Comparison of the nutritive value and economic benefit of straw treated with urea or anhydrous ammonia at different levels of supplementation

An experiment was carried out with 48 Chinese Yellow Cattle, age 12-14 months with an average weight of 180 kg, to investigate the production responses and feeding benefits of two different forms of treatment of wheat straw: 5% urea or 3% liquid ammonia; and three levels of supplementation with cottonseed cake (1.0, 1.5 and 2.0 kg/day). The feeding period was 60 days.

The live weight gains on urea-treated wheat straw at 1,1.5 and 2 kg/day cottonseed cake were 602,687 and 733 g/day, respectively; while for ammonia-treated straw the gains were 655, 728 and 786 g/day respectively. The difference between methods although small (average of 50 g/day) was significant (P<0.01). Straw intake was less for ammonia treatment.

The economic benefit was greater for ammonia-treated straw under the current market structure in China which has resulted in a high price for urea.

During the last decade a breakthrough has occurred in the application of urea-treated straw as ruminant feed by small farmers in China. Even so, only about 30% of the annual straw production of 570 million tonnes is utilized as livestock feed (Liu Jiang 1990). Some 15% is consumed by the paper industry and the remainder is either used as fuel to cook food in the remote areas or is burnt in the field after harvesting. Therefore, an enormous amount of straw and stover still waits to be exploited as a potential ruminant feed.

For the last two years, however, the urea treatment of straw has been confronted with economic constraints as the price subsidies for urea have been withdrawn and price is now determined by the market. In 1994 the price ratio between urea and concentrates (cereal grain) reached 2:1 in most of rural areas of China. Therefore, there are incentives to seek an alternative treatment technology, which will be more easily adopted in the new economic environment.

For some time anhydrous ammonia has not been subsidized in China, and its price does not fluctuate so much in the newly created market economy. Therefore it is oportune to reconsider the use of anhydrous ammonia for straw treatment which has been less used in practice previously in China (Guo and Yang 1994).

The objective of this study was therefore to investigate the efficiency of different methods of ammonia treatment method and to assess their economic feasibility, providing information for the better utilization of ammonia-treated straw feed in cattle production.

48 young bulls of the Chinese Yellow Cattle breed were used (initial live weight 160-210 kg and age 12-14 months). They were allocated at random to receive one of 6 treatments in a 2 x 3 factorial arrangement. There were 8 repetitions (individual animals) per treatment. The comparisons were:

The cattle in the feeding trial were all penned individually indoors on dry earth bedding and fed cottonseed cake together with a vitamin mineral mixture twice daily (9:00 am, 17:00 pm). The straw was fed ad libitum. Feed residues were removed and recorded once daily. The adaption period lasted 15 days and the feeding trial was continued for a further 60 days. The animals were weighed on three consecutive days at the beginning and the end, and once every 15 days during the experimental period.

The urea treatment method used in this experiment was with 5 kg urea and 80 kg water per 100 kg air dry wheat straw. After the urea was dissolved in the water, the solution was uniformly sprayed on the wheat straw. Then the straw was put into a cement pit and covered with plastic film, and the edges were sealed with mud. The ensiling period was 25 days at a temperature of 20-30 ?C. Before the feed was used, the ammoniated straw was allowed to aerate for one day to allow for the escape of volatile ammonia.

The stack method was used for treatment with anhydrous ammonia, the bales of straw being covered with plastic film. The delivery pipe from a small size anhydrous ammonia cylinder was then inserted into the stack and NH₃ added at the rate of 3 kg/100 kg air dry wheat straw. The treatment period lasted 25 days at a temperature of 20-30 ?C. The stack was aerated for one day before the straw was fed.

Before feeding, the cottonseed cake was soaked in iron sulphate solution for 24 hours to reduce toxicity (1 kg iron sulphate to 100 kg air dry cottonseed cake, which was a ratio of iron to gossypol of 1:1 (Zhang et al 1993)).

Table 1: Results of the chemical analysis ( % in DM) (AS anhydrous ammonia treated straw; US urea treated straw)

	N x 6.25	Fat	NDF	ADF	CF	Lig	Ash

Straw	3.4	0.57	80.0	54.3	44.2	12.0	7.2
AS	12.9	0.59	73.6	57.2	41.5	9.8	8.1
US	9.9	0.56	77.3	55.7	42.9	10.0	8.2
CSC	29.5	6.64	48.8	34.3	14.5	19.0	5.3

All the feeds were analyzed by chemical methods. The straw samples were collected before and after ammonia/urea treatment and passed through a laboratory hammer mill with a 1mm screen. The DM content was determined by drying the samples to constant weight at 103°C. Ash was determined by ignition in a muffle furnace at 550°C for 3 hours. The nitrogen (N) concentration of the feeds was determined according to the Kjeldahl method. Crude fibre (CF), neutral- and acid-detergent fibre (NDF and ADF) and acid-detergent lignin (ADL) were determined according to Van Soest and Wine (1967).

The DM degradabilities of the diets were measured by incubating 3 g air dry samples, which had passed through a laboratory hammer mill with a 3mm screen, in nylon bags in the rumen of three cannulated sheep and withdrawing them at either 8, 16, 24, 48, 72 or 96 h of incubation. The size of the bags was 140 x 90 mm and they were made from nylon filter cloth with a pore size of 45 to 55 1m (Zhang et al 1992).

An integral component of this analysis was the economic feasibility of the ammonia treatment. The production functions (response curves) were the basic tools for making these calculations.

Table 2: DM degradation characteristics of the different straws and cottonseed cake (RSD is the residual standard deviations)

	a	b	c	a+b	RSD	48h DMD

Straw	2.5	49.2	0.033	51.7	1.21	41.7
AS	6.9	67.7	0.028	74.6	1.80	58.4
US	4.6	58.4	0.035	63.0	1.27	51.3
CSC	29.2	47.9	0.032	77.1	0.90	66.7

It can be seen from Table 1 that anhydrous treatment resulted in a higher content of N in the straw than use of urea. There was little effect on cell wall constitutents other than a 20% reduction in lignin due to treatment.

Table 3: Mean values for live weight change and feed intake of cattle fed urea- or ammonia-treated wheat straw and different levels of cottonseed cake

	Straw	Live weight, kg
	(kg/d)	Initial	Final	Gain/d	FCE*

Ammoniated straw
CSM, kg/d
1.0	5.0	182±19	221±23	0.66±0.14	11.1
1.5	4.7	182±23	226±26	0.73±0.16	12.0
2.0	4.5	182±27	229±25	0.79±0.17	12.7
Urea-treated straw
CSM,kg/d
1.0	5.4	182±21	218±17	0.60±0.15	9.50
1.5	5.2	182±30	223±27	0.69±0.17	10.6
2.0	4.8	182±18	226±18	0.73±0.19	11.4

The degradation characteristics determined by the nylon bag method and represented by the equation P = a + b (1 - e-ct) (Orskov and McDonald 1979) are given in Table 2 together with the determined 48 hour degradability. As expected, both liquid ammonia and urea treatment brought about increases in the the a and b values as well as the 48 hour degradability while the constant c was lowest for anhydrous ammonia-treated straw. The ammoniated straw dry matter appeared to be digested to a greater extent than the urea-treated straw as reflected in higher values for (a+b) and for 48 hour degradability. However, the rate constant "c" was higher for the urea-treated straw.

The results of the feeding trial are given in Table 3. The mean difference in daily gain for the cattle fed straws treated with anhydrous ammonia and urea was about 50 g (P<0.01) with higher values for the anhydrous ammonia treatment. Straw intake (DM) was less on the anhydrous ammonia treatment and as a result the feed conversion rate was better than on the urea-treated straw. The straw DM intake tended to decrease in linear fashion with the increasing level of cottonseed cake. Liveweight gain and food conversion efficiency were linearly and positively related to the level of cottonseed cake.

The results of the economic evaluation are in Table 4. This analysis did not include housing, labour, and other sundry fees. The gross margin for the ammonia- compared with the urea-treated straw system was increased by 26.6%, 19.9% and 25.1% for the 1, 1.5 and 2 kg levels of cottonseed cake. concentrate supplement the gross profit for cattle fed anhydrous treated straw increased by compared with urea treated straw. The gross margin was the same for all levels of CSC supplementation.

Table 4: Effect of CSC supplement level on economic performance by cattle fed different treated straws

	Cost of feed inputs (Ø/day)			Output	Margin
	CSC	Straw	NH₃/Urea	(Ø/day)	(Ø/day)

Ammoniated straw
CSC (kg/d)
1.0	1.0	0.56	0.51	4.59	2.52
1.5	1.5	0.53	0.48	5.10	2.59
2.0	2.0	0.48	0.43	5.50	2.59
Urea-treated straw
1.0	1.0	0.61	0.61	4.21	1.99
1.5	1.5	0.57	0.58	4.81	2.16
2.0	2.0	0.53	0.53	5.13	2.07

Prices (Ø)/kg: CSC = 1.0, Straw = 0.1, Urea = 2.0, NH₃ = 3.
Beef cattle liveweight = 7.0

The results from chemical and in sacco feed evaluation and the animal production responses indicated that the anhydrous ammonia treatment was more effective and more economical than urea treatment. The chemical analysis data for the specific cell wall fractions and the degradability characteristics of the dry matter in nylon bags in the rumen were generally predictive of the direction but not the degree of animal responses in terms of weight gain and feed efficiency. Thus 48 hour degradability was 14% higher for ammonia-treated compared with urea-treated straw yet weight gain only differed by 7%. Feed conversion efficiency, by contrast, was improved by 20% by the anhydrous ammonia treatment mainly becauseof lower feed intake. Similar findings regarding lower intake and higher daily gain for cattle fed 3% anhydrous ammonia-treated straw compared to urea-treated straw have been reported before, and were attributed by Dolberg and Finlayson (1993) to the higher digestibility of anhydrous ammonia treated straw inducing satiation at a lower level of intake.

From a development, economic and resource perspective the cottonseed cake supplement level should not exceed 2 kg per day in the ammonia-treated straw feeding system. The fact that the margin over feed was the same for 1 as for 2 kg/d of cottonseed cake despite the higher gain on the latter emphasises the sensitivity of the feeding system to the price of the supplements. An increase in the price of cottonseed cake would make the lower level of 1 kg/d the most profitable. The growth rates of 0.6-0.65 kg/day for the 1 kg/day level of cottonseed cake are very satisfactory rates of production for smallholder farm conditions.

The anhydrous ammonia treatment technology has been in use in China for several years. Both FAO and the Agricultural Ministry of China have put much effort into such projects ( Orskov and Sundstol 1990); while the Beijing Agricultural Engineering University has done research on design of facilities for anhydrous ammonia treatmentdesign. Despite these activities, the impact of anhydrous ammonia technology has been less than that of urea mainly because of transportation and handling difficulties and also because the farmer could easily obtained urea at a subsidised price. It is probable that under the new market economy system the anhydrous ammonia treatment will prove to be more feasible. It is advocated that high priority should be given to research on the application of anhydrous ammonia treatment in China.