Urea versus ammonia for upgrading graminaceous materials

V. C. Mason
Animal and Grassland Research Institute,
Hurley, Maidenhead, Berks SL6 5LR, UK

E. Owen
Department of Agriculture and
Horticulture,
University of Reading, Reading RG6 2AT, UK

Summary
Introduction
Conclusion
References

Summary

Urea is a safe and cheap chemical, frequently assumed to be the equal of anhydrous or aqueous ammonia for upgrading cereal straws in the warmer regions of the world.

This assumption was tested with moist grass hays, whole crop wheat and barley straws in the temperate climate of the United Kingdom, the chemicals being compared at equal nitrogen applications. In each study urea was found to be inferior to anhydrous or aqueous ammonia for improving dry matter digestibility and also, where examined, voluntary feed intake in sheep.

Laboratory studies showed that one possible reason for this was the formation of ammonium 'carbonate' (i.e., ammonium carbamate/bicarbonate) through the reaction of ammonia and carbon dioxide in a confined space at temperatures of about 20° C. This corroborated findings of ammonium 'carbonate' crystals in a silo of whole crop wheat ineffectively treated with ammonia.

It was concluded that at low ambient temperatures the carbon dioxide released by the hydrolysis of urea is detrimental to the efficiency of the upgrading process.

Introduction

There is considerable interest in the use of urea as a precursor of ammonia for the upgrading of cereal straw (Kiangi et al, 1981; Saadullah et al, 1981). Indeed, in this context, this safe and cheap chemical is frequently assumed to be the equal of anhydrous or aqueous ammonia when hydrolysis conditions are optimal.

Sheep studies

This assumption has been examined in three sheep digestibility trials at our institutes. In one, moist crops of mature perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) hays were treated and stored for 2 months with 6% urea or 3.5% ammonia (DM basis) to elucidate their preservation and upgrading potentials, relative to those of field or barn-dried material (Tetlow et al, 1983/4). These hays were fed ad libitum to growing animals supplemented with a soybean meal/commercial vitamin-mineral mixture at a rate of 0.15% of L.W. In the second study (Deschard, 1983) whole crops of winter wheat (Maris Huntsman) were ensiled untreated, or following treatment with respectively 6% urea, 3.5% anhydrous ammonia, 7% ammonium hydroxide or 5% sodium hydroxide. After storage for 60 days, they were fed with 20 g vitamin-mineral mixture to growing sheep, urea being hand-mixed with the NaOH-treated silage to correct a possible deficiency in rumen degradable nitrogen. The final trial (Sherwood, 1985) involved diets of barley straw (ad libitum) and concentrates (restricted), the straw being fed: (a) untreated (chopped); (b) following ensilage with 7% urea at ambient temperatures (-2 to 5° C) for 11 weeks (chopped); (c) following ensilage with 7% urea at 25° C for 11 weeks (chopped); (d) following immersion (bales dipped) in a solution containing 32 g urea per litre for 45 minutes, with subsequent aerobic storage at -2 to 5° C for 4 weeks (fed in long form); or, (e) following ammoniation (baled form) in a sealed polythene sheeted stack (80 g NH₄OH/kg straw DM) for 11 weeks at -2 to 5° C (fed in chopped form).

As shown in Table 1, our studies indicate that in the temperate climate of Great Britain the efficiency of upgrading urea is somewhat inferior to that achieved with ammonia, irrespective of the type of crop and the conditions employed. Other work has shown that treatment with anhydrous or aqueous ammonia may itself be sub-optimal when applied to moist crops stored under plastic sheeting on commercial farms (Ibbotson et al, 1984).

In vitro studies

The reasons for these variations in response are probably several, but at least in our studies the inferiority of urea relative to ammonia was not caused by inefficient hydrolysis, though the rate of ammonia release is still poorly defined.

A clue to one possible factor was provided by a recent experiment, designed to compare sodium hydroxide, urea and ammonia as additives for the conservation and upgrading of whole-crop winter wheat harvested at a dry matter content of 580-740 g/kg (Deschard et al, 1984). It was intended to apply the ammonia to the moist crop in a clamp silo via perforated plastic tubing placed in its base. In the event, the ammonia was trapped by moisture in the immediate proximity of the pipe and the untreated material proceeded to ferment in a conventional manner. When the silo was opened, the surface of the silage was found to be covered with crystals of ammonium 'carbonate' (ie carbamate/bicarbonate).

Since urea is hydrolysed to ammonia and carbon dioxide and since the latter gas accumulates in sealed stacks of moist straws these observations suggested that the inferiority of urea treatments and certain commercial ammonia treatments could be due to reaction of ammonia with carbon dioxide to form ammonium 'carbonate'. To test this theory, the following experiment was devised: samples (15g) of grass hay (860 g DM/kg) were placed in Kilner jars and treated with (A) 0, (B) 15, (C) 30 or (D) 45 g CO₂/kg hay DM followed by 35 g NH₃/kg DM at 20° C. For treatment (E), hay was exposed to 35 g NH₃/kg DM produced in situ by mixing CaO (70 g/kg DM) and ammonium 'carbonate' (NH₄HCO₃.NH₃COONH₄, 31% NH₃; 110 g/kg DM) with water (120 g/kg DM) in a vial within the Kilner jar, the CO₂ being trapped as calcium 'carbonate'. Treatment (F) was untreated hay stored in a Kilner jar (Mason et al, 1985). All treatments were of 5 weeks duration.

Table 1. Upgrading of mature crops for sheep.

Crop type	Moisture content %	Treatment	DMD* %	DDMI* g/kg LW	Author
Perennial ryegrass hay	15	Field-dried	54	8.4	Tetlow et al 1983/4
" " "	16	Barn-dried	56	9.4
" " "	19	Field-dried (3.5% NH3)	62	14.1
" " "	36	Ensiled (6% urea)	62	13.1
" " "	36	Ensiled (3.5% NH3)	68	16.7
Tall fescue hay	14	Field-dried	54	9.2	Tetlow et al 1983/4
" " "	15	Barn-dried	55	9.1
" " "	19	Field-dried (3.5% NH3)	62	13.7
" " "	34	Ensiled (6% urea)	60	12.2
" " "	33	Ensiled (3.5% NH3)	65	17.2
Whole crop wheat	62	Ensiled (untreated)	55	9.1	Deschard 1983
" " "	58	Ensiled (6% urea)	63	10.8
" " "	55	Ensiled (3.5% NH3)	65	12.8
" " "	63	Ensiled (7% NH4OH)	65	13.0
" " "	60	Ensiled (5% NaOH)	68	15.0
Barley straw-concs. (75:25)+	15	Untreated	54	10.8	Sherwood 1985
" " (72:28)	50	Ensiled (7% urea) 1.5oC	60	10.8
" " (76:24)	50	Ensiled (7% urea) 25oC	63	13.2
" " (76:24)	70	Dip (7% urea) 1.5oC	58	12.2
" " (80:20)	50	Stack (8% NH4OH) 1.5oC	62	15.3

* Data uncorrected for residues of added chemicals.
+ Ratio of barley straw: concentrates (DM basis).

The main results are given in Table 2. Hays (A) and (E) darkened within a few minutes of treatment, but with (B), (C) and (D) the colour change was progressively less intense. Ammonium 'carbonate' crystals appeared on the jars in treatments (C) and (D) after a few days, but started to disappear towards the end of the storage period. In vitro organic matter digestibility (OMD) and cell wall degradability by 'cellulase' (Wd) were assessed after 5 weeks. Relative to the untreated hay, treatments (A) to (E) improved OMD by 142, 112, 91, 74 and 156 g/kg and Wd by 207, 112, 76, 74 and 202 g/kg. These results suggest that carbon dioxide can reduce the efficiency of ammoniation through the formation of ammonium 'carbonate'.

Table 2. Effect of CO₂ on efficiency of upgrading hay by ammoniation.

	Treatment						SED
	(A)	(B)	(C)	(D)	(E)	(F)
CO2 (g/kg hay DM)	0	15	30	45	45*	0
NH3 (g/kg hay DM)	35	35	35	35	35*	0
OMD (g/kg)	713	683	662	645	727	571	7.0
CWD (cellulase) (g/kg)	424	329	293	291	419	217	13.6

* (NH₄) 2CO₃-CO₂ absorbed by quicklime.
Source: Mason et al, 1985.

Conclusion

The stability of ammonium 'carbonate' is greatest at low ambient temperatures. Consequently, in the UK and other temperate zones one would expect the efficiency of upgrading with urea to be poorest with treatments applied during the damp, cool months of autumn and winter, when microbial activity is also minimal. In the tropics, however, where higher ambient temperatures stimulate microbial activity on the untreated crop, urea may be exploited for both its preservative properties (Pahlow, 1979) and its potential for upgrading (Sherwood, Table 1). Even so, wherever economic and technical conditions permit its effective use, best results are likely to be obtained with anhydrous or aqueous ammonia. Alternatively, hydrolysed urea may be exploited by treatment with quicklime to trap the carbon dioxide.

References

Deschard G. 1983. Alkali treatment of whole-crop cereal silage. PhD Thesis, University of Reading, UK.

Deschard G. Mason V C and Tetlow R M, 1984. Preliminary studies on intake and growth in steers fed whole-crop wheat treated with various chemicals. 7th Silage Conference. Silage production and utilization. The Queen's University of Belfast. Paper No 27.

Ibbotson C F. Mansbridge R and Adamson A H. 1984. Commercial experience of treating straw with ammonia. Animal feed science technology 10:223-228.

Kiangi E M I, Kategile J A and Sundstol F. 1981. Different sources of ammonia for improving the nutritive value of low quality forages. Animal feed science technology 6:377-386.

Mason V C, Wilson R F. Keene A S and Hartley R D, 1985. The effect of carbon dioxide on the efficiency of upgrading hay by ammoniation. Animal production 40:(3)533 Abst 52.

Pahlow G. 1979. Influence of the addition of urea on the stability of corn silage after unloading. Das Wirtschaftseigene Futter 25:30-35.

Saadullah M, Haque M and Dolberg F. 1981. Effectiveness of ammonification through urea in improving the feeding value of rice straw in ruminants. Tropical animal production 6:30-36.

Sherwood R S. 1985. Unpublished data, University of Reading, UK.

Tetlow R M and Walmsley R. 1983/4. The influence of crop species, maturity and conservation method on the feeding value of hay. Grassland Research Institute, Hurley, Berkshire, U.K. Annual Report 1983-84, 117-118.