Slow Ammonia Release from Urea: Rumen and Metabolism
Studies
F. N. Owens, K. S. Lusby, K. Mizwicki and O. Forero
J ANIM SCI 1980, 50:527-531.
The online version of this article, along with updated information and
services, is located on the World Wide Web at:
http://jas.fass.org/content/50/3/527
www.asas.org
Downloaded from jas.fass.org by guest on July 25, 2011
SLOW A M M O N I A R E L E A S E F R O M U R E A : R U M E N A N D
M E T A B O L I S M S T U D I E S 1'2
F. N. Owens 3 , K. S. Lusby 3 , K. Mizwickia and O. Forero s
Oklahoma Agricultural E x p e r i m e n t Station, Stillwater 74074
Summary
A new slow-release urea (SRU) made by
coating prilled urea with a tung oil-linseed oiltalc-catalyst mixture was evaluated for ammonia-nitrogen release rate, animal acceptability,
toxicity and effects on dry matter digestibility,
diet intake and nitrogen retention. When added
at a level equal to 1% urea in an 80% concentrate steer diet and fed twice daily, SRU gave a
ruminal ammonia-nitrogen peak 1 hr postfeeding of 32 mg/dl compared to a peak from
prilled urea of 53 mg/dl at 30 minutes. Bihourly feeding of prilled urea and SRU produced similar rumen ammonia-nitrogen levels
and demonstrated that SRU was almost completely hydrolyzed in the rumen. Steers fasted
for 26 hr and refed with supplements containing 10% urea from priUed urea had rumen ammonia levels of 120 ml/dl and showed muscle
tremors 35 min after feeding. Rumen ammonia
levels of steers fed equivalent urea from SRU
remained below 35 ml/dl and exhibited no
toxicity symptoms. Sheep fed ad libitum cottonseed hulls were offered a supplement containing 5% or 10% urea from urea or SRU once
daily. Intake of SRU supplement was 7 and
17% greater, while cottonseed hull intakes were
similar for sheep fed urea or SRU at the 5 and
10% levels. In a nitrogen balance trial, steers
were fed ad libitum cottonseed hulls unsupplemented or supplemented with isonitrogenous
amounts of SRU, prilled urea or soybean meal.
Added nitrogen from all sources increased cottonseed hull intake. Steers fed SRU consumed
t Journal Article 3651 of the Oklahoma Agri. Exp.
Sta., Oklahoma State Univ., Stillwater 74074.
more (P<.05) cottonseed hulls than steers fed
urea. Dry matter digestibility and nitrogen retention values tended to be highest for steers
fed soybean meal supplement with little difference noted between priUed urea and SRU supplements.
(Key Words: Urea, Slow Release, Feed Intake,
Digestibility.)
Introduction
Rapid ruminal hydrolysis of urea to ammonia can cause ammonia toxicity (Coombe et al.,
1960; Ohjen et al., 1963) and has been suggested as one cause of limited utilization of
urea as a nonprotein nitrogen (NPN) source for
microbial protein synthesis (Bloomfield et al.,
1960; Tudor and Morris, 1971; Romero et al.,
1976). Although many attempts have been
made to develop a "slow-release urea," most
products have either released ammonia too
rapidly or the nitrogen was so tightly complexed that little ammonia was released (Males
et al., 1979). A slow-release urea compound
should be useful to reduce toxicity and might
enhance acceptability of supplements and
utilization of urea.
The objective of this research was to evaluate ammonia release rate, toxicity potential,
palatability and effects on nitrogen balance and
digestibility of a commercial coated urea product.
ExperimentalProcedure
The slow-release urea compound (SRU)
used in these studies was manufactured by
Nipak Corp., Pryor, OK, as follows. Prilled
feed grade urea was mixed in a portable cement mixer with .5% talc. An oil mixture
consisting of 10% linseed oil, 89% tung oil, .5%
manganese octanoate and .5% cobalt octanoate
was slowly dripped onto the prilled urea and
talc as the mixer was rotating. Heated air was
blown constantly into the mixer during oil
addition to facilitate drying. Coated prills con-
2The research reported herein was partially supported by a grant from Nipak Corp., Pryor, OK.
3Anita. Sci. Dept.
4Present address: Dept. of Anim. Sci., Texas Tech
Univ., Lubbock 79409.
SPresent address: Carrera-43 A No. 105 -86,
Bogota, Colombia, South America.
527
JOURNAL OF ANIMAL SCIENCE, Vol. 50, No. 3, 1980
Downloaded from jas.fass.org by guest on July 25, 2011
528
OWENS ET AL.
tained 38% N and had a mean diameter of 3
millimeters.
Rate and extent of ruminal ammonia release
from SRU were estimated in trial 1 with nine
500-kg ruminally cannulated steers in three 3 x
3 Latin squares. The steers were held in individual pens and fed 3.15 kg dry matter of a
concentrate diet (table 1) every 12 hr with
either no urea added or with 30 g urea added as
prilled urea or SRU at each feeding. After 5
days of feeding, rumen samples were obtained
via cannula at 0, 89 1, 2, 4, 6 and 8 hr postfeeding and ammonia-nitrogen levels determined by
the method of Chaney and Marbach (1962).
For the last 2 days of each period, the steers
were fed 530 g dry matter of the above concentrate diet every 2 hours. Ruminal ammonianitrogen levels were determined at 89 and 1 hr
after each feeding to estimate relative extent of
ammonia release in the rumen.
Effects of SRU in reducing toxicity were
evaluated in trial 2 with five of the steers described above. Feed was withheld for 26 hr before two steers were offered ad libitum amount
of a supplement containing 10% prilled urea,
and three steers were offered a supplement with
10% SRU (table 1). Supplement intakes and
animal behavior were monitored. When toxicity
symptoms were observed, the rumen of the affected steer was evacuated via cannula and
rinsed with water. Ruminal ammonia-nitrogen
levels were determined at 0, 90 and 180 rain
postfeeding in steers not showing toxicity and
at the time of evacuation of intoxicated steers.
Palatability of SRU and its influence on forage intake as compared with prilled urea was
evaluated in trial 3 with six mature 65-kg
wether sheep. Sheep in individual pens were fed
cottonseed hulls ad libitum and offered 200 g
of the concentrate diet (table 1) with or without urea in the prilled form (0, 10 or 20 g per
day) or in the SRU form (10, 20 or 40 g urea
from SRU daily). A 6 x 6 Latin square design
was employed. Each supplement was offered
once each day for a period of 1 hr during each
14-day period, and intake for the final 5 days
was monitored. Fresh cottonseed hulls were
added daily with orts refed. Uneaten supplements were weighed and not refed.
Slow-release urea was further compared
with prilled urea and soybean meal as a supplemental N source in trial 4. Effects of supplement and cottonseed hull intakes, digestibility and nitrogen balance were monitored
using four 218-kg steers in a 4 x 4 Latin
square design. Steers were fed cottonseed hulls
ad libitum. Once daily, steers received 450 g
dried molasses (IRN 4-04-695) plus either 450
g of soybean meal (5-04-604) or an isocaloric,
isonitrogenous mixture of cracked corn (IRN
4-02-915) with prilled urea or SRU. As a negative control, unsupplemented cracked corn was
fed. Each period in the square lasted 14 days,
with total urine and feces collected the final 5
days.
Analysis of variance and means comparisons
by least-significant-difference followed those
TABLE 1. DIET COMPOSITIONS
Ingredient
Corn, rolled
Alfalfa hay, ground
Cottonseed hulls
Soybean meal
Cane molasses
Salt, trace rain.
Limestone
Dicalcium phosphate
Prilled urea
SRU
NaH~ PO4
Na~ SO4
Trace mineral mixa
Internat'l
Ref. No.
4-02-915
1-00-118
1-01-599
5-04-604
4-04-604
Concentrate diet
for ammonia
release studies
Supplements for toxicity trial
62.75
6.00
14.00
10.00
5.00
.50
.50
Prilled urea
SRU
43.60
43.30
15.00
15.00
19.40
7.00
19.10
6.00
1.00
1.00
8.90
.50
2.70
2.35
.50
10.50
2.70
2.35
.50
aComposed of 16% zinc, 12% iron, 6% manganese, 3% magnesium, 1% copper, 1% potassium, .6% iodine, .3%
cobalt and 1% mineral oil.
Downloaded from jas.fass.org by guest on July 25, 2011
SLOW-RELEASE UREA
80%
h
~o 60
e 50
9 Contro~ ~ 7kg/doy at an
concentrate rohon
12% protein
9 Urea : Control plus 60 G
urea / Ooy
~ SRU : Conlrol plus ?0 G
SRU/doy
| ~
z,
f
~40
',=
~
~
30
~x.~
IO
~
L
....
....
Urea
-~
sRu
----~o
0
i
2
4
6
Con rol
8
Time post feeding (nr)
Figure 1. Ruminal ammonia nitrogen levels of
steers fed urea or SRU.
procedures outlined by Snedecor and Cochran
(1967).
Results and
Discussion
Ruminal ammonia-nitrogen levels of steers
fed supplemental priUed urea or SRU are shown
in figure 1. Curves show that SRU produced a
peak lower and later than prilled urea. The
curve produced by SRU was higher than seen
with the control supplement but more closely
resembled the control curve than did prilled
urea. If absorption and utilization of ammonia
from SRU and urea were equal and only release
rate differed, the area between the SRU and
control curves should equal the area between
the urea and control curves. Only a crossover of
ruminal ammonia could make this possible. To
achieve this, ruminal ammonia concentrations
from SRU should have exceeded those from
prilled urea at some point. Such a crossover was
529
not observed, however, suggesting that some
SRU may have escaped intact to the lower gut
or that some SRU was so resistant to hydrolysis
that complete hydrolysis in the rumen occurred
after the end of the 8-hr sampling period.
Totality of release was evaluated by measuring ruminal ammonia of steers fed prilled urea
or SRU at 2-hr intervals. SRU and prilled urea
produced virtually equal ruminal ammonia concentrations. This observation suggests that an
equal amount of ammonia-nitrogen was eventually released from both SRU and priUed urea.
Alternatively, SRU degradation or ammonia
absorption and(or) utilization may differ between intermittent and continuous feeding. To
determine the extent of mastication damage of
SRU, boluses were caught at the cardia. Less
than 5% of SRU prills exhibited surface damage.
For toxicity testing (trial 2), SRU and urea
supplements were fed to five fasted steers (table
2). Amounts of urea of each form consumed
were equal. Muscle tremors were observed within 35 min of feeding in the two steers fed
prilled urea. Ruminal ammonia-nitrogen levels
at evacuation exceeded 120 rag/deciliter. Steers
fed SRU exhibited no abnormality, and tureen
nitrogen levels never exceeded 35 rag/deciliter.
Extrapolated from observed ammonia levels, an
intake of 900 g urea in the SRU form would be
required for toxicity. Such a level of SRU could
only be achieved by some nonphysiological
means such as dosing via fistula. Similar safety
for coated urea was reported by Ward and Cullison (1970). They fed prilled urea and ethyl
cellulose-coated urea to ewes in a toxicity
study and observed coated urea to be nontoxic
TABLE 2. STEER TOXICITY TRIAL
Form of urea
Item
Urea
Animal no.
Feed intake, g
Urea intake, g
Toxicity time, min
25
2450
218
32
SRU
29
1374
122
35
24
1459
129
none
28
2581
230
none
32
2013
179
none
Rumen ammonia, mg/dl
Time
-10
+35
+90
+180
rain
min
min
min
8.7
142.2
9.3
123.0
Evacuated
Evacuated
10.2
11.5
io.
ii.i
17.0
25.2
Downloaded from jas.fass.org by guest on July 25, 2011
9.0
2810
23.6
530
OWENS ET AL.
TABLE 3. SHEEP ACCEPTABILITY OF PRILLED UREA AND SRU
Nitrogen source
Item
Urea
Urea in supplement, %
Level, g/day
Supplement intake, % of fed
Cottonseed hulls intake, g/day
0
0
100 a
775 ab
SRU
5
10
88.5 b
861 a
10
20
67.0 c
839 ab
5
12
95.7 ab
839 ab
10
10
84.3 b
873 a
20
47
55.7 c
756 b
a'b'CMeans in a row with different superscripts differ significantly (P<.05).
w h e n fed at the same level as a toxic a m o u n t o f
u n c o a t e d urea.
Results of the palatability trial (trial 3) with
sheep are summarized in table 3. Intake of supp l e m e n t was i m p r o v e d b y coating in each c o m parison. S u p p l e m e n t intake f r o m S R U was
a b o u t 7% greater (P<.10) with the 5% level of
urea and 17% greater (P<.05) with the 10%
urea level. Even at a urea level of 20% f r o m
SRU, 55.7% of t h e s u p p l e m e n t was c o n s u m e d .
This observation indicates that S R U is quite
palatable. No s u p p l e m e n t with 20% prilled
urea was fed due to obvious acceptability
p r o b l e m s and danger of toxicity. C o t t o n s e e d
hull intakes t e n d e d (P>.05) to be higher w h e n
either S R U or prilled urea was offered at the 5
or 10% levels in the supplements t h a n w h e n no
N s u p p l e m e n t was offered. C o t t o n s e e d hull intakes by sheep fed S R U or prilled urea at t h e 5
or 10% level were similar. Feeding S R U at the
20% level in the s u p p l e m e n t reduced c o t t o n seed hull intake.
Results of the steer balance trial (trial 4) are
presented in table 4. Supplemental nitrogen
f r o m each source increased (P<.05) c o t t o n s e e d
hull intakes above the negative control. C o t t o n -
seed hull intakes of steers fed S R U were slightly
greater than with steers fed soybean meal and
were greater (P<.05) for steers fed priUed urea.
Dry m a t t e r and protein digestibility were increased (P<.05) with any nitrogen addition,
with soybean meal tending to produce higher
digestibility than S R U or prilled urea.
Digestible dry m a t t e r intake was greatest
with soybean meal and SRU. S R U p r o d u c e d
higher (P<.05) digestible dry m a t t e r intake
than did prilled urea. Nitrogen r e t e n t i o n
(grams/day) was similar b e t w e e n SRU and
prilled urea, although urea in either f o r m
t e n d e d to be inferior to soybean meal in p r o m o t i n g nitrogen retention. Others (Ward and
Cullison, 1970) have shown that while coating
of urea i m p r o v e d acceptability of urea in the
diet, coating did not increase nitrogen r e t e n tion.
Kropp e t al. (1977) simulated c o n t i n u o u s
release by feeding various levels of urea and
soybean meal to steers at h o u r l y intervals and
f o u n d that microbial protein synthesis appeared
to be limited by f e r m e n t a b l e energy and r u m e n
t u r n o v e r t i m e rather t h a n by nitrogen availability. It is possible that a sustained release o f
TABLE 4. DIET INTAKE, DIGESTIBILITY AND NITROGEN RETENTION
OF STEERS FED PRILLED UREA, SRU, SBM OR NO PROTEIN
Protein source
Item
Nitrogen intake, g/day
Cottonseed hulls, g/day
Digestibility, % dry matter
Protein
Digestible DM intake, g/day
Nitrogen retention, g/day
% of digested
None
47.0
4587 c
15.9 b
Jq.9 b
883 a
5.1 b
96.8 b
Soybean meal
89.8
7046 ab
37.2 a
30.6 a
2723b c
18.2 a
65.7 a
Urea
sRu
87.7
6587 b
32.2 a
26.4 a
2543 b
13.0 a
52.3 a
92.5
7300 a
35.3 a
24.2 a
2867 c
12.8 a
53.7 a
a'b'CMeans in a row with different superscripts differ significantly (P<.05).
Downloaded from jas.fass.org by guest on July 25, 2011
SLOW-RELEASE UREA
a m m o n i a alone in the r u m e n will n o t be sufficient to increase microbial protein p r o d u c t i o n
since a m m o n i a is n o t the only nitrogen source
required for microbial growth (Pilgrim e t al.,
1970; Salter e t al., 1979). In studies showing
benefit f r o m f r e q u e n t feeding o f urea, the c o m plete diet has generally been fed at f r e q u e n t
intervals, thus c o n f o u n d i n g the effects of stable
a m m o n i a release with m o r e c o n t i n u o u s availability o f f e r m e n t a b l e energy and natural p r o t e i n . Other benefits (reduced t o x i c i t y , increased
diet acceptability) remain unstudied.
E x p e r i m e n t s r e p o r t e d herein d e m o n s t r a t e
that S R U achieves a m o r e u n i f o r m release o f
a m m o n i a - n i t r o g e n into the ruminal fluid t h a n
does prilled urea and that S R U is unlikely to
produce toxic ruminal a m m o n i a levels. S u p p l e m e n t s containing S R U are c o n s u m e d m o r e
readily t h a n s u p p l e m e n t s containing prilled
urea although digestibility and balance trials
suggest that S R U m a y n o t improve dry m a t t e r
digestibility or nitrogen retention above t h a t
f o u n d with prilled urea.
Literature Cited
Bloomfield, R. A., G. B. Garner and M. E. Muhrer.
1960. Kinetics of urea metabolism in sheep. J.
Anita. Sci. 19:1248 (Abstr.).
Chaney, A. L. and E. P. Marbach. 1962. Modified
reagents for determination of urea and ammonia. Clin. Chem. 8:130.
Coombe, J. B., D. E. Tribe and J. W. C. Morrison.
1960. Some experimental observations on the
531
toxicity of urea to sheep. Australian J. Agr. Res.
11:247.
Kropp, J. R., R. R. Johnson, J. R. Males and F. N.
Owens. 1977. Microbial protein synthesis with
low quality roughage rations: Isonitrogenous
substitution of urea for soybean meal. J. Anim.
Sci. 45:837.
Males, J . R . , R . A . Munsinger and R . R . Johnson.
1979. In vitro and in vivo ammonia release from
"slow-release" urea supplements. J. Anim. Sci.
48:887.
Oltjen, R. R., G. R. Waller, A. B. Nelson and A. D.
Tillman. 1963. Ruminant studies with diammonium phosphate and urea. J. Anim. Sci. 22:36.
Pilgrim, A. F., F. V. Gray, R. A. Weller and C. B.
Belling. 1970. Synthesis of microbial protein
from ammonia in the sheep's rumen and the pro portion of dietary nitrogen converted into microbial nitrogen. Brit. J. Nutr. 24:589.
Romero, A. B.'D., B, D. Siebert and R. U. Murray.
1976. A study on the effect of frequency of urea
ingestion on the utilization of low quality roughage by steers. Australian J. Exp. Agr. and Anita.
Hush. 16:308.
Salter, D. N., K. Daneshvar and R. H. Smith. 1979.
The origin of nitrogen incorporated into compounds in the rumen bacteria of steers given
protein- and urea-containing diets. Brit. J. Nutr.
41:197.
Snedecor, G. W. and W. G. Cochran. 1967. Statistical
Methods (6th Ed.). The Iowa State University
Press, Ames.
Tudor, G. D. and J. G. Morris, 1971. The effect of
frequency of ingestion of urea on voluntary food
intake, organic matter digestibility and nitrogen
balance of sheep. Australian J. Exp. Agr. and
Anim. Husb. 11:483.
Ward, C. S. and A. E. Cullison. 1970. Effect of ethyl
cellulose coating on utilization of urea by ruminants. J. Anita. Sci. 30:331 (Abstr.).
Downloaded from jas.fass.org by guest on July 25, 2011
Citations
This article has been cited by 2
HighWire-hosted articles:
http://jas.fass.org/content/50/3/527#otherarticle
s
Downloaded from jas.fass.org by guest on July 25, 2011