Department of Animal Science Florida Agricultural
Mimeograph Series No. AN68-8 Experiment Station
December, 1967 Gainesville, Florida
LEVELS OF UREA SINGLY AND IN COMBINATION WITH FAT AND SODIUM CHLORIDE
FOR REGULATION OF FORAGE SUPPLEMENT INTAKE BY CATTLE.1/
J. F. Hentges, Jr. and J. E. Moore2/
The "feed-intake limiting" property of urea was observed during
the winter of 1966-67 as an unexpected response of beef cows to an
experimental supplement. At that time, the scientific literature con-
tained reports (Briggs et al., 1947; Dinning et al., 1949; Embry and
Gastler, 1955; Coombe et al., 1960; Stangel, 19617 Oltjen et al.,
1964; Baylock et al., 19657 of reduced feed intake as a consequence of
the addition of urea to cattle diets but a deliberate attempt to limit
the intake of a supplement of concentrates offered ad libitum to grazing
cattle or cattle whose diet was largely forage had not been reported.
It is known that ammonia is a normal product of diet utilization
by microorganisms in the rumen (forestomach) of cattle. If the rumen
ammonia concentration becomes too high, it will cause clinical and
toxic symptoms. Ammonia can be absorbed through the rumen wall and
carried by blood to the liver where some of it will be removed and
converted to urea for excretion in urine or recycling to the rumen.
When the liver's capacity to remove blood ammonia nitrogen is exceeded,
an abnormally high level will appear in the peripheral blood. The
central nervous system is sensitive to changing levels of ammonia in
the peripheral blood and "triggers" the responses which are seen during
"urea toxicity" (Dinning et al., 1948 and 1949; Gallup et l., 1953;
and Davis and Roberts, 1959). It is expected that other, perhaps sub-
clinical, responses which often are not recognized are occurring in
the form of reduced feed intake and lethargy.
Table 1 shows the composition of the supplement being fed to beef
cows when the "feed-intake limiting" effect of urea came to our atten-
i/Supported in part by research contract with Animal Husbandry Research
Division, A.R.S., U.S.D.A.
-./Animal Nutritionist and Associate Animal Nutritionist, respectively.
Acknowledgement is made of the assistance of John Easley, Assistant
Animal Nutritionist and Johnar.hon Smith, Gradiate Research Assistant.
COMPOSITION OF FORAGE SUPPLEMENT OFFERED BEEF'
COWS FIELD OBSERVATION
Cane molasses, standard, U.S.S.C.1/ 66.6
Concentrate premix, 60% crude protein 33.3
15% Urea, 282V c,p, equivalent
30% Soybean oil meal, 50% c.p.
30% Corn meal
10% Alfalfa meal, dehyd., 17% c.p.
10% Defluorinated rbck phosphate, 18% P
5% Trace mineralized salt (Careys Flo-Min)
1/U. S. Sugar Corporation: standard blackstrap molasses averaging 9.8%
The supplement was offered to the cows by pouring molasses in long
metal feed boxes and evenly distributing the concentrate premix over
the molasses. The quantity offered was adjusted to two pounds of
molasses and one pound of concentrate premix per cow. Three pounds of
this supplement provided 0.8 lb. crude protein (or equivalent) which
was estimated to be minimal for beef cows which had free access to
frosted grass pasture forage and corn silage. The response of the
herds of cows to this small quantity of supplement was unexpected.
Unlike their usual practice of immediately consuming all of the sup-
plement that was offered, they ate only a small quantity initially
but frequently returned to the feed box throughout the day. Three herds
of lactating cows, including one herd of Brahmans, were offered this
supplement for about five months. The response of each herd to the sup-
plement was the same. The high level of urea in the supplement was not
only causing the cows to distribute their supplement consumption over a
6 to 8 hour period of time, but it also proved to be an effective
"limiter" of supplement intake when larger quantities were offered
during the breeding season. An offering to a cow herd of a quantity of
supplement which provided 1 lb. concentrate premix plus 2 lb. molasses
was consumed within 6 to 8 hours. Increasing the offering to 1 lb.
concentrate premix and 2 lb. molasses resulted in lengthening of the
consumption period to more than 8 but less than 24 hours. When! the
offering of supplement was increased to 2 lb. concentrate premix plus
2 lb. molasses, all of it was not consumed within 24 hours. Increasing
the ratio to 4 lb. molasses per 2 lb. concentrate premix did slightly
increase the consumption of concentrate premix but not to a daily
intake of more than 2 Ib. These observations on supplement consumption
which gave an intake of about 0.3 lb. (136 gm.) urea or .135 lb. (61.3
gm.) urea nitrogen by lactating cows averaging 1000 lb. in body weight
apparently implicated one of two or more factors. Either the cows
- 3 -
could detect the urea in the supplement by some effect on sensory
receptors or the blood level of ammonia was "triggering" a central
nervous system action to control consumption of the supplement. The
performance and reproduction records of these herds were comparable
to records from previous years. No unusual behavior or health symptoms
were observed. Tissues were not sampled for nitrogen content. These
field observations prompted the experiments which are partially reported
This experiment was a comparison of two levels of urea in a forage
supplement which was fed at the rate of 4 lb. per day to wintering
500-lb. beef calves. Previously, a forage supplement containing 4% urea
had been fed successfully with corn silage offered ad libitum to similar
calves. Because the supplement had to be limited to 4 lb. per day, a
great deal of labor was used in daily feeding. This experiment was an
attempt to find a way to self-feed these supplements through limitation
of supplement intake by increasing urea content of the supplement to 6,.
The composition of the two forage supplements is shown in table 2.
COMPOSITION OF FORAGE SUPPLEMENTS TO CORN SILAGE OFFERED AD LIBITUM
TO GROWING BEEF CALVES EXPERIMENT I
Ingredients I II
Corn meal 58 68
Soybean oil meal, 50% 12 O
Urea, 45% nitrogen 4 6
Alfalfa meal, dehyd., 17% c.p. 10 10
Defluorinated rock phosphate, 18% P 3 3
Trace mineralized salt (Careys Flo-Min) 3 3
Molasses, cane, std., U.S.S.C. 10 10
Total crude protein (equivalent), % 23 24
Crude protein from urea, % 49 70
Eight beef calves averaging about 500 lb. in live weight were
allotted to the two dietary treatments on basis of initial weight.
They were group-fed for 126 days in shaded 1-acre drylot pens.
The results of Experiment I are summarized in table 3.
RESULTS FROM COMPARISON OF FORAGE SUPPLEMENTS CONTAINING
4 and 6% UREA EXPERIMENT I
Percent urea in supplement 4 6
Number of steers 8 8
Days in experiment 126 126
Av. initial wt., lb. 513 518
Av. final wt., lb. 697 684
Av. daily gain, lb. 1.5 1.3
Av. daily feed intake, lb.1/ 14.3 14.1
Corn silage 10.3 10.2
Supplement 4.0 3.9
Av. daily urea intake, lb.2/ 0.16 0.23
S/Air dry (12% moisture) basis. Silage contained 76% moisture.
/Estimated average daily intake per steer. Weight in grams shown in
The most noticeable difference between the two lots of cattle was
their pattern of eating the supplements. The supplement containing 4%
urea was eaten quickly while the one containing 6% urea was consumed
very slowly and intermittently throughout the day and night. Frequently,
the supplement containing 6% urea was not entirely consumed; therefore,
this supplement was almost available on an ad libitum or self-fed basis.
No signs of unusual behavior or cases of sickness were observed.
- 5 -
Procedure *. -
This experiment was a comparison of five forage supplements which
contained three levels of urea and combinations of urea + waste fat and
urea + waste fat + sodium chloride. To extend the research done in
Experiment I, the supplements were offered in quantities which provided
ad libitum access to supplement in all but the control (2% urea) lots.
To extend the research reported previously (Animal Science Mimeo Series
AN 68-4 and AN68-7) on the supplement-limiting effects of stabilized
waste fat and sodium chloride, the supplement containing 6% urea was
compared with ones containing 6% urea + 10% waste fat and 6% urea + 10%
waste fat + 20% sodium chloride. The steers in this experiment were
older and heavier but in a thinner condition-than those in Experiment I.
Compensatory weight gain was evident in the large weight gains whcch
were recorded. The composition of the forage supplements is shown in
COMPOSITION OF FORAGE SUPPLEMENTS TO CORN
TO GROWING YEARLING STEERS -
SILAGE OFFERED AD LIBITUM
Supplements by no.
I II III IV V
% % % Ib. Tb. -
Corn meal 56 68 79- 79 71.8 79 60.8
Soybean oil meal, 50% 27 13 -- -- --
Urea, 45% nitrogen 2 4 6- 6 5.5 6 4.6
Stabilized waste fatl/ -- -- 10 9.1 10 7.7
Salt, white, mixing -- -- -- O --- 20 15.4
Cane molasses, std. 10 10 10 10 9.1 10 7.7
Trace mineralized salt2/ 2 2 2 2 1.8 2 1.5
Defluorinated phosphate3/ 2 2 2 2 1.8 2 1.5
Vitamin premix-2 1 1 1 1 0.9 1 0.8
Quantity offered/day, lb. 10 10 10 11 ---- 13 ----
Estimated crude protein, %/ 25 25 25 22 -- 19 ---
Crude protein as NPN, % 23 45 68 68 ---- 68 ---
1/Stabilized rendered animal fat, Florida Soap Co., Jacksonville, Fla.
2/Careys Flo-Min, sulfur content 0.10% minimum.
3/CDP defluorinated rock phosphate, 18% P minimum.
--Calculated to provide at least 30,000 I.U. vitamin A and 3000 U.S.P.
units vitamin D2 in 2270 gm. which was anticipated to be minimum
daily intake of supplement.
./Formulated to avoid protein deficiency its daily supplement intake
was sharply reduced.
The results of Experiment II are summarized in table 5.
RESULTS FROM COMPARISON OF FIVE FORAGE SUPPLEMENTS
EMPLOYING UREA, WASTE FAT AND SODIUM CHLORIDE
AS "INTAKE LIMITERS" EXPERIMENT II
I II III IV V
Limiter, % ";
Urea 2 4 6 6 6
Fat 10 10
No,.of steers 6 6 6 6 6
Days inexperiment 42 42 42 42 42
Av. initial wt., lb 693 677 684 675 668
Av., final w'., lb.!/ 834 826 826 .790 .780
Av. da:. gain, b. 2.8 3.0 2.8 2.2 2.1
Av. ration, ib.2/ 18.6 18.4 17.2 14.4 13.7
corn siage 8.9 9.4 10.9 9.1 10.6
Supplement 9.7 9.0 6.3 5.1 3.1
Av. da. urea.intake, lb.4/ 0.19 0.36 0.38 0.31. 0.19
(86) (163) (173) (141)' (86)
I/Feedlot weight shrunk 3%.
Air-dry (12% moisture) basis. Silage was 741 moisture.
3Act~ually did n6tjlimit intake below average for III except first two
weeks; was accepted gradually.
4/Estimated average daily intake per steer, Weight in grams shown in
The intake of supplement was most markedly reduced by the combi-
nation of 6% urea, 10% f and 20%' salt,. Only. 3.1 lb..of the 10 lb.
offered daily was- the, average quantity .consummed;by this lot of steers.
The acceptance of. the mixture was slow during'the first two weeks.(av.
intake of 2.1 lb.).and was attributed to.the fat content.. After the
cattle became adjusted to the mixture during the initial 2 weeks, the
average daily intake was 3.5 lb. A fortunate observation was the.
absence of a detrimental effect of the intake-limiters in the supple-
ment on the intake of corn silage. Apparently, the smaller weight gains
by steers fed supplements IV and V were a reflection of a limited
ration and smaller available energy intake.
- .6 -
The intake of supplement was not reduced by combining 10 fat with
6% urea, except during the first 2 weeks when only 2 of the 10 lb. being
offered was the average daily consumption. Following adjustment to this
mixture curing the initial two weeks, daily intake averaged 6.6 lb. Which
was comparable to supplement III which contained 6% urea without fat.
The intake of.the supplement containing 6% urea was limited to 650
and 70%, respectively, of the quantities consumed by the lots offered
supplements containing 2% and 4% urea. The intake of urea (173 gi.) per
day by the steers offered the supplement containing 6% urea was larger
than the quantity consumed (106 gm.) by .the smaller calves in Experiment
I. When the intakes were adjusted to grams per kilogram of liveweight,
the magnitude of the differences appears smaller: 0.5 for Experiment II
and 0.4 for Experiment I. In the field trial with cows, 136 grams of
urea or about 0.3 gm. per kg. liveweight, was the maximum quantity that
was consumed voluntarily. Without question, urea content was the factor
limiting consumption of the supplements in the cow trial and in Experiment
It is interesting that an anonymous author wrote in the Queensland
Agricultural Journal 87:463, 1961, that for trough feeding of urea to
cattle, the most successful level appeared to be 3 oz. (85 gm.) to 1 lb.
molasses per day per animal. The 3 oz. level was preferred over lower
levels because the low palatability prevented overconsumption to the
extent that the urea might become toxic. Another Australian report
(Coombe et al., 1960) stated that sheep readily consumed up to 100 gn.
urea per day provided that the concentration of urea in the ration did
not exceed 6%; at higher levels, food intake was markedly reduced. He
postulated that rumen acidity and rate of ammonia absorption might be
In opposition to the Australian reports and the data reported
herein are numerous reports of cattle and sheep deaths from consumption
of diets containing urea. The explanation for these opposing observa-
tions might be based on the pattern of consumption of the urea-containing
feeds during especially the first day and probably the initial 3 weeks of
access to the feed. It is known that very high intakes of urea can be
tolerated by cattle which are gradually adapted to urea over a period of
months. The largest continuous daily consumption of urea reported for
cattle are 400 gm. (Dinning et al., 1949), 520 gm. (Gallup et al., 1953),
272 gm. (Lassiter et al., 1958), 585 gm. (Huber et al., 196 7 and diets
fed by Virtanen (1966) in which urea furnished 95 of the dietary nitro-
gen. In each of these studies, the urea content was a small percentage
of a predominantly concentrate ration; hence, it is difficult to compare
these data with those obtained from cattle consuming small quantities
of forage supplements with predominantly forage rations.
This experiment was conducted to further test the intake-limiting
properties of urea at levels of 2, 4 and 6% in a self-fed, concentrate
mixture which, with the exception of water, comprised the entire ration.
Ten recently-weaned beef calves were allotted on basis of initial weight
to each of three groups and were self-fed the supplements shown in
COMPOSITION OF DIETS OFFERED AD LIBITUM TO
GROWING BEEF CALVES EXPERIMENT III
Ingredients I II III
Citrus pulp, dried 41.0 39.0 37.0
Rice bran, o.p. 20.5 20.5 20.5
Wheat middlings 20.5 20.5 20.5
Molasses, cane, std. 10.0 10.0 10.0
Alfalfa meal, dehyd., 17% 4.0 4.0 4.0
Ua;', 45% nitrogen 2.0 4.0 6.0
Trace mineralized salt 1.0 1.0 1.0
Defluorinated phosphate 1.0 1.0 1.0
Crude protein, ola 15.5 20.1 27.2
Crude protein from urea, % 36.0 56.0 62.0
1/The study of levels of urea as intake-limiters with a mimimal alter-
ation of the ingredient formula was desired; consequently, percent-
ages of ration crude protein (equivalent) were widely different.
Blood samples were withdrawn during the final two
experiment to determine blood urea nitrogen content.
was terminated after 70 days because of the continuous
by one group of calves.
weeks of the
loss of weight
A summary of some of the results of this experiment is shown in
- 9 -
RESULTS FROM COMPARISON OF 2, 4 AND 6% LEVELS OF UREA IN RATIOS
FOR GROWING BEEF CALVES EXPERIMEiT III
I II III
Ration urea, % 2 4 6
No. calves 10 10 10
Days in experiment 70 70 70
Av. initial wt., lb. 424 420 418
Av. final wt., lb. 457 420 375
Av. da. wt. change, lb. 0.48 0.0 -0.61
Av. daily feed intake, lb./ 11.1 9.3 6.0
Av. daily urea intake, lb./ 0.22 0.37 0.36
(100) (168) (163)
Av. blood urea nitrogen, mg. % 13.9 18.6 26.7
-LAir-dry (12% moisture) basis.
2/Estimated average daily intake per steer. Weight in grams shown in
Again, diet urea content proved to be a regulator of feed intake.
The daily urea intakes of the groups fed 4 and 6% levels of dietary
urea were 168 and 163 grams, respectively. Conversion to average daily
urea intake per unit of body weight makes these values become 0.88 and
0.91 gm. per kg. live weight. These values are considerably larger than
those recorded in previous trials. Apparently, the level of urea in the
feed is the only factor with a consistent relationship to cattle perfor-
mance. Not shown in the summary is the pattern of consumption during
the experiment. From the first contact of the calves with the self-
feeder until termination of the experiment, the eating pattern among the
groups was the same. The level of urea in the different diets was
detected or sensed immediately upon first contact and was reflected by
limitation of diet intake in accord with urea level. Calves consuming
the diet containing 6% urea appeared listless and some individuals were
more lethargic than others. This may have been a reflection of contin-
uous weight loss or high bloodurea nitrogen or both.
Research on this subject is continuing and must provide more con-
sistent data before drawing conclusions or making recommendations for
feeding practices. The effects of high levels of blood urea nitrogen
over extended periods of time may prove to be injurious to health and a
cause of sub-normal performance; consequently, the practice of using
high levels of urea to regulate and limit the daily consumption of con-
centrate mixtures offered ad libitum as supplements to forage cannot be
recommended at this time for cattle.
- 10 -
Blaylock, L. G., L. H. Neagle and J. H. Goihl. 1965. Dietary protein
and urea levels in beef cattle rations. J. Animal Sci. 24: 875
Briggs, H. M., W. D. Gallup, A. E. Darlow, D. F. Stephens and C. Kinney.
1947. Urea as an extender of protein when fed to cattle. J.
Animal Sci. 6: 445.
Coombe, J. B., D. E. Tribe and J. W. C. Morrison. 1960. Some experi-
mental observations on the toxicity of urea to sheep. Australian
J. Agr. Res. 11: 247.
Davis, G. K. and H. F. Roberts. 1959. Urea toxicity in cattle. Fla.
Agr. Exp. Sta. Bull. 611.
Dinning, J. S., H. M. Briggs, W. D. Gallup, H. W. Orr and R. Butler.
1948. Effect of orally administered urea on the ammonia and urea
concentration in the blood of cattle and sheep with observations on
blood ammonia levels associated with symptoms of alkalososis. Am.
J. Physiol. 153: 41.
Dinning, J. S., H. M. Briggs, W. D. Gallup, 0. B. Ross, L. H. Moe and
R. L. Butler. 1949. Urea in livestock feeding. J. Am. Vet. Med.
Assn. 114: 90.
Embry, L. G. and G. F. Gastler. 1955. Influence of urea on ration
digestibility by cattle and sheep. Proc. So. Dakota Acad. Sci.
Gallup, W. D.1 L. S. Pope and C. K. Whitehair. 1953. Urea in rations
for cattle and sheep: summary of experiments from 1944-1952.
Okla. Agr. Exp. Sta. Bull. B-409.
Huber, J. T., R. A. Sandy, E. E. Polan, H. T. Bryant and R. E. Blaser.
1967. Varying levels of urea for dairy cows fed corn silage as
the only forage. J. Dairy Sci. 50: 1241.
Lassiter, C. A., R. M. Grimes, C. W. Duncan and C. F. Huffman. 1958.
High level urea feeding to dairy cattle. III. Effect on per-
formance and metabolism of lactating dairy cows. Michigan Agr.
Exp. Sta. Quart. Bull. 41: 326.
Lowrey, R. S. and B. L. Southwell. 1967. Proc. Ga. hNutrition Conference,
U. of Ga., Athens.
Minson, D. J. and W. J. Pidgen.
urea on utilization of low
Effect of continuous supply of
forages. J. Animal Sci. 20: 962
Oltjen, R. R., J. D. Robbins and.R. E. Davis. 1964.
the use of glutamic acid in.ruminant nutrition.
J. Animal Sci. 23:
Virtanen, A. I. 1966. Milk production of cows on protein-free feeds.
Science 153: 1603.