Urea Toxicity in Dairy Cows
UREA POISONING
Urea (NH2-CO-NH2, 46.7% N) is a useful cheap source of non-protein nitrogen (NPN) for ruminants, in which 100 g urea is equivalent to 220 g digestible crude protein. Fed in two or more feeds daily, urea-N can usefully replace up to 16% of total dietary-N for dairy cows but much higher levels can be fed safely if other animal- and dietary- factors are optimal. |
| urea toxicity |
Sources of urea
Plants
usually contain little or no urea. The most common source of poisoning
of cattle and sheep is via ingestion of toxic levels in concentrate
feed, due to an error in urea addition or due to uneven mixing. A less
common source of poisoning is via direct ingestion from liquid
preparations of urea, from bags of urea, or of urea granules
concentrated on pasture due to spillage or uneven spread of fertilizer.
Urea
is also used as a means of ammoniation of hay, straw or whole-crop
cereals. Excessive addition of urea or failure to convert to ammonia
(NH3) in the treated feed can lead to urea poisoning in animals.
Toxicity of urea
Rumen
microbes convert urea to microbial protein and NH3. The microbial
protein flows past the rumen, for digestion and absorption mainly in the
small intestine. The NH3 is absorbed in the rumen, reticulum and
omasum. It is carried in the portal vein to the liver, where it is
detoxified to urea, amino-acids, etc. Some NH3 escapes from the liver
into the general circulation.
Oral urea poisoning is
basically poisoning due to excessive NH3 production in the rumen.
Normal levels of NH3-N are 60-680 mg/L in rumen fluid and 0.8-2.5 mg/L
in blood. When NH3 levels in rumen fluid exceed 500-800 mg/L, NH3 levels
increase in
the
peripheral blood. Thus, after ingestion of urea, levels of NH3 in rumen
fluid and NH3 and urea in blood increase markedly and rapidly. When
NH3-N levels exceed 6 mg/L in peripheral blood, signs of poisoning may
occur in cattle. Clinical signs correlate roughly
with
blood NH3-N level: ataxia occurs at levels >20 mg/L and death occurs
later at levels >49 mg/L. In sheep, deaths occur at blood NH3-N
levels >32 mg/L. In both species, the
pH of rumen fluid (post-mortem) is alkaline (7.1-7.9) and has high NH3 levels.
(The pH is much lower (6.4-7.1) in healthy sheep and cattle).
Tolerable and toxic doses of urea: It is difficult to define a toxic level of dietary urea
for ruminants, as their ability to metabolise large amounts of dietary urea safely depends
on many factors. These include: adaptation of the ruminal microbes; specific dietary factors and other factors. If the amount of urea entering the rumen is excessive over a short period of time, or if the ruminal environment favours very rapid conversion of urea to NH3, levels of NH3 in rumen fluid and NH3 and urea in blood may reach toxic levels.
for ruminants, as their ability to metabolise large amounts of dietary urea safely depends
on many factors. These include: adaptation of the ruminal microbes; specific dietary factors and other factors. If the amount of urea entering the rumen is excessive over a short period of time, or if the ruminal environment favours very rapid conversion of urea to NH3, levels of NH3 in rumen fluid and NH3 and urea in blood may reach toxic levels.
Thus, the toxic dose of urea varies.
A single oral dose of 330 mg urea/kg live weight (LW) causes increased
NH3 levels in blood of starved naive cattle. Assuming a dietary total
dry matter (DM) intake (TDMI) of 3% LW/d, this is equivalent to about
1.1% urea in DM if the TDMI is eaten in a few hours. An oral dose of
200-500 mg/kg LW (about 0.67-1.67% urea in
DM, if eaten quickly) can cause signs of poisoning in naive cattle.
Single doses of 1000-1500 mg/kg LW are lethal (about 3.3-5.0% urea in
DM, if eaten quickly).
Assuming
that the diet of dairy cows has a total crude protein content of 16% DM
(thus a total N content of 2.56% DM), urea-N can be fed safely up to
0.41% dietary DM, or 0.878% urea in dietary DM. This is equivalent to
circa 132 g urea/cow/day in 15 kg dietary DM. Assuming that concentrates
supply 41% of the total DMI (i.e. 7 kg concentrate (6.16 kg DM) in 15
kg TDMI) and that the urea is mixed in the concentrates (fed frequently
during the day), the concentrates safely could contain up to 1.9% urea.
If the urea intake is increased to 280 g/day (equivalent to 4% urea in 7 kg concentrate), milk yield may fall but other signs of toxicity may be absent (Anon 1977). Under optimal conditions, up to 600 mg urea/kg LW can be fed safely to fattening cattle (Anon 1977) but up to 1500 mg/kg LW can be eaten safely on well balanced diets, if eaten over the whole day. Assuming a TDMI by fattening cattle of 2.2% LW, urea levels of 600-1500 mg/kg LW are equivalent to 2.7-6.7% in TDMI. Given as a drench, a dose of 8-10 g urea can kill sheep, especially if they are malnourished or have liver damage.
(LW : Live weight, TDMI: total dry matter intake)
Adaptation of rumen microbes: If the dietary content of urea is increased gradually, healthy cattle on a good diet can tolerate up to 5% urea in the TDMI.
Healthy sheep on a good diet can tolerate urea up to 6% in feed DM, provided it is well mixed with forage and is fed over the whole day. If, however, unadapted animals ingest a high level over a short period (see below), the result can be acute or subacute poisoning. Tolerance is lost rapidly; animals which receive no urea for 3 days again become susceptible to poisoning.
Specific dietary factors: A single dose of 25-45 g urea may kill sheep within 1 hour of ingestion. The presence of adequate amounts of fermentable carbohydrate in the rumen is essential for safe utilisation of urea. A urea-molasses mixture can be sprayed on roughage. Sheep given a mixture of urea-molasses can tolerate up to 18 g urea/day. A ad libitum intake of up to 10% urea in molasses in ball-lick feeders is safe for sheep or cattle.
In other trials, sheep tolerated up to 100 g urea/day (up to 6.7% feed DM) without clinical signs of toxicity (Bartik & Piskac 1981). Feeding of lucerne increases tolerance to urea but fasting for 24 hours, starvation or a low-protein diet reduces tolerance. High-nitrate feeds (see nitrate poisoning, below) increase rumen NH3 levels and increase the risk of urea toxicity.
If the urea intake is increased to 280 g/day (equivalent to 4% urea in 7 kg concentrate), milk yield may fall but other signs of toxicity may be absent (Anon 1977). Under optimal conditions, up to 600 mg urea/kg LW can be fed safely to fattening cattle (Anon 1977) but up to 1500 mg/kg LW can be eaten safely on well balanced diets, if eaten over the whole day. Assuming a TDMI by fattening cattle of 2.2% LW, urea levels of 600-1500 mg/kg LW are equivalent to 2.7-6.7% in TDMI. Given as a drench, a dose of 8-10 g urea can kill sheep, especially if they are malnourished or have liver damage.
(LW : Live weight, TDMI: total dry matter intake)
Adaptation of rumen microbes: If the dietary content of urea is increased gradually, healthy cattle on a good diet can tolerate up to 5% urea in the TDMI.
Healthy sheep on a good diet can tolerate urea up to 6% in feed DM, provided it is well mixed with forage and is fed over the whole day. If, however, unadapted animals ingest a high level over a short period (see below), the result can be acute or subacute poisoning. Tolerance is lost rapidly; animals which receive no urea for 3 days again become susceptible to poisoning.
Specific dietary factors: A single dose of 25-45 g urea may kill sheep within 1 hour of ingestion. The presence of adequate amounts of fermentable carbohydrate in the rumen is essential for safe utilisation of urea. A urea-molasses mixture can be sprayed on roughage. Sheep given a mixture of urea-molasses can tolerate up to 18 g urea/day. A ad libitum intake of up to 10% urea in molasses in ball-lick feeders is safe for sheep or cattle.
In other trials, sheep tolerated up to 100 g urea/day (up to 6.7% feed DM) without clinical signs of toxicity (Bartik & Piskac 1981). Feeding of lucerne increases tolerance to urea but fasting for 24 hours, starvation or a low-protein diet reduces tolerance. High-nitrate feeds (see nitrate poisoning, below) increase rumen NH3 levels and increase the risk of urea toxicity.
Other factors: Soya meal contains urease. Urea mixed with soya meal is rapidly converted to NH3 and can be dangerous.
Individual susceptibility: Intercurrent disease, malnutrition and liver damage can increase susceptibility to urea poisoning.
Treatment aims to reduce or neutralise ruminal and blood NH3 level and to treat the irritation in the digestive tract, liver, kidney and other tissues, caused by the alkaline-corrosive effect of NH3. The only reliable treatment is to empty the rumen (by prompt rumenotomy or by large-bore stomach tube plus weak-acid lavage). Weak acids act as chemical antidotes to alkaline poisons. The dose is 3-5 l of table vinegar or 2% acetic acid
by stomach tube to adult cattle; 0.5-1.0 l to sheep. Repeated doses may be needed, as signs may recur after 30 minutes. Demulcents, sedatives and stimulants may be used to control specific clinical signs. Treatment often fails.
Such feeds should be fed in restricted amounts, preferably spread over 12 hours or more of each day. Mixing urea with molasses, fodder/sugar beet or good quality silage reduces the risk of poisoning. Urea mixing in the feed should be thorough.
Fertilisers and feeds containing urea should be stored, used and disposed of properly. Spillage should be cleaned up or dispersed as it arises. Pasture should be rested for an adequate time before grazing or cutting after N-application.
Individual susceptibility: Intercurrent disease, malnutrition and liver damage can increase susceptibility to urea poisoning.
Clinical signs: The
main signs of urea poisoning are due to the alkaline-corrosive effects
of the high NH3 levels in the digestive tract and other tissues and the
neuro-toxic effect on the brain. Signs of poisoning arise within 10-30
minutes of ingestion of toxic doses. There are three main types of
poisoning: acute (severe), subacute (moderate) and chronic (mild). Acute
(severe) signs include salivation, severe abdominal colic, bloat,
forced rapid breathing, muscle tremor, incoordination, bellowing,
convulsions, violent struggling, weakness, recumbency, coma and death
within 4 h of ingestion. Subacute (moderate) signs include abdominal
colic and milder nervous and circulatory signs. Chronic (mild) signs
include poor appetite, listlessness and lethargy.
Treatment
Once urea poisoning is suspected, the source should be withdrawn immediately and veterinary help should be sought.Treatment aims to reduce or neutralise ruminal and blood NH3 level and to treat the irritation in the digestive tract, liver, kidney and other tissues, caused by the alkaline-corrosive effect of NH3. The only reliable treatment is to empty the rumen (by prompt rumenotomy or by large-bore stomach tube plus weak-acid lavage). Weak acids act as chemical antidotes to alkaline poisons. The dose is 3-5 l of table vinegar or 2% acetic acid
by stomach tube to adult cattle; 0.5-1.0 l to sheep. Repeated doses may be needed, as signs may recur after 30 minutes. Demulcents, sedatives and stimulants may be used to control specific clinical signs. Treatment often fails.
Prevention
Urea should not be given to fasting or malnourished animals, or those with damaged livers. If urea is added to ruminant diets, it should be introduced gradually and increased to a safe level. Concentrate feeds with 3% urea or more in the DM should be regarded as high-urea feeds.Such feeds should be fed in restricted amounts, preferably spread over 12 hours or more of each day. Mixing urea with molasses, fodder/sugar beet or good quality silage reduces the risk of poisoning. Urea mixing in the feed should be thorough.
Fertilisers and feeds containing urea should be stored, used and disposed of properly. Spillage should be cleaned up or dispersed as it arises. Pasture should be rested for an adequate time before grazing or cutting after N-application.
Tags:
Toxicology