DCAB value key to keeping milk fever in check
DCAB value key to keeping milk fever in check
By Richard Allison
FEEDING grass silage which has an unbalanced mineral content during the last three weeks of pregnancy can cause more cases of milk fever and retained placenta in dairy cows.
There is a widespread belief that a low calcium dry cow ration is the most important factor in prevention of milk fever, says Glos-based vet and recent Bath and West scholar James Husband. "But low calcium levels can be difficult to achieve, rarely being less than 50-60g a day using standard dry cow rations."
Instead, the most important aspect is the dietary cation-anion balance (DCAB). The relevant cations are sodium and potassium, while the main anions are chloride and sulphate. A highly positive value is associated with more cases of milk fever and an increased incidence of retained placenta.
When a ration has a high DCAB value, grass silage is the most likely cause as values can approach +1000meq/kg dry matter. Grass silage is highly variable and should always be analysed before being included in the pre-calving ration. Silage analysis only costs about £25 a sample, he adds.
In contrast, maize silage is consistently low at about +200meq/kg dry matter, and is, therefore, a better forage for milk fever prevention, says Mr Husband.
"The reason for this variability in grass silage is differences in soil potassium levels. DCAB values are normally higher for first cut silage and cuts taken from grassland receiving large amounts of dung or potash."
To better understand the benefits of lowering DCAB by feeding anionic-salts, Mr Husband carried out two on-farm trials funded by the British Cattle Vet Association.
The first study looked at feeding 100g a day each of magnesium sulphate and ammonium chloride in a mixed ration to reduce DCAB to about -150meq/kg DM. Feeding these anionic-salts caused a mild metabolic acidosis, resulting in more acidic urine compared with cows fed a standard dry cow ration, he says.
"But the standard ration already had a relatively low DCAB of +169meq/kg dry matter, which resulted in very few cases of milk fever. Cows fed anionic-salts had no milk fever and blood calcium levels were greater at calving."
He set up a similar study on a separate farm, but fed a premix on top of grass silage instead of using a mixed ration. Cows were able to avoid the anionic-salts, which resulted in variable mineral intakes, says Mr Husband.
Producers without mixer wagons can still take action without using anionic-salts, he believes. "Dry cow ration DCAB can be reduced to less than -150meq/kg DM by simply avoiding high DCAB silage."
A common strategy to combat milk fever is to add magnesium chloride, an anionic salt, to water troughs or feed. But it can make the situation worse when large quantities are used without appropriate forage analysis and monitoring, warns Mr Husband.
"The problem is that urinary calcium losses are up to 10 times higher for cows fed diets with strongly negative DCAB values. Therefore, urine pH should be monitored so calcium can be added when required."
Another problem encountered when feeding anionic-salts is reduced ration palatability. To maximise palatability, US producers routinely feed a mixture of salts and select forages with low values to minimise the amount of salt needed."
To build on his research, Mr Husband was awarded a travel scholarship to the US by the Royal Bath and West Society.
He visited several California-based producers with extensive experience of feeding anionic salts to dry cows.
One unit visited was River Ranch, which has 3000 Holstein cows averaging 11,000 litres on three-times-a-day milking. The units vet Philip Jardon is closely involved in herd nutrition and health, says Mr Husband.
"Dry cows are fed anionic-salts in their last three weeks of gestation and the units large size means the dry group normally contains about 150 cows. This makes dry cow management much easier than on UK farms."
As part of their strategy, lucerne hay is analysed and only sources with a DCAB of less than 100meq/kg dry matter are used. Urine pH of dry cows is also routinely monitored, this task is made easy with lockable feeding yolks.
"Urine pH data is recorded and inspected by Mr Jardon during his weekly visit and compared with numbers of milk fever and retained placenta cases. Increased urine pH quickly alerts you to errors in ration mixing."
Using this approach, the ranch has achieved milk fever rates of less than 3%, he adds. *
By Richard Allison
FEEDING grass silage which has an unbalanced mineral content during the last three weeks of pregnancy can cause more cases of milk fever and retained placenta in dairy cows.
There is a widespread belief that a low calcium dry cow ration is the most important factor in prevention of milk fever, says Glos-based vet and recent Bath and West scholar James Husband. "But low calcium levels can be difficult to achieve, rarely being less than 50-60g a day using standard dry cow rations."
Instead, the most important aspect is the dietary cation-anion balance (DCAB). The relevant cations are sodium and potassium, while the main anions are chloride and sulphate. A highly positive value is associated with more cases of milk fever and an increased incidence of retained placenta.
When a ration has a high DCAB value, grass silage is the most likely cause as values can approach +1000meq/kg dry matter. Grass silage is highly variable and should always be analysed before being included in the pre-calving ration. Silage analysis only costs about £25 a sample, he adds.
In contrast, maize silage is consistently low at about +200meq/kg dry matter, and is, therefore, a better forage for milk fever prevention, says Mr Husband.
"The reason for this variability in grass silage is differences in soil potassium levels. DCAB values are normally higher for first cut silage and cuts taken from grassland receiving large amounts of dung or potash."
To better understand the benefits of lowering DCAB by feeding anionic-salts, Mr Husband carried out two on-farm trials funded by the British Cattle Vet Association.
The first study looked at feeding 100g a day each of magnesium sulphate and ammonium chloride in a mixed ration to reduce DCAB to about -150meq/kg DM. Feeding these anionic-salts caused a mild metabolic acidosis, resulting in more acidic urine compared with cows fed a standard dry cow ration, he says.
"But the standard ration already had a relatively low DCAB of +169meq/kg dry matter, which resulted in very few cases of milk fever. Cows fed anionic-salts had no milk fever and blood calcium levels were greater at calving."
He set up a similar study on a separate farm, but fed a premix on top of grass silage instead of using a mixed ration. Cows were able to avoid the anionic-salts, which resulted in variable mineral intakes, says Mr Husband.
Producers without mixer wagons can still take action without using anionic-salts, he believes. "Dry cow ration DCAB can be reduced to less than -150meq/kg DM by simply avoiding high DCAB silage."
A common strategy to combat milk fever is to add magnesium chloride, an anionic salt, to water troughs or feed. But it can make the situation worse when large quantities are used without appropriate forage analysis and monitoring, warns Mr Husband.
"The problem is that urinary calcium losses are up to 10 times higher for cows fed diets with strongly negative DCAB values. Therefore, urine pH should be monitored so calcium can be added when required."
Another problem encountered when feeding anionic-salts is reduced ration palatability. To maximise palatability, US producers routinely feed a mixture of salts and select forages with low values to minimise the amount of salt needed."
To build on his research, Mr Husband was awarded a travel scholarship to the US by the Royal Bath and West Society.
He visited several California-based producers with extensive experience of feeding anionic salts to dry cows.
One unit visited was River Ranch, which has 3000 Holstein cows averaging 11,000 litres on three-times-a-day milking. The units vet Philip Jardon is closely involved in herd nutrition and health, says Mr Husband.
"Dry cows are fed anionic-salts in their last three weeks of gestation and the units large size means the dry group normally contains about 150 cows. This makes dry cow management much easier than on UK farms."
As part of their strategy, lucerne hay is analysed and only sources with a DCAB of less than 100meq/kg dry matter are used. Urine pH of dry cows is also routinely monitored, this task is made easy with lockable feeding yolks.
"Urine pH data is recorded and inspected by Mr Jardon during his weekly visit and compared with numbers of milk fever and retained placenta cases. Increased urine pH quickly alerts you to errors in ration mixing."
Using this approach, the ranch has achieved milk fever rates of less than 3%, he adds. *