A string of mysterious cow deaths across the dark brown soil zone of Saskatchewan during the 2007 and 2008 calving seasons left 11 producers short 46 cows. Molybdenum toxicity was determined to be the cause.
Patty Smith, business development manager with Blair’s Fertilizer at Lanigan, Sask., says that once the problem had been identified, other producers came forward with reports of similar circumstances on their farms. More than 60 herds in all were found to have high molybdenum levels in their forages. Knowing this, the producers were able to take proactive measures to correct the mineral imbalance.
“Now there is a much greater understanding of molybdenum and overall awareness in the area than there was two years ago,” Smith adds. “We can do things to manage it before it gets to this stage, which is an easier situation to deal with.”
She became involved with the investigation when one of the first producers who had lost a number of cows within a short period of time came to her in search of clues he may have overlooked.
Because the cows went down and died at or near calving time and exhibited symptoms similar to those of milk fever induced by a calcium/phosphorous imbalance, they had been treated with subcutaneous calcium drips. This was unsuccessful.
Blair’s Fertilizers is a Cargill Animal Nutrition retailer. Working with Cargill’s animal nutritionist Shannon Borden, Smith collected feed samples from the farm. The tests came back showing nothing out of the usual. When they requested a molybdenum test, the reading came back off the charts. Testing for molybdenum and copper levels is standard in Blair’s mineral program. If you’re sending feed samples off to the lab yourself, you’ll have to specifi-cally request these tests.
Though none of the producers with whom they consulted had tested their feeds in the fall, the cattle diets were found to be generally adequate. Some were on a mineral program and others were providing minerals for only part of the year.
Molybdenum became suspect when it was learned that the symptoms of molybdenum toxicity range from acute scouring, faded and rough hair coats, swollen and stiff joints (particularly in the hind legs, and loss of body condition, to reduced milk production reflected in decreased weaning weights, poor reproductive performance year after year, and downer cows.
The affected cows in all of the herds were thin, with the majority of deaths being among younger and older animals and continental breeds. Other cows showed less severe signs. Those that went down never did recover in these particular cases. They succumbed at or near calving because that’s when nutritional requirements are the greatest for calving, lactation and self maintenance — something had to give.
“A degree of molybdenum toxicity is very common. We see more clinical cases in herds mainly on alfalfa diets with continental breeds, and less in British breeds, particularly if the herd is not on a 100 per cent forage-based diet,” Smith explains. “Even then we see sub-clinical signs, such as the rough hair coat or low body condition score. Producers can easily miss the signs because they see their cows every day.”
The first sign of any mineral defi-ciency or imbalance is a decrease in immunity, followed by a decrease in maximum growth or fertility, which gradually leads to a decrease in normal growth and fertility before any clinical signs become evident.
Causes of molybdenum poisoning
The symptoms of molybdenum toxicity mimic those of copper deficiency in beef cattle. This is because molybdenum ties up copper in the rumen interfering with its absorption into the bloodstream. In effect, this creates a copper deficiency (which can be fatal as well) even though the dietary intake of copper may be adequate.
It doesn’t take an excessive level of molybdenum in the diet to trigger toxicity. The safe level is less than three ppm (parts per million) on a dry matter basis, however, as little as one ppm can lead to toxicity when dietary copper is deficient. Molybdenum intake in excess of 10 ppm can cause toxicity regardless of copper intake.
The key is maintaining the balance. An ideal copper-to-molybdenum ratio is 6:1; borderline is 3:1; toxic is less than 2:1. The condition is most commonly seen when dietary copper levels fall below six to nine ppm and molyb-
denum levels are up in the three to five ppm range.
An incidental source of molybdenum is ingestion of soil due to overgrazing. Sulfates in the water are also implicated because they interfere with copper absorption, thereby increasing the potential for feeds with relatively low levels of molybdenum to become toxic.
“The area affected had experienced a cold spring and forage clippings taken early in the year tended to be high in molybdenum,” Smith explains. Molybdenum accumulates in plant tissue as the growing season progresses. Molybdenum content in the hay samples taken from the affected farms ranged from one ppm to 12 ppm, while the copper level ranged from zero to 13 ppm.
Random samples of forages collected by Blair’s for testing from the dark brown soil zone across Saskatchewan in 2007 and 2008 showed that 100 per cent were deficient in zinc, 80 per cent were deficient in copper, 100 per cent were deficient in selenium, 50 per cent were deficient in manganese, and 80 per cent had molybdenum concerns.
With the exception of zinc, the deficiencies were far more pronounced in the dark brown soil zone than they are in general across Western Canada. A Cargill Animal Nutrition evaluation of 352 forage samples taken from across Western Canada showed that 97 per cent of the samples were deficient in zinc, 36 per cent were deficient in copper, 20 per cent were deficient in selenium, 24 per cent were deficient in manganese and 60 per cent had molybdenum concerns.
Molybdenum content in plants varies in response to molybdenum content in the soil, soil pH, and the season. Typically it is a concern on peaty and poorly drained soils because molybdenum follows the water table. Tap-rooted plants absorb more molybdenum than shallow-rooted plants. Neutral and alkaline soil conditions favour uptake of molybdenum by the plants, so alkaline sloughs can be hot spots.
On the basis of post-mortem results and feed testing, the herd veterinarians made the decision not to treat the downer cows with copper injections in these cases. Adding more copper to the diet was risky because it could have triggered copper toxicity.
The veterinarians and nutritionist put together an action plan that included removing or blending the high-molybdenum forage, feeding pellets or adding grain to the ration, and providing readily-absorbed chelated minerals to get through the crisis period.
In the long term, producers are testing water samples annually and feed testing so they can manage feeds with high levels of molybdenum by mixing them off with low-molybdenum feeds or feeding them earlier in the gestation period.
“When we are formulating rations it comes down to the fact that producers have to use feedstuffs at their disposal. Only a few have had to purchased some hay or pellets. Others have blended in straw. There are always answers. The big thing is to know what you have,” Smith says.
To get representative feed samples for her clients, she waits until at least 14 days after baling, then probes for core samples from about eight bales on every quarter section. Sampling the bales in the field is preferred because the molybdenum level in the forages varies from field to field. This way, if there are bales with molybdenum concerns, you’ll be able to keep them separate for feed management purposes.
Providing a balanced mineral program year round is crucial, given that many producers no longer feed grain to cows and cattle on 100 per cent forage rations are most at risk for developing molybdenum toxicity.
Tips for free-choice feeding
The social order in the herd may restrict some animals from getting their fair share of minerals. One feeder for every 25 to 30 cows is recommended.
Intake should be about four ounces per day for a 1,300-pound cow. Uptake will increase when forage quality and/or quantity are low and decrease on pastures with alkaline soils. Salt content greater than 20 per cent may limit intake.
Monitor body condition score, per cent open cows, and calf weaning weights for a measurement of your mineral program’s effectiveness.
Patty Smith can be reached at Blair’s Fertilizers, 306-528-7592.