This means less nitrogen is made available to the grass plants so their growth is reduced.
It also means there is a lower proportion of clover in the pasture that animals graze so there is a smaller amount of high-quality feed for growing lambs.
It was in the 1950s that molybdenum was first shown to be an important trace element in nitrogen fixation and, therefore, pasture production.
The initial trial plots used an application rate of 175 grams of sodium molybdate per hectare. As a result, it was recommended farmers use sodium molybdate at 175g/ha every four to five years.
Farmers found the clover and pasture responses were amazing, so some repeated it the next year.
Soon there were cases of molybdenum-induced copper deficiency occurring in cattle and in sheep. This led many farmers to stop using molybdenum and the fear of it remains with many today.
In the 1970s the research division of MAF set up a series of field trials to determine the optimum rate at which to apply molybdenum.
These trials demonstrated that at 50g of sodium molybdate/ha the pasture and clover response was the same as that at 175g/ha.
Based on that research the fertiliser companies reduced the amount of molybdenum recommended to the equivalent of 50g of sodium molybdate/ha.
Since that reduction in the application rate of molybdenum the cases of molybdenum-induced copper deficiency have fallen right away.
In 40 years of giving fertiliser advice to farmers in the southern half of the South Island, I have not seen a case of it.
But I have seen a lot of farms where production has improved significantly with molybdenum application.
It is not uncommon to meet sheep farmers whose lamb carcase weights have increased by between 1kg and 1.5kg where molybdenum has been applied after 15 or more years of no molybdenum input.
The MAF research also showed increases of up to 20% in annual pasture growth where molybdenum had not been applied for 15 years or more.
One of the effects of lime application is to release minute amounts of molybdenum that are locked up in the soil.
Lamb carcase weights have increased by between 1kg and 1.5kg after molybdenum was applied for the first time in 15 or more years.
So soils with a high pH because of high inputs of lime might not need more molybdenum applied.
This is best checked out with a herbage test of actively growing clover. Where the herbage test shows molybdenum at less than 0.1 ppm and nitrogen at less than 4.5%, applying molybdenum at the recommended rate is extremely unlikely to induce copper deficiency but will give a significant increase in pasture growth and clover content.
In some instances where the pH is below 6.0 and there have been many years of no molybdenum input, the molybdenum levels are so low that the laboratory equipment cannot detect any molybdenum in the clover herbage.
The cost of molybdenum is very low. It is less than $1 a hectare a year. Granular molybdenum can be added to any fertiliser so it is applied at 200g/ha. This supplies the same amount of molybdenum as in 50 grams of sodium molybdate.
Granular molybdenum added to any fertiliser costs about $3.75/ha and supplies sufficient molybdenum for five years. No other plant nutrient has the same cost-benefit ratio.
For low-fertility farmland in the southern South Island, the cost-benefit of plant nutrients from highest to lowest is likely to be: molybdenum, sulphur, phosphate.
There are recent examples where spending about $100/ha on these nutrients, combined with good grazing management, has changed store country into finishing country through the dramatic increase in clover.
- Bruce Beckingsale is a field consultant for Ballance
