Andrew Swallow

Pamper pasture in autumn to set up a successful spring because that is the key period that drives profit on most sheep and beef farms, a long-time AgResearch pastoral scientist, now working independently, stressed at a recent Beef + Lamb NZ field day.

However, while it was easy for him to say, it is not so easily done, particularly after a drought, Tom Fraser admitted to visitors to Tamar Farms, Mid Canterbury, just before the Covid-19 lockdowns kicked in.

“We’ve been sold the lucerne story, that you have to let it flower once per year,” he said.

“Really what you are doing is giving it a spell so that it can rebuild its reserves and grass is no different. It needs a spell from time to time.”

That’s particularly the case following drought when the temptation is to put stock back on to browned-off paddocks as soon as they turn green. However, that’s a recipe for disaster, he warned.

“A lot of that initial growth is from reserves and if you take it off, particularly with sheep that will graze it to ground level, it has to grow from reserves again. Do that two or three times and the plants say ‘bugger that’ and die!”

What’s more, the new growth could well be laden with parasite larvae so stock going on to it will quickly face a severe challenge.

Much better to hold stock back for a few weeks on a sacrifice area, if necessary on bought-in feed, so that your best paddocks have a few weeks to recover before they’re grazed.

“I know that’s bloody hard to do and very easy for me to say, but what you need to do is go around and score your paddocks one to four. Keep the better ones back for the last graze, or even don’t graze them at all. These are the paddocks you want to treat with kid gloves because they’re the engine room for the spring on a sheep and beef farm.”

If it’s still warm enough for growth, a little nitrogen fertiliser in autumn can help rebuild the covers, he added.

“But remember, nitrogen fertiliser should go under your feed budget, not your fertiliser budget, because it’s a feed multiplier. That’s all it is.”

Speaking to Country-Wide after the field day Fraser said as a guide, soil temperatures need to still be 8C or above to get a worthwhile nitrogen response. He also acknowledged the point made by field day hosts Richard and Chrissy Wright that nitrogen seems to provide the pasture with some frost protection.

“Farmers often say that and no-one can refute it.”

Ideally, pasture cover should be 1500 to 1600kg drymatter (DM)/ha going into winter, though many farms would have to make do with 1000-1200kg DM/ha this year. That could get to 1500-1600 by spring if grazing could be avoided, and that should be the aim at least for the blocks that will be allocated to twin-lambing ewes.

Being able to do that is a key benefit of growing winter crops, he added.

“I know there’s been a lot of controversy about them but actually I still don’t think we grow enough of them in our sheep and beef systems… Winter crops allow you to go into spring with better grass cover.”

In turn that means better lactation from ewes, more lambs away at weaning, and, prices being equal year to year, more profit.

Molybdenum the forgotten fertiliser

not thriving how it should in your pasture? It’s possibly molybdenum deficient.

“In the 1960s through to the 1980s a lot of molybdenum was used but since then it’s been rather forgotten about,” Tom Fraser, told the field day (see main story).

“If you haven’t used any in the past 10 years then you probably need to get a herbage test done.”

Blended into a fertiliser application it’s “dirt cheap” to apply, at about $6/ha, and it only needs doing once every four or five years.

However, Fraser warned that just because it’s cheap, it shouldn’t be applied “just in case” without testing to determine need.

“Don’t be silly with it, because it will lock-up copper if you use too much of it. Get some expert advice on what to do if the pasture tests do come back deficient.”

Herbage tests should be from clover only because that’s the pasture species the molybdenum directly benefits, boosting nitrogen fixation by rhizobia in the clover’s root nodules.

“If molybdenum is deficient, the clover won’t be fixing nitrogen to its full potential.”

Ballance AgriNutrients’ Jim Risk recently reviewed the need for molybdenum in the co-operative’s Grow publication, drawing on research from the 1950s, 60s, 80s and 90s.

Given the risk of inducing copper deficiency in livestock, he told Country-Wide he recommends taking three or four clover herbage samples from across the farm when the clover is growing well: ie when soil is warm and not too wet or dry, to determine if the plant is short of the nutrient.

“Beware of historic differences in management. If you were just concerned with animal health then you would look at the mixed pasture analysis of molybdenum and copper.”

AgKnowledge’s Doug Edmeades says his business is also finding renewed incidence of molybdenum deficiency, particularly in the South Island.

“If you notice a lack of clover in your paddocks it’s most likely one of three reasons. Most frequently it’s lack of potassium, followed by lack of sulphur, and then molybdenum.”

Jeff Morton from MortonAg, who with John Morrison researched pasture requirement and response to molybdenum in the 1990s, says nitrogen content of the clover also needs to be checked.

“If it was low in nitrogen and molybdenum, then we measured a response to applying molybdenum but if the clover contained adequate nitrogen then we did not measure one, even if the [molybdenum] analysis came back low.”

Another point to be aware of is that on many soils molybdenum increases with soil pH, so liming may also correct a herbage deficiency.

Where both molybdenum and nitrogen are deficient in clover, that’s below 0.1 ppm and 4.5% respectively, Ballance recommends 2kg/ha of its 1% molybdenum product Nutrimax every four or five years, applied in a blend with a regular maintenance fertiliser.

“Sometimes the full pasture production response takes a number of years to come through because if the clover is really struggling it will take a year for it to start to flourish and only then does the extra nitrogen fixed start to cycle in the pasture and you see the ryegrass and other species respond too,” notes Risk.

However, the response in clover vigour is usually pretty immediate, notes Morton, who adds that he’d push that threshold for a molybdenum application up to 0.3ppm as test levels can vary over time and the risk of complications with copper only start when mixed pasture sample results for molybdenum are well over 1.0ppm.

In his research with Morrison in the 1990s total pasture responses of nearly 3.5t DM/ha were recorded in the second year after application on one deficient site in inland Otago. Total pasture responses were more modest at other deficient sites, but all were statistically significant, as were increases in clover production.

Molybdenum reminders

  • Essential for N-fixation by legumes.
  • Potentially big responses where deficient.
  • Too much may induce copper deficiency in ruminants.
  • Herbage test clover to check need.
  • A little (20g Mo/ha) lasts a long time (4-5 years).