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“If you see anything new, get it identified,” stresses AgResearch senior scientist Trevor James. “It might be on the roadside near your property or in your pasture, but wherever it is, talk to somebody to find out what it is. The regional council is often the first place to go, or ask someone like me.”
High quality cameras on smartphones allow rapid, easy picture taking and sending for identification, he points out, but making sure the person asked to identify the problem knows their stuff is also important.
“I know of one farm where five or six years ago a new species was dismissed as ‘summer grass’ by a local advisor. Six years later the farmer was having to import 180t of feed to maintain milk production at levels he used to get off pasture.”
In that case, the problem was Yellow Bristle Grass (YBG), an annual weed first identified as a problem in Waikato 15 years ago, now established in most of the northern North Island and now reaching into Taranaki and Manawatu.
It only impacts production January to March when it has much lower energy content and palatability than ryegrass. Consequently, stock avoid it and production off pasture drops. In the case of dairy pasture with 13% YBG, it’s calculated $343/ha has to be spent on supplement to maintain milk production, says James.
“It’s an annual so it’s not there in winter. It just affects summer grazing.”
Importing contaminated hay or, when YBG is seeding, livestock from a contaminated area are two means of spread, hence the need to be vigilant about such imports. Similarly, silage-making machinery coming from paddocks or farms with YBG problems is also a risk.
“There is a spray that will kill it in pasture (Puma S, fenoxaprop-p-ethyl) but you’ll often have to hit it twice due to its staggered emergence and not let it seed. To get rid of a seedbank you need to go through a pasture renewal process with ideally two years of summer crop.”
A just completed study by AgResearch found YBG seed remains viable at least five years if ploughed down so if possible make a shallow, stale seedbed prior to any cropping to stimulate germination then take out YBG and whatever else germinates with glyphosate, suggests James.
“Near the surface the seed doesn’t persist too long, and we found it persists longer in dry, sandy soils longer than it does in heavier clay soils, which to some extent explains where we’re seeing it become a problem.”
Yellow Bristle Grass is not the only problem pasture weed to watch for. In the northern half of the North Island Alligator Weed “turns up in all sorts of places,” says James.
“We used to think it was a semi-aquatic weed but it’s all over the place now, probably carried there by cultivation equipment and pond area clean outs.”
Once established, control is particularly difficult due to an extensive root system. In Waikato the regional council has taken over control. “Report it and they will come in and try to control it.”
Giant buttercup is similarly a problem in the north, but also further south in Taranaki and Golden Bay. In those latter regions there’s the added complication of populations that have developed resistance to the two main in-pasture herbicides used in its control: flumetsulam and MCPA, meaning a pasture renewal phase, preferably with a crop between pastures, is needed to hit it with an effective herbicide.
Across the central North Island toxic low-growing shrub Tutsan is making a comeback as a strain resistant to rust spreads.
“The rust arrived from Australia about 30 years ago and gave reasonable control but in the past five years a strain’s emerged in the King Country that’s tolerant to the rust and it’s spreading like crazy,” notes James.
The red, later black-berried plant is “tolerant of all but the toughest brush killer-type herbicides” and has very small seeds. People picking the plant for decoration or garden use has aided spread, as has roadside mowing.
Stock generally avoid living plants but cut stems or berries pose a risk.
In Northland Gravel Groundsel is encroaching on pasture from coastal areas.
“It’s a fairly new one we don’t know how to control yet... Northland Regional Council has an application in with MBIE for funding to look at it.”
As the name suggests, the weed favours light, free draining ground and while it’s not toxic, stock avoid it, impacting production off infested paddocks.
In Horizons, Field Horsetail has moved out of riverbeds to become a pasture problem, often through movement of material for roading.
“There’s a lot of it on the West Coast but it’s only really become a pasture problem in the Rangitikei flood plain so far.”
While James admits he’s less familiar with the South Island’s problem weeds, he still rattles off a list: Viper’s Bugloss; Russell Lupins; Nassella Tussock; Chilean Needle Grass (see sidebar).
Whatever the weed, a good approach is to avoid getting it in the first place. Grazing “the long acre” and/or importing feed are often import pathways.
“The problem often comes in drought. When you’ve got hungry stock to feed you don’t always have much of an option about where you buy your feed, but if you do buy in feed, try to keep where you feed it to a limited area and go back there the following year and look for anything unusual.”
Besides minimising the risk of weed spread, keeping stock to a limited “sacrifice” area during such dry times will also limit damage to pasture elsewhere on the farm, he notes.
