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Rural Diary
1 February 2020 Matthew Crampton

Perennial grass weed control of twitch

Perennial grass weeds are a problem on South Island farms and can build up over time, replacing higher producing grasses in your pastures. The one I come across a lot on-farm is the grass weed called twitch (Elytrigia repens), also known as couch.

Twitch spreads by its wiry creeping rhizomes and is commonly found as a problem throughout New Zealand’s home gardens, pastures and roadsides. Twitch is a problem because it isn’t productive compared to other pasture species, so as it dominates an area of pasture, the overall productivity drops. Like all weeds, it is also using valuable nutrients and moisture which are better utilised by desirable pasture species. 

Once identified, this weed requires a concerted effort and good timing to control it. But, let’s start with identification. From above ground, twitch looks similar to most pasture grasses, but give the leaf a pull and it shows its real colours by exposing the plant’s root system. The plant produces extensive rhizomes horizontally underground so as you pull up the grass plant some of the rhizomes come with it (you may have to gently dig out the root so it doesn’t snap off). The plant can be easily identified by its rhizomes which are bright white and grow horizontally about 10 cm below the surface. These rhizomes are a key characteristic of twitch (Image 1).

These rhizomes, which help with identification, are also the reason this weed is so successful. They provide a large reserve for the plant to bounce back after attempts at control, and also have the ability to survive being cut into pieces during cultivation, therefore dividing and producing a new plant from each piece. These two characteristics contribute to making this grass weed a problem on-farm, but by taking a few things into consideration control, is possible by depleting the rhizome’s reserves and eliminating each plant.

The best time to control any weed is when it has leaf area to take up the herbicide when actively growing, which for twitch is in the spring or autumn. Make sure you avoid the winter when twitch is dormant. In the South Island most pasture and crops are sown in spring and if there is a lot of twitch you can struggle to get control with this timing. In problem paddocks, alter your rotation so an autumn application of glyphosate is applied to actively growing twitch; this gives a great result. During your cropping phase, there is also the option of using a selective grass killer in some crops to help in the battle against this weed.

Consult with your local PGG Wrightson Technical Field Representative for advice around product selection and timing to ensure you get the result you are after and control twitch on your farm. Or for help identifying the weed, visit the PGG Wrightson YouTube channel and watch my ‘Grass Weeds: Twitch/Couch’ video.

Matthew Crampton

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Getting lime on early for next season's spring sown crops

01 February 2020

In New Zealand, our pastoral/cropping soils are usually slightly acidic. Over time, soils become more acidic due to a variety of factors including plant uptake of essential nutrients, leaching, decomposition of organic matter by microbes and the application of certain fertilisers such as elemental sulphur. 

When soils become too acidic, essential nutrients like phosphorus (P) and molybdenum (Mo) may become less available to plants. Conversely elements like aluminium (Al) become more available and toxic to plants. An excessively acidic soil can have a negative impact on the potential yield of the crop planted in it. This is where lime comes into a cropping programme and has a critical role to play in maintaining soil fertility. Adding the right amount of lime to a soil can neutralise high levels of Al and enable plants to use most essential nutrients more efficiently, and can stimulate microbial activity.  

The hydrogen ion (H+) concentration in soils, the sole cause of soil acidity, is measured in a pH scale ranging from 0.0 to 14.0 Depending on which end of the scale the pH is on, a soil can be either acidic or alkali. Acidic soils have a pH level between 0.0 and 7.0 and alkali soils have a pH value ranging from 7.0 to 14.0. The pH levels of most New Zealand soils generally falls within the range of 5.0 to 7.0, and most crops have an optimum soil pH between 5.8 to 6.4 (crop dependant). In simple terms, lime adds carbonate to the soil that reacts with water producing an alkali (OH-) and neutralising acid (H+). As acidity is neutralised, the pH increases, and the more carbonate added, the more acid is neutralised and the greater the pH increase.

A well-known, handy ‘rule of thumb’ is that 1 tonne per ha of good quality lime, for instance 80 percent calcium carbonate or better, raises the pH of a typical soil by 0.1 units. This rule of thumb is for pastoral soils that are soil tested to a depth of 7.5 cm. For crops that are sampled to a 15 cm depth, this ‘rule of thumb’ needs to be approximately doubled to 2 tonne per ha for a 0.1 unit change. 

Lime is sparingly soluble so it takes time (minimum of six and up to 24 months depending on climate and soil type) to dissolve and move down the soil profile. For this reason, it is best to apply lime to the soil at least six months prior to planting a spring-sown crop. The best way to evaluate the need for liming is to identify cropping paddocks early and have these soil tested (to 15 cm) to identify any potential pH issue. Then lime can be applied in a timely manner to rectify any issues with soil acidity. 

Talk to your local PGG Wrightson Technical Field Representative today about soil testing potential cropping paddocks and suitable liming products. 

Controlling pests in brassicas

01 February 2020

Forage brassicas form the cornerstone of forage cropping in New Zealand, because they are fast to establish and relatively cheap to grow. 

The ability to grow a fast establishing abundant feed, high in protein, energy and digestibility for livestock, as well as, in many situations, forming a useful break in the pasture cycle allows the paddock to have a period of time not growing grass or clover to control various weeds and bugs.

Brassicas may sound like the perfect crop, but there are a few issues to contend with to maximise their success in delivering valuable livestock feed. Whilst seedbed preparation, plant nutrition and weed control are all important, the one area that can have a devastating effect is pests. This is due to the yield value diluting the growing costs which can have a massive economic impact on the crop.

As farmers, we see a growing crop as feed for our livestock. If we plant and grow fresh, succulent leaves for our animals, there are millions of other animals that are also grateful for having a feed source that, in many cases, is essential for them to complete their lifecycle. A crop can cope with some amount of feed being lost to these other species, but when they start to have an economic impact on the crop, they become a pest and have to be dealt with.

The growth stage of the crop and weather patterns have an influence on what pests are likely to attack your crop. This allows you to be more focussed on what to look out for and formulate a control plan. At establishment, we need to be observant of pests, such as slugs, snails, nysius and springtail. At this time of year, though, our crops are more likely to be infested by caterpillars (diamondback moth and white butterfly), aphids and fly larvae (leaf miner). 

There are also a large number of other species that make their home in the crop, and some of these actually feed on the pests and do the control for us. These are called beneficial species and they can negate the requirement for spraying with an insecticide. Whilst it is tempting and easy to say ‘just spray with a broad spectrum insecticide like an organophosphate (OP)’, this is not environmentally responsible nor is it the safest option. It also increases the risk of the insect pest you are wanting to control becoming resistant to the spray and, with few new products being registered year-on-year, we may lose the ability to control them in the future.

This is why monitoring of pest and beneficial species is important so that an informed decision can be made on whether that pest is having an economic effect on the crop and whether an insecticide is needed. 

The correct identification of the pest is critical if a spray is required because it enables us to use an insecticide with more selective activity thus leaving the beneficials safely behind. This, in turn, reduces the likelihood of the pest numbers increasing again because the beneficial numbers are higher and control them for you. This process is called Integrated Pest Management (IPM for short).

To learn more about IPM and the beneficials in your brassica crop, contact your local PGG Wrightson Technical Field Representative, or visit the PGG Wrightson YouTube channel.

Controlling diamondback moth

01 January 2020

Mild winters can result in higher insect pressure in spring and if these conditions turn into a hot and dry summer, then Diamondback Moth (DBM) thrives. Unfortunately, this can mean damage to the leaves of valuable winter feed crops.

DBM is a pest which feeds on brassica plants including forage crops. In a hot dry year, DBM can build to plague proportions and destroy leaf area of feed crops. Using what we know about its lifecycle and beneficial insects, we can make better decisions to reduce the impact of this pest.

The lifecycle of DBM consists of four stages:

  • Adult
  • Egg
  • Larvae (caterpillar)
  • Pupae

The damaging stage is stage three. The caterpillar has the job of eating as much leaf material as it can to allow it to pupate into the adult moth. Just like a monarch butterfly, the caterpillar builds a pupae around itself and transforms into the adult inside it. This eating results in all of the visible leaf damage that we see. 

Knowing which stage causes all the damage allows us to target control options and minimise these effects. The last few years have seen the introduction of two new insecticides which are safer for the applicator and on beneficial insects than previous chemistry. Beneficial insects help us with controlling crop pests by feeding on the pests themselves, so we need to protect them wherever possible. An example of this is the lacewing. The larval stage of the lacewing walks around the leaf feeding on aphids on the crop.

Monitor your crop for the first signs of DBM. If a selective pesticide needs to be applied, the timing should be based on identification of the DBM and lifecycle stage. Identification of this first incursion is hard because the eggs are small and the adult moth doesn’t sit still long enough to easily identify the distinctive diamond pattern on its back. An easier way to identify if the pest is present is visible damage to the crops foliage. DBM damage can be easily identified by the distinctive windows formed in the leaves and/or holes which are away from the edge of the leaf. This is when to time your application for maximum benefit. As the caterpillars ingest the plant material, they also ingest the insecticide. 

If dry conditions continue through late summer, more than one application may be required. Each adult can lay more than 100 eggs, so keep an eye out for another flight going through the crop then monitor for another batch of eggs hatching. 

To get the best control of DBM, contact your local PGG Wrightson Technical Field Representative. 


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