Autumn Agronomy

Putting in the ground work

When it comes to harvest, regrassing or nurturing crops ready for winter feed; it's the little things that count. You're local PGG Wrightson Technical Field Representative, and store have the products and support to help your crops perform.


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Controlling diamondback moth

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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Controlling pests in brassicas

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.

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

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. 

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Don't let caterpillars steal your summer feed

When growing forage crops, there are usually weeds, pests and disease competing, consuming or destroying valuable feed.

The natural ecosystem keeps pests in check and stops them becoming an economic issue. However, sometimes with favourable conditions, they increase in numbers and start having an economic impact on the crop so need to be managed.

The plantain moth is a member of the carpet moth family and is made up of two species (Scopula rubraria and Epyaxa rosearia). While very little is known about these two species, they are commonly found in New Zealand and Australia and are of the type called “loopers”. The adult moths are quite small with a wingspan of 20 mm, both species look very similar and are light brown in colour with darker brown spots and a dark brown band towards the end of the wings.

With the increase in plantain as a stock feed, either as a stand-alone crop with or without clover, or as a constituent of pasture of another crop, caterpillars are rarely an issue in the establishment year, but numbers can grow to epidemic proportions in the second and subsequent summers. If you experience large numbers of moths through the summer (starting in December), then you can expect a large number of eggs laid which hatch quite quickly into caterpillars with a voracious appetite for plantain leaves. The lifecycle of these caterpillars is short, pupating and emerging as adults to lay more eggs and therefore more caterpillars several times through the summer months. In winter, they largely disappear, probably overwintering in the soil as a pupae.

To monitor for these pests through the summer months, walk through your plantain paddocks in the evening. If there are large numbers of small brown moths flying up in front of you, then look closer in the leaf litter and see if you can find small curled up caterpillars and evidence of their feeding on the plantain leaf. In years where there have been large numbers of moths, some farmers have observed 90 percent leaf area of plantain plants consumed by these caterpillars.

If you have identified moths in your crop, contact your local PGG Wrightson Technical Field Representative for a control plan.

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Autumn Agronomy

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