Rural Diary

What is the most important nutrient that a plant needs? The answer to this question is: the one that is the most limiting, also known as the ‘law of the minimum’. This law states that growth is dictated not by total quantity of nutrients that are available, but by the scarcest nutrient.

Crops require 16 essential nutrients to complete their life cycle. Essential plant nutrients are divided into macro and micronutrient groups. Macronutrients are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S). 

Micronutrients include zinc (Zn), copper (Cu), iron (Fe), manganese (Mn), boron (B), molybdenum (Mo) and chlorine (Cl).

Plants require macronutrients in greater amounts compared to micronutrients, although all nutrients are important for plant growth. The first three macronutrients (C, H, and O) are supplied to plants majority by air so their supply to plants is not usually a problem. Ideally, the remaining 13 nutrients should be present in the soil in adequate amounts and proportion for optimum plant growth. 

Nutrient deficiency can occur for a number of reasons: when there is insufficient nutrient in the soil, where pH is significantly high or low, where a significant nutrient imbalance exists, or when they cannot be absorbed and utilised by plants as the result of unfavourable environmental conditions. The impacts of nutrient deficiency can cause many side effects, such as reducing a plant’s ability to fight pests and diseases and interfering with the conversion of nutrients into the proteins that are required for plant growth. 

Crop nutritional needs are supplied via the soil by applying capital and maintenance fertiliser. The quantity of solid fertilisers applied is usually based on soil test results, but these are mainly useful for macronutrients (soil testing for micronutrients is unreliable) and only report what is available for the growing 

crop instead of what the crop has actually taken up. There may be a need to look at complementary alternatives, such as foliar nutrition, to remedy nutrient deficiencies. Foliar nutrition is mainly based on plant tissue tests, which identifies nutrient deficiencies in the crop at the time of testing.

The purpose of foliar nutrition is not to replace soil fertiliser, as this is the most effective and economical means of supplying a plant’s macronutrient needs. The benefit of foliar nutrition is that it is an excellent method of supplying plant requirements of micronutrients (such as Mn and Mo), while supplementing any N, P and K needs for short-term growth. It is also a quick and effective management tool to favourably influence the growth of plants that have been compromised by environmentally induced stresses, such as adverse growing conditions and/or poor nutrient availability. Some of the main benefits of foliar nutrition are: 

• Rapid correction of nutrient deficiency with immediate uptake of applied nutrients.
• Accurate and even application of small amounts of key elements.  
• Providing plants with certain micronutrients that may not be readily available in the soil.
• Can be combined with other foliar chemicals, minimising application expenses.

Contact your local PGG Wrightson Technical Field Representative for advice or help with foliar nutrition.

Maize gets a number of diseases and they can all affect the final yield, but one of the most damaging is Northern Leaf Blight (NLB) caused by the fungus Exserohilium turcicum. 
NLB overwinters in infected maize leaves, husks and trash from the previous crop. The NLB fungal spores are produced when environmental conditions are favourable in the spring and early summer; then with the aid of rain splash and wind, NLB infects the new plant growing in the same or neighbouring paddock.

New developing leaves that are exposed to periods exceeding 12 hours of wet or damp conditions and temperatures between 18 to 27 degrees are susceptible to infection from NLB if the fungal spores are present. Then heavy dews and warm humid conditions spread the disease rapidly and move the infection to the lower leaves and cob. The most common and most damaging time for infection in New Zealand is December to January.

Yield loss from NLB is driven by the loss of leaf area. With weather conditions favouring infection from the early tassel stage in the crop, yield can be impacted by as much as 30 percent. With an infection later than this stage, the impact is considerably less and there is an increased risk of lodging.

So what can you do to manage the risk?

1. Crop rotation
   Growing maize crops back-to-back in the same paddock increases the amount of inoculum passed from one crop to the next, so try to avoid this.
2. Maize grain
   Crops of maize grown after a maize grain crop are more susceptible to infection than after a silage maize crop because more stova is left after harvest.
3. Cultivation
   If you fully cultivate after harvesting the maize, burying all of the residue, then there is less plant material to infect. Direct drilling, minimal cultivation and strip tilling leave trash on the surface can lead to an infection risk to the next crop.
4. Hybrid selection
   All major seed breeders publish the susceptibility of each hybrid to NLB, so when choosing your hybrid, make sure you take this into account.
5. Planting date
   Late planted crops are at higher risk from developing NLB as there is more inoculum around in the environment from earlier planted and infected crops. The later planted crops are more likely to be at an earlier growth stage when infected.
6. Fungicides
   There are a few products with activity on NLB but they have to be sprayed either before or at early infection, protecting the plant from infection rather than trying to cure it. The best time to prevent the worst impact of disease is at early tassel stage; usually this has to be done by an aircraft because of the crop height.
7. Monitor 
   Ensure you monitor the crop through the high risk infection period and growth stage.

For more advice on how to manage and control NLB, contact your local PGG Wrightson Technical Field Representative.

The days of spraying broad spectrum insecticides every time insects are seen in a crop are over. We now have the ability to make informed decisions using an Integrated Pest Management (IPM) approach and achieve better crops.

In previous years if insects were seen in a crop, it was sprayed with a broad spectrum insecticide. This was all that was available to us, but with modern science and more selective insecticides, we now have the ability to alter farming techniques and rotations to discourage pest insects and use selective insecticides as a precision tool to target them. 

Most of the insects in your crops are carnivorous insects that eat other insects, which means most of the insects are beneficial to our crops. These are termed ‘beneficial insects’. What we do on-farm affects the populations of both beneficial and pest insects. An example is caterpillars which are a problem every year in forage brassica crops. Diamondback moth caterpillar populations are affected by the number of eggs that the moth lays. Some weeds flowering in a crop can act as a source of nectar to the diamondback moth, increasing the number of eggs they lay and so the number of caterpillars in your crop. Controlling weeds can help reduce caterpillar populations in your forage brassica crops.

When planning your farm rotations, take into account that pests build up in crop paddocks so don’t plant the same crop in the same paddock year after year and plan a good crop rotation. It is a simple decision and doesn’t usually cost much but can have a big impact on pest populations on your farm. 

Beneficial insects are everywhere and forage brassicas are no exception. Some of the ones I see when walking crops are lacewings, hoverfly larvae and parasitic wasps. All of these are sensitive to broad spectrum insecticides, but by using a product which only targets a pest insect, we can protect these beneficial insects while preventing any further damage to the crop. 

One example of how you find out if your crop contains beneficial insects is to look for parasitized aphids. Parasitized aphids are much larger than healthy aphids because they contain the parasitic wasp which is growing inside them. A large number of parasitized aphids and only a few healthy aphids tells us we won’t need to use an insecticide for aphid control and can then focus on what the caterpillar population is doing, reducing the impact on the environment. 

If you would like to get more information about IPM in your forage brassica crops, contact your local PGG Wrightson Technical Field Representative who has access to the latest information and products to suit an IPM approach.

Controlling problem weeds

Some weed populations build up on-farm over time, and it takes a concerted effort to get on top of them. Examples I have come across in New Zealand are horehound, ragwort, Californian thistle, nasella tussock and yellow bristle grass. Use the following three suggestions to help prevent problem weeds getting established on areas of your farm.

• Step 1: 
  Control the plants the first time you see them on your farm. The stock yards are a good place to look, also watch feeding areas if you have brought in feed.
• Step 2: 
  Prevent any new weeds from flowering and seeding. Each plant can produce hundreds to thousands of seeds each year so a small amount of prevention is a good idea.
• Step 3: 
  Find out the best control options for the target weed. This could be spraying or sometimes manual removal.

If you need help identifying a plant you don’t recognise, contact your local PGG Wrightson Technical Field Representative.

Managing Facial Eczema (FE) requires a multipronged approach that involves reducing spore intake and providing zinc as a prophylaxis and therapy. 

FE is caused by a mycotoxin produced by the fungus Pithomyces chartarum growing in pasture dead matter. This fungus multiplies rapidly when overnight temperatures are above 13 degrees Celsius and when there is moisture available. The fungal spores are ingested by livestock and release a toxin called sporidesmin, which is concentrated in the bile of the liver causing significant liver damage. This causes phylloerythrin, a breakdown product of chlorophyll which builds up in the blood stream and reacts with sunlight causing photosensitivity and the lesions we recognise as FE. By the time you see the physical signs of FE, there is already significant liver damage in a large proportion of the mob. 

Zinc helps to reduce FE incidence. The mycotoxin in sporidesmin is a thiol compound. Thiols react with certain metals to form stable compounds called mercaptides that do not cause tissue damage. Zinc is a mercaptide forming metal and is able to prevent and limit tissue damage from the sporidesmin. It is important to note that chelated zinc does not have this protective effect. 

Zinc prevention needs to occur two to three weeks prior to the risk period to build up protective levels in the animal (see Zinc dosing steps). When spore counts are rising towards 30,000, begin zinc supplementation so that by the time counts actually reach 30,000, the animals are receiving 2 g of zinc per 100 kg. 

The most effective treatment for preventing facial eczema is the use of The Time Capsule®. It contains zinc oxide, providing four weeks protection in cattle and six weeks in sheep. Repeated treatments extend this protection period. It is a safe and easy-to-use bolus, that leaves no residue and removes the guess work from dosing.

In a 2019 trial¹, The Time Capsule was administered to cattle on two farms over a period of two months. Animals were blood sampled and weighed each week to determine liver function and serum zinc levels. The Time Capsule performed well with serum zinc levels rising quickly in all animals to be greater than the protective level of 18 umol per L at the first sampling one week after administration. The levels remained consistently protective through the eight week study.

Liver function in the animals remained well within the normal parameters indicating that the cattle were protected. There was no relationship seen between the serum zinc levels and weight gain. Weight gain was not affected by the zinc treatment on the two farms.

If animals are affected with clinical FE, it is still worthwhile treating them with zinc. Treatment helps to reduce the continuing liver damage and allow for recovery. Zinc can also be supplied through supplementation in water and feed using products like Zinc Oxide, Zinc Sulphate Heptahydrate and Zinc Sulphate Monohydrate. If you are going to supply the zinc through water or feed, give animals an oral drench to lift levels quickly and safely. The zinc content of supplements varies, so check with your PGG Wrightson store or Technical Field Representative to calculate the correct dose for the product you are using. Blood samples can be analysed to check that the animals are receiving a protective dose.

For more advice around FE treatment or prevention, contact your local PGG Wrightson Technical Field Representative or store. 

Zinc dosing: Steps for when to start treatment 

• Step 1: 
  Monitor regional spore counts
  When regional counts trend upwards to reach 20,000, continue to Step 2.
• Step 2: 
  Monitor farm specific counts
  Choose four paddocks that are representative of the farm. When farm specific counts rise to reach 30,000 continue to Step 3.
• Step 3: 
  Weigh a selection of cattle
  Accurate live weight measurement is essential to ensure the correct bolus size or mixing rate is being used. Guesswork leads to error!
• Step 4: 
  Dose cattle with full zinc rates
  If spore counts are over 30,000, cattle should be dosed with a full dose of zinc: 2 g of elemental zinc per 100 kg liveweight per day.
• Step 5: 
  Is my programme working?
  Have you checked your dosing method is effective? Recent New Zealand trial work¹ has highlighted that approximately 70 percent of zinc dosing programmes don’t achieve protective levels. Measure serum zinc and GGT three to six weeks into your programme.

 ¹ 2019 New Zealand trial approved by an animal ethics commitee, conducted by an independent research company.

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