Monitoring your legume crop
It is incredibly important, particularly when planting a new legume species, to monitor it closely and observe any under performing areas, as well as the areas that are performing well. Elders agronomist Nick Eyres advises on what you should be looking for and why.
If you find holes in the crop, make sure you have a good look around to diagnose why.
Firstly, look closely at the landscape. Any water movement or soil type changes can indicate a change in interaction with any pre-emergent chemicals, or possibly with insects.
Insects and insect damage
A lot of legume species are extremely attractive to the hungry insect, which is no surprise as a lot of pulses or legumes have been developed as a feed for grazing animals or high protein food source for us humans (think lentils, chickpeas).
Watch for early damage to the stem and leaves as they emerge from the ground, then equally critically during flowering to pod set.
Legumes are typically moderate to deep green in colour (with various shades), given good chlorophyll production typical of non-nitrogen limiting scenarios. If you find patches,or whole paddocks of off-colour legumes there are a few items you can check.
Firstly, that there was adequate inoculant used at seeding put close to the seed. As legumes form a bacterial-plant symbiosis, you must ensure they have the most apt bacterial species in and around their root system to aid in nitrogen fixing at seeding.
This is easy to check, as if you are unable to find any root nodules with apt red colouring when split open on a dug up root system, you can deduce there is poor nodulation and/or poor inoculation, both of which will be leaving the plant N-hungry and not fulfilling its duty.
Chemical carryover can also induce similar symptoms and is a significant cause of preventable legume failure in cropping systems.
Virus and diseases
Like all cropping plants, disease is an issue for most legumes to varying degrees. Diagnosis is incredibly important for management decisions, both in-season and for future seasons.
Any discolouring of plant tissue, namely reddening, is likely to be caused by a virus. Necrotic or darkened areas of plant tissue is more likely to be caused by a fungal disease.
How and when these are distributed can tell you about the source of the pathogen and what it is likely to be. Make a note if they are in hotspots around the paddock or evenly distributed, or if you have any disease vectors like aphids present in the paddock.
This is a kicker for diagnosing issues in legume production. Somewhere along the line there is an environmental factor contributing to other-than-ideal legume nodulation which can have variable impacts on production.
If the plant is otherwise healthy, there is clearly significant background nitrates to produce the plant proteins required. This is most likely from residual N from the previous crop or pasture, resulting in reduced nodulation.
If there is significant reduction in plant performance where there are low nodule numbers, nodule formation has been inhibited by an environmental factor. This can often correlate to chemical residues such as Group B (such as Monza, Ally) or significant levels of lontrel carryover. Good records are essential for managing legume production.
The other major nodulation issue often seen by legume growers are the lacklustre colouration of nodules. In an ideal nodule, we have a rich red colouration from the leghaemoglobin proteins which indicate beneficial bacteria have inhabited that nodule and are actively fixing N. Often we find nodules can be an off green or low levels of pink, which indicate that we no longer have a nodule that is actively fixing N.
Managing a legume for success is of utmost importance and recognising potential issues to mitigate can aid in performance in any one year. The variability of seasons can influence management practices in legume production and keeping a close eye on them is the most important practice to implement.
Haven’t committed to sowing legumes yet? Read Nick’s article on things you need to consider when adding legumes to your crop rotation.