Lucerne: effects of crop residues and wheat as a nurse crop

Lucerne is commonly used as a pasture crop in long-rotation systems in the southern Cape region, where it is rotated with wheat, barley, canola and other crops. The unpredictability of rainfall and the delicate nature of lucerne seedlings create hurdles for successful establishment of lucerne in dryland regions. The authors have explored potential solutions. Their recent study tested the effects of wheat as a nurse crop and crop residue on lucerne establishment.

What is a nurse crop?
By planting an annual companion crop alongside lucerne during its initial growth phase, support and protection to the young seedlings can be provided. Wheat, canola, and oats are among the popular choices for nurse crops, offering benefits such as efficient space utilisation, weed suppression, and protection from environmental stressors like wind and excessive sunlight.

In the southern Cape region, where conservation agriculture practices prevail, retaining crop residues as soil cover is a common strategy. While crop residues offer advantages such as soil water conservation and weed suppression, their high loads can pose challenges for small-seeded crops like lucerne. Finding the balance between reaping the benefits of residue cover and avoiding its hindrance to lucerne germination is a key aspect of sustainable farming practices.

The experiment took place near Swellendam in the Overberg region. Different treatments were tested to see how they affect lucerne establishment. Half of the site was covered with crop residue (4 t/ha), while the other half had no crop residue on the soil surface. This was to test for the effects of crop residue – common in conservation agriculture systems – on lucerne establishment. To test the nurse crop effect, wheat was planted without lucerne; lucerne without wheat; and a wheat-lucerne mixture. The wheat cultivar SST 0166 was planted at a seeding rate of 80 kg/ha (± 200 seeds/m²) and the lucerne cultivar Aurora was planted at a seeding rate of 10 kg/ha (± 450 seeds/m²).

Results
The research on effects of crop residues and wheat as a nurse crop for establishment of lucerne was published in the African Journal of Range & Forage Science. Here follows a summary of the results:

• Lucerne plants thrived better without crop residue, with over 60 plants/m² compared to only 23 to 37 plants/m² when residue was present. Having a sufficient plant population is crucial for lucerne productivity and longevity.
• Crop residue helped increase biomass production of wheat and lucerne where intercropped. This could be due to the water conservation benefits associated with residue presence.
• Wheat yield remained unchanged regardless of the treatments, but protein content varied. Wheat grown alone with no residue had the highest protein content, while wheat grown with lucerne and residue had the lowest. Wheat thousand kernel mass was highest when wheat was planted with a nurse crop and without residue. However, all treatments resulted in satisfactory wheat quality.
• Intercropping wheat and lucerne did not significantly differ in productivity compared to planting them individually.

To summarise, when wheat and lucerne are planted together, the crops compete with each other, affecting various growth parameters. Wheat thrives better in pure stands, exhibiting higher grain quality, while lucerne shows superior performance when grown without wheat. Moreover, high levels of crop residue reduced lucerne germination, which emphasise the importance of residue management during establishment.

While the nurse cropping concept holds promise, the research suggests caution, particularly in the Mediterranean climate region of South Africa. Avoiding the use of wheat as a nurse crop for lucerne in such conditions could optimise establishment success and ensure sustainable crop growth.

Source
Le Roux, S & Swanepoel, PA. 2023. Lucerne establishment in dryland conditions: effects of crop residues and wheat as a nurse crop. African Journal of Range & Forage Science, 1–6. https://doi.org/10.2989/10220119.2023.2253285.