Yellowing of crops and pastures

Over the last four to six weeks, I have had several enquiries from producers who have seen yellowing in forage crops and pastures. There are several potential causes of this discolouration, which we can classify into five broad groups:

1. The first group is diseases, such as fungi, viruses, bacteria and nematodes
2. Secondly, environmental stresses like large diurnal (day to night) changes in temperature, frosts and wind
3. A third group is pest damage
4. The fourth includes management impacts such as herbicides, for example
5. The fifth group, which many producers presume will be the likely cause, is nutrient deficiencies and toxicities

One of the main stresses presently causing yellowing falls into the environmental group: waterlogging. The effects may be observed in individual plants, in patches or over broader areas.

Waterlogging tends to be seen in drainage or low-lying areas. In some cases, the duplex soils are so wet that the root systems are sitting in the perched water table or in the saturated clay layer. The waterlogged plants can look like they are nitrogen deficient (and can be too) – often with yellow or pale leaves and (particularly in forage cereals) poor tillering.

Over time, these plants may become brown and die. In some cases, forage crops have not produced their secondary roots, or their root tips have ceased growing. This problem may not show up until spring, when the warmer weather and faster plant growth leaves the underdeveloped roots unable to keep up with the plants increased water requirements.

While waterlogging is the most likely cause of yellowing at present, it is also worthwhile mentioning nitrogen and sulfur.

Nitrogen (N) is important for protein and chlorophyll. When N is lacking, crop and pasture growth and production can be compromised. Nitrogen and organic matter are linked, and the release of N from the organic pool can be dependent on temperature, moisture and soil type. Nitrogen deficiency can be seen in the older leaves usually as pale green to yellow leaves.

On the other hand, a deficiency of sulfur (S) is expressed as a yellowing of the younger leaves first. Sulfur is also important in protein and integral to chlorophyll production. Deficiency of S can affect growth and lower the protein content. Sulfur deficiency is more likely to be found in sandy soils, in soils with lower organic matter and in irrigation or high rainfall areas. Low S can also impact legume persistence and nodulation.

There are other nutrients that may cause pale colour or yellowing of leaves. For further information on adequate plant nutrient levels and deficiency symptoms (in pastures) refer to page 30 of Fertiliser for Pasture 2021 booklet.