Grazing after summer rain

Tuesday, January 30, 2024

Stock Journal Technical Column - February 2024

Author: Emma Shattock, Livestock Production Advisor,  Elders

What was originally shaping up to be a long, dry summer has turned around, with much of the state receiving some summer rain. This presents a range of challenges for managing grazing sheep, from the changes in feed quality to the added presence of undesirable or toxic plants. It is critical to understand the potential issues summer rain can bring and how to manage sheep accordingly.

The first of these issues is the sudden and dramatic change in the quality of dry feed. Once a pasture has dried off it slowly degrades, losing digestibility, energy and protein over time. Rain on dry feed speeds this process up, resulting in dramatic changes to feed quality relatively quickly.

Rain washes out water soluble carbohydrates, the main energy source in dry feed. The result is a high fibre, low digestibility and low energy feed. For sheep, this means that pasture which would have sustained a dry ewe may no longer. Lambs will be facing an even larger energy deficit due to their smaller rumen and relatively higher energy requirements.

Then comes the new, green pick, be it in the form of summer weeds, crop regrowth or perennials such as lucerne. These are often the complete opposite in feed value; low in fibre and high in digestibility, energy and nitrates. It takes time for a rumen to adapt to a new type of feed and the sudden change can lead to issues such as pulpy kidney and green feed scours. Many summer weeds are particularly high in nitrates, exacerbating the challenge to stock.

It is critical to smooth out this transition with roughage and mineral supplementation, while avoiding adding more nitrogen through urea. Make sure sheep have been vaccinated recently (within 2-3 months) for pulpy kidney before a change on rich feed.

Toxic weeds such as heliotrope (potato weed) and caltrop may also appear. Many of these are poorly palatable and only eaten when stock are hungry and have little choice. Ingestion can lead to health issues such as liver damage, photosensitisation, jaundice and death.

While not from the plant itself, another big risk for toxicity after summer rain is found on lupin stubbles. The fungus Diaporthe toxica grows after rain and produces toxins that cause lupinosis. Affected stock appear lethargic, jaundiced, lose weight and die. Best practice is to graze lupin stubbles early and avoid where possible after a rain. Contact your local livestock advisor for more help here.

Good grazing management after rain comes from awareness and preparation. Assess pasture quality and quantity, smooth out feed transitions and monitor stock closely through high-risk periods. Balance the diet where necessary with supplementation and be on the lookout for sources of toxins. With extra awareness and management strategies, you can make the most of a summer rain.