Grazing of canola (Brassica napus) crops during winter has been adopted by many mixed farming enterprises in south-eastern Australia. Seasonal conditions in 2018 led to canola crops being cut for hay followed by late spring rainfall which facilitated regrowth of crops. Opportunistic grazing of livestock on canola crops must be managed appropriately to avoid animal health issues and subsequent economic losses, along with the transition from crop to any new feed source.
During early December 2018 on a property at Barellan in the Riverina of NSW, mixed sex autumn-drop crossbred lambs were grazing oaten stubble. They also had access to canola hay, barley and cereal-based pellets in self-feeders. A commercial mineral loose-lick was available to lambs in lick feeders in the paddock. The lambs had been vaccinated with 6-in-1 one month prior.
The owner had moved the lambs from the oaten stubble into a paddock of canola which had been cut for hay in September 2018 but had reshot following over 45 mm rainfall in the region throughout November 2018. The canola flowered again (Figure 1).
The owner moved the lambs into the canola paddock and trail-fed barley, however no hay was provided in this paddock. Within 24 hours the owner found three lambs recumbent in the paddock and approximately another 20 appearing lethargic. The mob was removed from the canola paddock back onto cereal stubble with grain and pellet access. Four lambs died over the next three days.
After the lambs were returned to the cereal stubble, they were observed to be scouring and appeared to have a staggering, tender-footed gait. Two lambs were examined in the paddock. They were each mildly pyrexic, dehydrated and scouring. One of the lambs had marked abdominal distension. Blood samples were collected for haematology and biochemistry on these lambs. D-lactate levels were elevated and results indicated dehydration and inflammation in both animals.
A post-mortem was completed on one lamb found dead in the paddock. There were copious amounts of what appeared to be canola plants and some grain present in the rumen (Figure 2). D-lactate, nitrate and nitrite levels were within normal limits.
A random sample of the fresh canola crop was collected for analysis. Nitrate levels in this sample were 3074 mg/kg, Neutral Detergent Fibre (NDF) 44%, Water Soluble Carbohydrate (WSC) <4%.
The clinical signs and D-lactate levels in both living lambs after they had been moved off the canola crop were consistent with ruminal acidosis, which was supported by access to supplementary grain when moved back onto cereal stubble. This complicated the case and without collecting samples from lambs while they were directly grazing the canola it is difficult to determine the exact cause of morbidity and mortality for lambs that died before their feed was changed again. There are several potential animal health issues that can occur when grazing brassicas such as canola. Samples taken from live lambs and canola plants make nitrate/nitrite poisoning less likely in this case, as well as an absence of haemolytic anaemia caused by excess levels of S-methyl-Cysteine Sulphoxide (SMCO)1.
Potential causes of disease in the animals which died before being moved off the canola include: ruminal stasis due to poor adaptation of majority-canola diet; bloat due to a rapid release of plant cell contents into the rumen, or enterotoxaemia (pulpy kidney), the last of which is less likely given vaccination history2,3.
Grazing canola in winter before seed production, removing livestock prior to bud elongation, is a relatively recently developed practice in south-eastern Australia4. The late-season rainfall in this case and regrowth of canola following being cut for hay is an uncommon occurrence. Without prior experience of such canola growth patterns and minimal green feed available elsewhere on the property, grazing management decisions were made without knowledge of how animal health would be affected.
The crash-grazing strategy and subsequent quick transition back to cereal grain and pellets that was employed in this case complicated not just the differential diagnoses but the potential for management of affected animals. Instead a preventative approach should be taken to manage initial grazing of reshot canola as a novel feed source and would have negated the second change in diet for this mob. Animals should be introduced to all feed sources gradually, increasing quantity of feed or time on feed each day until accustomed to the crop or supplementary feed2. Roughage such as hay should be provided prior to being introduced to any crop, between times on the crop, and available in the paddock at all times. A stepwise adaptation of livestock to a change in diet assists to establish a stable microbial population in the rumen and helps improve feed efficiency5. A diet consisting of both canola and roughage is important for rumen function and sheep and cattle fed sole diets of brassicas have been found to have low growth rates1.
A method for practically managing the introduction of livestock to grazing canola consists of grazing between 10am-2pm each day for seven days. After seven days animals can be left on the crop provided they have ongoing access to hay. While the owner in this case acted correctly in removing lambs from the canola paddock immediately after noticing clinical signs, the transition back to a predominantly cereal and pellet-based diet should also have been managed gradually. A slow reintroduction of these readily fermentable carbohydrates would have reduced the incidence of subsequent ruminal acidosis.
Key learnings from this case were the importance of careful transition onto new feed sources and that at times the remedy (in this case moving the animals off the canola paddock) when mismanaged can also contribute to disease.
Dr Judy Ellem, Dr Bruce Watt, Dr Elizabeth Braddon & Dr Emily Stearman.