Irrigated pastures require strategic management to avoid sheep health challenges
By Paul Nilon, Nilon Farm Health
Irrigation provides abundant lush, highly-digestible forage and high animal growth rates. On the downside, it also creates ideal conditions for some novel sheep diseases and exacerbates the potential for others.
The burden of worms
Parasitism is the biggest challenge of irrigated pastures. Barber’s pole worm (Haemonchus spp.) is an emerging problem on irrigated pastures in particular, with the potential to cause major production losses.
While all irrigated forages can sustain contamination, grass-dominant perennial pastures become relentlessly contaminated with larvae under irrigation. Legumes, while not worm-free, are parasitically much more robust.
Worm control in sheep grazing irrigated pastures centres on keeping forages clean for as long as possible after establishment. Rotational grazing with cattle has some benefit. Heavily-contaminated pastures can be managed with the strategic use of long-acting drenches.
Sudden death syndromes
Diseases such as redgut, clostridial disease and bloat can be regarded as diseases that occur due to lack of fibre.
Redgut: While the cause of redgut is poorly understood it occurs almost uniquely on lucerne, and very occasionally on clover. Current theories favour overactivity of the small intestine in the presence of high nitrogen levels. Others claim it is the result of overgrowth of atypical clostridial bacteria. Regardless, the terminal event is (sometimes) twisting of the lower intestine and death of the gut through lack of blood supply.
Allowing the lucerne to mature a bit more before grazing, providing additional fibre in the paddock through hay, and controlled grazing during introduction to the pasture can alleviate the risk. Multi-vitamin and mineral supplements are widely peddled and testimonialled, but there is no published information to support their efficacy.
Pulpy kidney (Clostridium perfringens): Pulpy kidney commonly occurs on grass and legume crops and occasionally on brassicas (e.g. fodder rape). Lambs need to be vaccinated at least twice for protection, with a booster every four months while on irrigated forages. Very occasionally clostridial-like deaths will still occur in the face of a strategic vaccination program.
Using an 8 in 1 Clostridial vaccine may help.
Bloat: Bloat is most common in stock grazing irrigated clover, seen occasionally on lucerne and sometimes on brassicas. Generally, providing some fibre controls the problem. Occasionally antibloat preparations are required.
Bloat in sheep and cattle (NSW DPI)
Metabolic diseases: While metabolic diseases, notably hypocalcaemia, are not confined to irrigated pastures, they are particularly prevalent on irrigated grasses and winter cereals.
Mid-to-late pregnancy ewes are vulnerable when moved onto lush pastures from dry winter pastures. Calcium supplementation will always prevent this, provided it is given quickly.
Hypocalcaemia fact sheet (Making More from Sheep)