Bloat has been estimated to cost the beef cattle industry in southern Australia about 47 million dollars per year making it the most significant disease in this region (Sackett et al 2006). Costs not only include deaths but also preventative measures and the avoidance of highly productive pastures due to the risk of bloat. Bloat normally occurs on high legume content pastures however the late winter and early spring of 2007 there were widespread reports of bloat on unusual pastures.
In the spring of 2007 many cattle across the central tablelands died of frothy bloat. Most of these occurred predictably on predominately legume pastures. However in late July and early August 2007 I was called to investigate deaths in cattle on three properties in which the cattle were either grazing oats or were fed wheaten hay.
In case 1, two mature Angus cows died suddenly six weeks after introduction onto a paddock of grazing oats. The deaths were very sudden and occurred on a paddock that had been top dressed with urea 6 weeks previously. The cattle had been vaccinated recently with 5:1. Both cows were moderately autolysed on post-mortem but aqueous humour testing was negative for nitrates and nitrites and calcium and magnesium levels were normal. Initially bloat was not suspected until I saw case 2. The owners used bloat oil in troughs and placed bloat blocks in the paddock and no further deaths occurred.
In case 2, of seven purchased Charolais steers and 30 Red Angus steers, two Red Angus and one Charolais died suddenly over a ten day period in early August 2007. The deaths commenced 9 weeks after they were introduced onto grazing Nile oats. The steers were treated with Moxidectin, a vitamin-mineral drench and were vaccinated with 5:1 before they were introduced to the oats. No steers were post-mortemed as the most recent death was autolysed but about 20-30% of the mob had marked high left sided abdominal distension. The oat crop was very clean having previously been sprayed with a broadleaf herbicide and was fresh and dark green. The paddocks received 40 tonnes per hectare of biosolids prior to sowing and 110 kg of single super was sown with the crop.
The owners elected to use bloat blocks (bloat capsules being temporarily unobtainable) and reported that they were consumed readily. No further deaths occurred and only one steer showed signs of rumenal distension.
Case 3. Nine Poll Hereford cattle died suddenly in early August 2007 over a 3 week period. The cattle of both sexes and of mixed ages were grazing on a very short virtually inaccessible clover, ryegrass pasture and so were supplemented with wheaten hay. The owner noticed that the cattle became bloated shortly after consuming the wheaten hay and that the deaths occurred close to the trail of hay. Post-mortems were conducted both by me and by private practitioner Andrew Denman and were consistent with death from frothy bloat. Aqueous samples were negative for nitrates and nitrites and calcium and magnesium levels were normal. In the case shown below the rumen was filled with frothy green ingesta and there was marked anterior congestion and posterior blanching of the carcase.
The owner commenced treating the hay with bloat oil and put bloat oil in the water troughs and reported that bloating and deaths ceased by mid-August 2007.
I have not seen bloating associated with cereal hay previously. A sample of the hay was initially tested for nitrates. Levels were reported as under 200 mg of nitrate and under 20 mg of nitrite per kg wet weight, well below toxic levels. Samples of hay were then submitted for routine feed analysis.
The tests showed a dry matter content of 93.8%, a crude protein of 11.6%, a dry matter digestibility (DMD) of 64% and an ME of 9.2 MJ/kgDM.
While it seems highly likely that the deaths in case 1 and 2 were due to bloat this was not confirmed. The oats crops were not tested for protein levels but it is noteworthy that both crops had supplementary nitrogen either as urea or as biosolids. While in case 3 the post-mortem findings were highly suggestive of bloat it is difficult to explain. The hay at a crude protein of 11.8 is well within the normal range for cereal hay and well below legume hay. FEEDTEST based at Hamilton found an average crude protein of 9.3 with a range of 2.2 to 22.1 for 2,397 samples of cereal hay tested between August 2006 and January 2007. ME levels averaged 10.2 (range 5.1 to 12.9) and DMD averaged 68.7 (range 39.1 to 84.5).