The Condition
General
Bloat is over-distension of the rumen caused by the accumulation of fermentation gases in the rumen. Primary bloat or frothy bloat is the more common form of the condition, and usually occurs as an outbreak in several animals on pasture that contains high levels of leguminous plants, clover in particular. Primary bloat can also occur in feedlot cattle. Secondary bloat or gaseous bloat is more rare and occurs sporadically in an individual animal as a secondary manifestation of another pathological condition (e.g. physical obstruction of oesophagus, injury, tetanus, rumen impaction with cereal etc.). This section is devoted to primary pasture bloat.
Risk factors
Pasture factors
The main risk factor in pasture bloat is the rapid ingestion of immature/fast-growing legumes in pre-flowering stages. Alfalfa and ladino clover are the most "dangerous" legumes, while red and white clover are only moderately so. Sporadic bloat outbreaks have also been reported on grazed cereal crops, rape, cabbages, peas and beans.
Ingestion of only the most succulent parts of the plant in set grazing systems is an important risk factor, in addition to the sward type. Frost and growth of alfalfa at low temperatures have been shown to increase bloat risk by increasing the leaf cell constituents related to pasture bloat (MacAdam and Whitesides, 1996). High levels of nitrogen fertilisation have been associated with both high and low incidence of bloat (Ledgard et al., 1990; Reason et al., 1989).
Wetness of the pasture has also been suspected to be a risk factor for bloat. It is, however, more likely that the real risk factor is the fast growth brought on by wet and favourable weather.
Animal factors
It is fairly well established that young animals are more susceptible to acute and severe bloat than older animals, and it is suspected that animals get used to eating bloating pastures and are less susceptible after exposure.
It has also been established that some cows can be classified according to their susceptibility to pasture bloat and that a number of inherited characteristics are related to bloat. These characteristics include the composition of salivary proteins and salivation rate and rumen capacity/girth measurement and motility (McIntosh et al., 1988).
The condition
Once the froth has formed in the rumen and the natural erucitation is prevented, the rumen motility is initially increased, causing further frothing. Finally there is a loss of muscle tone and rumen motility. Death is a result of several factors, including the depressive effect of rumen distension on the heart and lungs and absorption of toxins from the rumen.
As there are individual differences in the ability of cattle to tolerate rumen distension and in the presence of contributory factors in any given situation, some animals only suffer sub-clinical or mild bloating on clover-rich pastures. Whilst the toleration of mild bloat allows adaptation to new pastures, sub-clinical and mild bloat have been recognised as causing major losses on clover dominant pastures in the form of reduced feed intake and subsequent lower weight gains (Latimori et al., 1992; Rossi et al., 1997)
Occurrence
Bloat has long been recognised as a major problem in countries like New Zealand, where clover forms an important part of the pastures (Carruthers et al., 1987). In the UK, in conventional cattle herds, bloat occurs sporadically in outbreaks on lush spring or autumn pastures. The reported outbreaks have not been necessarily associated with clover pastures.
Due to its association with clover, bloat has been considered a risk factor on organic farms, where clover often constitutes more than 50% of the sward content. However, research both in the UK and elsewhere in Europe on organic farms suggests that the incidence of clinical bloat is not higher on organic than on conventional farms (Weller and Cooper, 1996; Frankow-Linberg and Danielson, 1997).