Bacterial populations of some grazed pastures in Hawke's Bay

Abstract

Changes in the bacterial populations of soil and herbage of pastures associated with three distinct soil types have been examined in relation to climate and season, the physical and physiological condition of the herbage, the soil type, and the effects of grazing. The flora of the associated drinking water and the gut of the grazing animal have also been examined.

Three principal groups are distinguished in the herbage populations:

(a) orange and yellow chromagens, the most common and typical epithytic bacteria, generally well-represented on most types of herbage;

(b) pink and red chromagens, which tend to become dominant on dry grass and litter;

(c) non-pigmented bacteria, generally favoured by periods of rapid plant growth. They include Aerobacter and pseudomonads, and in part, at least, are probably of soil origin.

Goliform bacteria, other than free-living types (Aerobacter), occur mainly on the herbage and in the drinking water. It is likely that they are of faecal origin. Attention is drawn to the failure of sheep coliforms to grow in MacConkey's broth at 44°c.

It is suggested that there is an interrelation between the microbial populations, in particular the bacteria and moulds. The relative dominance of one or other is largely a function of microclimate, which, in turn, is significantly affected by the grazing history of the pasture and the soil type. The possible relations between the microbial population and stock health are discussed.