Microbial feed additives for ruminants

Microbial feed additives are used in ruminant feeds for three different purposes. The first is for the same reason that probiotics are used in non-ruminants, namely stabilisation of the intestinal flora; this is applicable only in young, pre-ruminant animals, however, where lactobacilli, enterococci and yeast have been reported to be helpful in preventing diarrhoea and in improving live weight gain in calves and lambs. The second aim in young animals is to enhance the development of the adult rumen microflora, because this stimulates development of rumen structure and accelerates weaning. The same organisms and Aspergillus oryzae fermentation extract appear to achieve this objective, and inoculations with rumen microorganisms have also been found to be effective experimentally. In the adult ruminant, only yeast and fungal cultures have been widely reported to be effective. Responses in meat and milk production are highly variable, with an average response of 7 to 8% in each case to both products. Increased feed intake usually appears to drive the response. In turn, improved intake occurs because of a more rapid breakdown of fibre in the rumen, and an enhanced microbial protein flow from the rumen has been reported. Both types of additive cause a marked increase in the viable count of anaerobes recovered from rumen fluid, but for different reasons. Yeast appears to function by removing traces of oxygen that may be toxic to rumen bacteria: non-respiratory yeasts did not stimulate numbers. In contrast, the mode of action of A. oryzae appears to be associated with its enzyme activity. Neither yeast nor A. oryzae grows to a significant extent in the rumen.