The effects of fishmeal and postruminal glucose on the intake and digestion of low-quality herbages by grazing cattle in southern Australia

Abstract

The intake and digestion of barley stubble and mature pasture were studied in grazing cattle given no supplement or supplements of fishmeal (800 g day-1 per rumen) or fishmeal (800 g day-1 per rumen) plus glucose (500 g day-1 per abomasum). Herbage intake was unaffected by supplements but was greater on pasture than stubble (P<0.001). While fishmeal increased ammonia levels above the basal level of 90 mg 1-1 , the flow of bacterial nitrogen was not increased. Bacterial efficiency (g bacterial nitrogen per kg dry matter apparently digested in the rumen) was 26 and 15 at pasture and stubble, respectively (P< 0.008), and was uninfluenced by supplementation with fishmeal. Similarly, the bacterial nitrogen content and the ratio of diaminopimelic acid to total nitrogen in the bacteria was unaffected by supplementation. Total non-ammonia nitrogen flow and the flow of essential amino acids from the rumen were greater at pasture than stubble (P<0.003) and were increased by fishmeal supplement (P<0.01). Plasma glucose was significantly affected by herbage type but not by supplement (P>0.05). Plasma insulin was not significantly altered by either herbage or supplement. It is concluded that large quantities of postruminal energy and protein supplements can be given to grazing cattle with no detrimental effects on herbage intake and utilization. Further work should be conducted to identify the cause of an apparent inefficiency of bacterial efficiency in cattle on low-quality stubble. It did not appear to result from a deficiency of either ammonia or branch-chain fatty acids.