Effects of fermentation products of silage on its intake by cattle
The end-products of silage fermentation have been implicated as factors limiting its intake by ruminant animals, although the contribution of individual chemical components is not clearly understood. A series of experiments were conducted to investigate the effect of short-term intra-ruminal infusions of silage fermentation end products on roughage intake by cattle. Infusions of lactic acid, the predominant organic acid found in well preserved silages were found to reduce the short-term intake of roughage by both steers and dairy cows. The short-term intake of hay- and silage- fed steers was reduced by infusions of 32 g lactic acid/kgDMI. The reduction in intake was greater when the acid was infused over two hours as opposed to one hour. The same amount of lactic acid (32 g/kgDMI) reduced the intake of silage fed dairy cows. These results were substantiated by a further experiment in which intra-ruminal infusions of 16, 32 or 48 g lactic acid/kgDMI over a two hour period reduced the short-term intake of silage-fed steers in a dose related manner. The mechanism by which lactic acid reduces intake was not clear from these trials. Infusions of acetic and propionic acids in the molar proportions to which lactic acid is metabolised in the rumen depressed silage intake by dairy cows, but this was attributed to a fall in rumen pH. Urea, used to mimic silage ammonia had no effect on voluntary intake of hayand silage- fed cattle. This supports the theory that rather than ammonia per se limiting intake it is other fermentation end-products, such as silage amines, produced in similar conditions to ammonia that are responsible for poor intakes. Amines were detected in noticeable amounts in silages made at Hurley. The predominant one being gamma amino-n-butyric acid (Gaba). Infusion of Gaba intraruminally, reduced the intake of silage-fed steers by up to 22%. Given in combination with putrescine the depressing effect of intake depression was doubled (44%), although these results were not significant. Physical gut fill was found to have little effect in the limitation of short-term silage intake. Rumen emptying studies showed that the maximum amount of digesta present within the rumen, in terms of DM, OM and NDF occurred after the cessation of the first meal.