Control of feed intake in dairy cattle: evolution of theories

摘要

Early investigations of the control of feed intake indairy cattle focused on physical limitations.This followed the suggestion by Lehman (1941) that feed intake is limited by the ballast of undigested feed residues in the gastrointestinal tract.Research related to physical limitations to feed intake has focused on forage characteristics that affect gut fill,the site of distention,and the mechanism by which distention limits intake.Feed intake was positively related to forage digestibility with a decreased response as digestibility increased (Blaxter et al.,1961) suggesting that distention from gut fill limits feed intake less as digestibility increases.Conrad et al.(1964) proposed that as diet digestibility increases,there is a breakpoint in digestibility at which limitation of feed intake by physical fill in the gastrointestinal tract is replaced by satisfaction of energy demand.It was subsequently shown that the form of energy affected energy intake; various fuels derived from the fermentation and digestion of feeds hadvery different effects on feed intake.Research with laboratory speciesin the 1980's showed that feeding behavior is related to the oxidation of fuels and that the signal was conveyed to brain feeding centers by hepatic vagal afferents.Further research provided evidence that the signal from the liver to brain feeding centers is related to the balance between the production and utilization of energy by the liver (Friedman,1997).We call this the Hepatic Oxidation Theory (HOT) of the control of feed intake and have applied it to dairy cattle and other ruminants (Allen etal.,2009).Because the liver utilizes fuels from the diet as well as those mobilized from tissues,the control of feed intake and energy partitioning are inextricably linked.This paper will discuss the evolution of ideas,hypotheses,and theories of the control of feed intake over the last half century as they pertain to lactating dairy cattle.