In any supplementation program, it is essential that the forage resource be stocked in such a manner so there is adequate forage quantity available per animal unit. If forage quantity is insufficient, then the supplementation program will be ineffective. The object of supplementation programs (usually protein supplements) are to match deficiencies in forage quality in such a manner as to increase passage rate of forage and thus increase forage intake of the cow.
Forage intake of the cow declines with decreased forage quality. Cellulose content in mature forage increases and requires increased rumen residence time for rumen microbes to break down chemical bonds. Also, protein content of mature forage decreases, allowing less protein to be available for making new rumen microbes. The net effect is for passage rate of forage to decline as well as forage intake (Table 1).
A general rule is for daily protein supplementation to be limited to around 2 lbs. a day in order to avoid forage substitution effects. If energy supplements are fed, then it is generally expected that negative forage substitution effects will occur.