Effects of supplementation on voluntary forage intake, diet digestibility, and animal performance.

A data base was constructed to describe and estimate supplementation effects in nonlactating cattle consuming forage ad libitum. The data base included 66 publications on 126 forages (73 harvested and 53 grazed) and a total of 444 comparisons between a control, unsupplemented treatment and a supplemented treatment. Daily gains were reported for 301 comparisons and voluntary intake for 258. Direct measures of forage digestibility were reported for 202 comparisons, and total diet digestibility for 150. Supplements did not increase gain in all cases. Change in ADG due to supplement was not related closely to intake of supplemental TDN. Lowest increases in ADG were with native forages supplemented with molasses alone or with low intakes of molasses containing high levels of NPN. Greatest increases in gain were with improved forages, supplements with > 60% TDN, and supplemental CP intake > .05% of BW. Supplements decreased voluntary forage intake (VFI) when supplemental TDN intake was > .7% of BW, forage TDN:CP ratio was < 7 (adequate N), or VFI when fed alone was > 1.75% of BW. When supplements increased VFI, forage TDN: CP ratio was > 7 (N deficit), and VFI when fed alone was often low. There was little relationship between change in VFI and sources of supplemental CP and TDN. Supplements caused total diet TDN concentration to deviate from expected values by -10 to +5% of OM. When supplemental TDN intake was > .7% of BW, diet TDN concentration was always less than expected. There was little relationship between deviation from expected total diet TDN and type or composition of forages or supplements. Empirical multiple regression equations were developed to estimate effects of supplements on VFI and total diet TDN concentration. The most acceptable intake equation estimated VFI when fed with supplement (r2 = .84) That equation included VFI when fed alone, supplement intake, CP and TDN concentrations in forage and supplement, and classification codes describing forages and supplemental energy. The most acceptable equation for estimating total diet TDN concentration included only the expected total diet TDN concentration (r2 = .87). These equations may be used in nutritional models to account for associative effects.