Rapid proteolysis of alfalfa during ensiling and rumen fermentation impairs protein use by dairy cattle. The severity of conditioning may alter protein degradability in forages, particularly if protein-binding tannins and o-diphenols are present. In 2002 and 2003, cuttings of alfalfa, low to high tannin birdsfoot trefoil, and red clover with o-diphenols and polyphenol oxidase were conventionally conditioned or macerated and then conserved in minisilos for 90 d. Forage protein was partitioned with buffer and detergent solutions into rapidly (AB1), moderately (B2), and slowly (B3) degraded and nondegradable (C) fractions. Averaged over years and harvests, protein in conditioned alfalfa was comprised of 734 g/kg of AB1, 198 g/kg of B2, 34 g/kg of B3, and 34 g/kg of C. High tannin trefoil had 74 kg/g less AB1, 69 g/kg more B2, similar B3, and 6 kg/g more C than alfalfa. Red clover had 220 kg/g less AB1, 143 g/kg more B2, 78 g/kg more B3, and similar C compared to alfalfa. Compared to conditioning, maceration decreased AB1 by 161 g/kg in alfalfa, 191 g/kg in high tannin trefoil, and 144 g/kg in red clover. Maceration increased B2 by 132 g/kg in alfalfa, 100 g/kg in high tannin trefoil, and 24 g/kg in red clover. Maceration also increased B3 by 27 g/kg in alfalfa, 78 g/kg in high tannin trefoil, and 113 g/kg in red clover, and increased C by 12 g/kg in high tannin trefoil and 6 g/kg in red clover without altering C in alfalfa. The results indicate that polyphenols and maceration shifted protein fractions from rapid to slowly degraded forms. Maceration enhanced the action of polyphenols, shifting protein to the slowly degraded B3 and nondegradable C fractions. The impact of these shifts on milk production and nitrogen utilization by dairy farms will be evaluated using nutrition and whole-farm models.