Flatpea (Lathyrus sylvestris L.) is a long-lived, stress-tolerant, perennial legume that is generally used as a conservation cover crop. The abundant forage produced by this species is rich in protein, but it also contains 2,4-diaminobutyric acid (DABA), a neurotoxic nonprotein amino acid. No signs of flatpea toxicity, however, have been reported for cattle consuming flatpea herbage. A study was conducted to compare the performance of growing cattle (Bos taurus) fed a diet containing early-bloom stage flatpea silage with that of cattle fed a corresponding diet containing alfalfa (Medicago saliva L.). In a randomized complete block, unbalanced design with unequal replication, 52 crossbred cattle (steers and heifers) that had not previously consumed flatpea were blocked by sex and assigned by weight to pens (four pens of steers and three pens of heifers) of seven or eight animals each. Animals had ad libitum access to watcr and diets consisting of 30% (dry matter [DM] basis) legume ('Lathco' flatpea or 'Vernal' alfalfa) silage, 30% timothy (Phleum pratense L. 'Climax') silage, and 40% grain (67% oats [Avena sativa L.], 33% corn [Zea mays L.]) for 86 d. Alfalfa and flatpea silages were of similar chemical composition and in vitro organic matter disappearance (IVOMD). Flatpea silage contained 1.1% DABA (DM basis), which did not adversely affect intake of the diet. At the conclusion of the trial (d 86), average daily gain (ADG) was approximately 10% less (P 0.10), requiring 7 to 10% more feed (on an as-fed basis) per pound of gain than animals offered the alfalfa diet. The observed weight gains and the absence of signs of flatpea toxicity during the study suggest that flatpea could be a satisfactory substitute for alfalfa in diets for growing beef cattle in regions where edaphic constraints limit alfalfa production. protect erodible surfaces. Its potential as a forage species has long been recognized (Smyth, 1892), but flatpea has had limited use as a livestock feed. Flatpea herbage contains DABA, a neurotoxic amino acid, that can occur at levels approaching 3.0% of the DM (Ressler, 1964; Foster, 1990a; Turner et al., 1994). Feeding studies have suggested that flatpea is toxic to sheep (Ovis aries) when DABA levels are greater than approximately 1.0% of the herbage DM (Rasmussen et al., 1992; Rasmussen et al., 1993; Rowe et al., 1993; Turner et al., 1994). Clinical signs of flatpea toxicosis included lethargy and diminished responsiveness; pacing, circling, and uncoordinated, often violent, spasmatic movements; a tendency to press the head against hard objects; excessive salivation; teeth grinding; seizures; and death (Rasmussen et al., 1993; Rowe et al., 1993). Such signs have not been reported for cattle consuming flatpea herbage (Foster, 1990a), even when the diet included flatpea hay (2.5% DABA, DM basis) or silage (3.2% DABA, DM basis) that was lethal to sheep (Turner et al., 1994; Foster, 1994, unpublished data). Average daily gain by steers and heifers grazed continuously on a flatpea pasture in western Washington was high (2.3-2.9 lb/head per d) early in the season, but decreased rapidly as the season advanced (Grunder and Dickson, 1948). Conserving flatpea forage as hay can be difficult because the large volume of herbage is slow to cure. In general, ensiling direct-cut or wilted herbage is an alternative to hay making. The objective of our study was to assess the performance by growing cattle offered diets containing early-bloom stage flatpea silage to determine whether flatpea could be a viable substitute for alfalfa in diets for growing beef cattle.