Corn (Zea mays) and alfalfa (Medicago sativa) silages are used extensively in Virginia. A survey was conducted to determine chemical composition of these two forage silages grown in five geographical regions of Virginia; Eastern Virginia (EV), Northern Piedmont (NP), Southern Piedmont (SP), Shenandoah Valley (SV), and South-Western Virginia (SWV). A total of 889 samples of corn silage, 106 of ammoniated corn silage and 247 of alfalfa silage collected during 1988 and 1989 from 76 counties, were analyzed for fiber, N, and macro- and micro-nutrients. Chemical composition of the silages was correlated with S applied in fertilizer or manure. Data were compared with critical levels of mineral requirements of various classes of livestock. Alfalfa silage was higher (P < 0.05) in crude protein (CP), P, K, Ca, Mg, S, Mn, and Fe than com silage. Ammoniated corn silage was higher (P < 0.01) in CP and NS ratio, and lower in P, S (P < 0.01) and K (P < 0.05) concentrations than non-ammoniated corn silage. For lactating dairy cows, 86 and 95% of corn silage and ammoniated corn silage, respectively, grown throughout the State were deficient in P. Information supplied by farmers suggested that manure application increased P concentration of these forages. Over 90% of all corn silage would not have met the Ca requirements of dairy cows, however, 97% of the alfalfa silage was excessive in Ca concentration for dairy cows and could have served as a Ca supplement to the diet. Nitrogen:S ratio indicated S deficiency (N:S ratio > 12) in 34, 89 and 41% of samples of corn silage, ammoniated corn silage and alfalfa silage for dairy cattle and in 85, 96 and 91% of the respective silages for sheep (N:S ratio > 10). Based on S concentrations, 96% of corn silage and ammoniated corn silage grown throughout Virginia were S deficient for dairy cows while 72% of corn silage and 86% of ammoniated corn silage were deficient in S for sheep. Sulphur concentrations in silages did not indicate S deficiencies for plant growth. Over 60% of corn and alfalfa silages would not have met nutritional requirements for Zn and Cu in lactating dairy cows but requirements for Mg and Mn would have been supplied by more than half of the silages produced in Virginia. Regional/ geographical variations in almost all the nutrients were observed for both forages. Generally, corn silage grown in EV was lower in CP, TDN, Mg, and Mn and was higher in ADF compared to silage grown in the rest of the State. Lower CP, Ca, and S were observed in alfalfa silage grown in EV compared to the mean of other regions. Generally, higher N:S ratio in corn and alfalfa silages and lower P were found in alfalfa silage grown in Western Highlands compared to Piedmont region. Also CP and Ca were lower in corn silage grown in SWV compared to SV while Mg was lower in either silage grown in SV compared to SWV region. In general, concentrations of P, Ca, S, Zn, and Cu in corn silage and ammoniated corn silage were widely deficient ( > 70% samples deficient) for dairy cattle, and deficient in S for sheep. Magnesium deficiencies were less frequent. In alfalfa silage concentrations of Zn, and Cu were low for dairy cows. Nitrogen:S ratios indicated S deficiency for livestock, particularly in sheep and lactating dairy cows. / Master of Science