The objective of this study was to evaluate the cost of diets for high-producing cows including either corn silage (CS) or sorghum silage (SS) as the main forage source. A database was generated for the nutritional composition of SS, and included: dry matter (DM; n = 22), ash (n = 16), crude protein (CP; n = 23), ether extract (EE; n = 13), neutral detergent fiber (NDF; n = 25), acid detergent fiber (ADF; n = 21), acid detergent lignin (ADL; n = 18), starch (n = 11), and in vitro dry matter digestibility (IVDMD; n = 5). The nutritional composition of CS was obtained from the dairy NRC (2001). Diets were formulated with CPM Dairy Ration Analyzer using least cost optimization. Diets were formulated for a 635-kg Holstein cow producing 40 kg of milk (3.5% fat, 3.1% protein). Formulation constraints included 100% of predicted DM intake, 100 to 110% of metabolizable energy requirement, 95 to 103% of metabolizable protein requirement, 28 to 33% dietary NDF, 30 to 40% dietary non-fiber carbohydrates, and 50 to 60% dietary forage. Ration formulation was performed under 7 scenarios (Table 1): very low, low, middle, and high grain prices, and considering the price of SS to be either 85, 70, or 55% of the price of CS. When the price of SS was 85% of that of corn, it was cheaper to include CS in the diets, likely explained by the greater inclusion of expensive grain in diets including SS. When the price of SS was 70% of that of CS, marginal differences in diets costs were observed between CS and SS. When the price of SS was 55% of that of corn, it was more expensive to include CS in the diets. In conclusion, SS had to be 30% cheaper than CS to obtain diets of similar composition and cost. Table 1. Daily costs of diets including corn silage (CS) or sorghum silage (SS) as affected by corn grain prices and the relative silage prices

Corn grain	CS			85% CS			70% CS			55% CS
	CS price	Diet cost		SS price	Diet cost		SS price	Diet cost		SS price	Diet cost
$2.7/bu	$27/ton	$3.87/d		$23/ton	$3.99/d		$19/ton	$3.89/d		$15/ton	$3.80/d
$3.7/bu	$37/ton	$5.36/d		$31/ton	$5.50/d		$26/ton	$5.39/d		$20/ton	$5.24/d
$4.6/bu	$46/ton	$6.59/d		$39/ton	$6.79/d		$31/ton	$6.60/d		$23/ton	$6.41/d
$5.8/bu	$58/ton	$8.31/d		$49/ton	$8.56/d		$41/ton	$8.37/d		$32/ton	$8.16/d