This brigade, of three regiments , was encamped on the Hamburg and Purdy road, its right on the high ground near Owl Creek, in the following order from left to right: Fortieth Illinois, Forty-sixth Ohio, Sixth Iowa. At the first alarm Sunday morning, April 6, 1862, each regiment formed upon its color line. Two companies of the Sixth Iowa, with one gun of Behr's battery, were on guard at the bridge over Owl Creek. About 8 a.m. the brigade was advanced to the brow of the hill overlooking Shiloh Branch, the Fortieth Illinois joining the right of Buckland's brigade. After a skirmish with Pond's brigade McDowell was ordered at 10 a.m. to retire to the Purdy road and move to the left to connect with Buckland's brigade near the crossroads. In obedience to this order the brigade abandoned its camps without a contest and moved by the left flank past McDowell's headquarters, when it was discovered that the Confederates occupied the road between this bridge and Buckland's. McDowell then moved directly north and put his brigade in line on west side of Crescent field, facing east, where he engaged and drove back the force of the enemy moving into said field. The brigade them moved northeasterly across Crescent Field and into Sowell Field, facing south, its left at Sowell house, where it connected with McClernand at 11:30 a.m., and advanced with him to the center of Marsh's brigade camp. Here Sixth Iowa was transferred from the right to center of brigade, and Thirteenth Missouri placed between the Fortieth Illinois and Sixth Iowa, the Forty-sixth Ohio slightly in rear and to the extreme right of the line. At about 12m. the brigade was attacked on its right flank by Trabue. In an engagement lasting until 1:30 p.m. the Sixth Iowa had 52 killed - they were buried in one grave where they fell; they Forty-sixth Ohio had 246 killed and wounded, and the Fortieth Illinois 216 killed and wounded. The brigade commander was thrown form his horse and disabled. At 2:30 p.m. the brigade retired to the Landing and later formed behind Hurlbut. On Monday, the Sixth Iowa and Fortieth Illinois were attached to Garfield's brigade of Army of the Ohio, and remained with him until Wednesday, but were not engaged.
The above is from Reed's history of the battle. To read McDowell's report, click here. Unfortunately I cannot find a picture of McDowell