These are books I have successfully used in toddler storytime for the "farm" theme. Find more "farm" book suggestions for all ages on the Colorado Libraries for Early Literacy (CLEL) website.
My Farm Friends by Wendell Minor
Mrs. Wishy-Washy by Joy Cowley (out of print, but I made this flannel board and the kids love it http://storytiming.com/2012/02/10/flannel-friday-mrs-wishy-washy/)
Clip Clop by Nicola Smee
This is the Farmer by Nancy Tafuri
Big Red Farm by Christianne C. Jones
Maisy’s Morning On the Farm by Lucy Cousins