If corn appears in the manure of cows on corn stalks then research data would suggest that the diet being consumed is about 9.2% crude protein. This is above the requirement for cows in mid and late gestation. The quality of the stalks is lower when the corn is removed leaving the husk and leaves which are closer to 6% crude protein and TDN ranges from the mid to low 50%. At this point, energy content is not adequate for late gestating 1st-calf heifers. Either move to a new, fresh stock field or begin supplementation.
