Drought damaged corn silage would be considered a high quality forage at 64% TDN. A gestating cow will easily consume about 2.5% of her body weight on a D.M. basis of corn silage. If the cow weighs 1200 pounds, she could consume on a daily basis 30 pounds of silage on a D.M. basis. If the silage is 35% D.M., this 1200 pound cow could eat 86 lb of silage daily. If she is fed a full-feed of silage and the silage is 64.8% TDN, she could consume 19 lb of TDN daily (30 lb of D.M. daily x .648 = 19.44 lb of TDN). If the 1200 lb cow is in the last 1/3 of gestation, her TDN needs are between 12 and 14 pounds of TDN, average about 13 lb of TDN. So at a full feed of silage, she is consuming more silage than is needed to meet her requirement. You could “limit-feed corn silage to meet her energy requirement. In this example, feed her 57.5 lb daily (20.1 lb of dry mater of silage x .648 = 13.0 lb of TDN D.M. daily needed to meet TDN requirement; then 20.1 lb of dry matter/.35 D.M. of silage = 57.4 lf of silage “as-fed”) to meet her TDN requirement.
You probably don’t want to design a totally limit-fed diet for late gestating cows to this degree. If you added 5 lb of ground corn stalks (50% TDN, 5.5% crude protein, 88% D.M.)on a D.M. basis as a filler, these cows should gain body condition under typical winter conditions. In addition, energy requirement will be exceeded (13 lb requirement, 13 lb from silage, 2.2 lb from corn stalks) and crude protein requirement would be very close to being met (about 2.1 lb per day required, 1.81 lb from silage, .24 lb from corn stalks) that I would not add any extra to the diet. Total D.M. intake would be 24.5 lb/hd/da. Add vitamins and minerals and, under typical weather conditions, the herd should perform well. After calving more energy and protein will be needed in the ration.
Corn grown in drought conditions could potentially be high in nitrates. The ensiling process will reduce nitrates 40 to 60 percent. Before opening the silage pit for feeding make sure you have allowed it to go through the 21 to 28 day fermentation phase. If you are still concerned about nitrates, take a sample and send it in for a nitrate analysis.