Feeding Corn to chickens is fine as long as there getting good poultry feed also, it is nothing like feeding twinkies to a pregnant lady. If your feeding a 18% protein feed you could feed up to 15%/20% of there diet in treats like corn, scraps, fruits and vegetables. 
Corn has a part in a feed ration and has proteins, fat, fiber, energy, vitamins and minerals it also is more digestible nutrition than Oats, Wheat, and Sorghum.
 
TDN = Total Digestible Nutrients
NEm = Net Energy of Maintenance
NEg = Net Energy of Gain
CP = Crude Protein
NDF = Neutral Detergent Fiber
ADF = Acid Detergent Fiber
 
Table 1. Nutrient content of various feed grains (NRC, 1996).
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                           Corn   Barley   Wheat   Oats  Sorghum 
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TDN, %                     90      88      88      77      82 
NEm, Mcal/lb                1.02    0.94    0.99    0.84    0.91 
NEg, Mcal/lb                0.70    0.64    0.68    0.55    0.61 
CP, %                       9.8    13.2    14.2    13.6    12.6 
Escape Protein, % of CP    55      27      23      17      57 
NDF, %                     10.8    18.1    11.8    29.3    16.1 
ADF, %                      3.3     5.8     4.2    14.0     6.4 
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Table 2. Mineral content of major feed grains (NRC, 1996). 
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                   Corn  Barley   Wheat   Oats   Sorghum 
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Calcium, %         0.03    0.05    0.05    0.01    0.04 
Phosphorus, %      0.32    0.35    0.44    0.41    0.34 
Potassium, %       0.44    0.57    0.40    0.51    0.44 
Magnesium, %       0.12    0.12    0.13    0.16    0.17 
Sodium, %          0.01    0.01    0.01    0.02    0.01 
Sulfur, %          0.11    0.15    0.14    0.21    0.14 
Copper, ppm        2.5     5.3     6.5     8.6     4.7 
Iron, ppm         54.5    59.5    45.1    94.1    80.8 
Manganese, ppm     7.9    18.3    36.6    40.3    15.4 
Selenium, ppm      0.14    -       0.05    0.24    0.46 
Zinc, ppm         24.2    13.0    38.1    40.8     0.99 
Cobalt, ppm          -     0.35     -      0.06     - 
Molybdenum, ppm    0.60    1.16    0.12    1.70     - 
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Table 3. Nutrient content of corn using different harvest, storage, 
or processing methods. 
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                                                                 Escape 
                           Dry    TDN,   NEm,     NEg,           Protein, 
Corn Type                 Matter   %   Mcal/lb  Mcal/lb  CP, %   % of CP 
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Dry Rolled Corn              86    90    1.02    0.70     9.8      60 
Ear Corn                     87    83    0.92    0.62     9.0      60 
Steam Flaked Corn            82    94    1.06    0.73    10.0      45 
High Moisture Corn           75    90    1.02    0.70    10.0      40 
High Moisture Ear Corn       75    83    0.92    0.62     8.7      40 
High Moisture Snapped Corn   74    81    0.90    0.59     8.8      40 
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