High grain yield and low grain protein indicate fertility (primarily nitrogen and/or sulfur) was limiting grain protein accumulation, particularly for a hard wheat crop. This is confirmed if the test weight of the grain is acceptable (≥ 60 lbs/bushel). Low fertility could be due to inadequate fertilization, loss of nutrients due to leaching (movement of nitrogen or sulfur below the root zone by water), denitrification (loss of nitrogen as a gas in water-saturated soils), or volatilization (loss of nitrogen as ammonia gas from the soil surface). Reevaluate your fertility program in the future and pay attention to soil nutrient levels and fertilizer rates when developing a fertility program.
In the absence of major stress events during grain filling, grain protein is a reasonable indicator of the nutrient status of the crop and whether or not you reached maximum yield potential. The following are critical grain protein levels associated with maximum yields of wheat by market class:
- Hard red spring wheat: maximum yield is achieved at 12.5% grain protein
- Hard white sprint wheat: maximum yield is achieved at 11.5% grain protein
- Hard white winter wheat: maximum yield is achieved at 11% grain protein
- Hard red winter wheat: maximum yield is achieved at 11% grain protein
- Soft white wheat (winter or spring): maximum yield is achieved at 10% grain protein
For example, if your soft white wheat came in at 8% protein and moderately high yield, the crop likely had insufficient nitrogen and 10-20% of the crop’s full yield potential was left in the field.