How are hybrid differences accounted for in the model?
Corn relative maturity (RM) ratings attempt to illustrate differences in when a given product would be ready to harvest relative to another product. While it is often referred to as “days” to maturity, it is important to understand that the system does not refer to actual calendar days from planting to maturity. A 108 RM product takes somewhere around 150 calendar days from planting to harvest in the central corn belt and that varies considerably from year to year, location to location, and even within a field. A RM rating of 5 units (that is, days) later only means that product will likely be ready for harvest somewhat later than the reference. The Interpreting Corn Hybrid Maturity Ratings article from Purdue University discusses issues with maturity rating systems.
We rate the silking and physiological maturity of products using the same numbering system. An example illustrates how this works practically. The following table has the RM ratings and GDU requirements for three 111 CRM hybrids that have differing RMs to silk and physiological maturity (black layer). P1151 silks like a 105 CRM hybrid and reaches maturity like a 107 CRM hybrid so it has a shorter vegetative growth and grain fill period than the “typical” 111 CRM hybrid, P1168, but takes longer to dry down. Conversely, P1197 silks and black layers more like a 113 CRM hybrid (longer vegetative and grain fill periods), but dries down more rapidly.
| Hybrid | Silk CRM | Physical Maturing CRM | Harvest CRM | GDU Vegetative | GDU Grainfill | GDU Drydown | GDUs to Maturing | GDU at Harvest |
| P1151 | 106 | 107 | 111 | 1320 | 1260 | 400 | 2580 | 2980 |
| P1168 | 111 | 111 | 111 | 1380 | 1300 | 300 | 2680 | 2980 |
| P1197 | 113 | 113 | 111 | 1400 | 1330 | 250 | 2730 | 2980 |
The GDUs required to reach a given phenological stage can vary by 10-20% between years and locations so are not necessarily more exact. Factors like solar radiation, weather conditions, soil characteristics, altitude, latitude, and longitude also impact crop growth and development. The six Silk RM units difference between P1151 and P1197 on average translates to 1-3 calendar days difference. With differing local and field conditions, P1197 might actually silk from a few days earlier to 7-8 days later than P1151.
The nitrogen model differentiates hybrids based on the GDUs to silk. This was validated against silking date data collected at research studies collected across the country and is within +/- 4 days for most locations. The target date for competitive products is based on the average GDU to silk of Pioneer hybrids with the same harvest maturity CRM.
