Growing Degree Days (GDD) have long been used by farmers and agronomists to track crop development based on accumulated heat. In forage systems, where timing of planting, cutting, and grazing directly affects yield and feed quality, temperature-based planning is especially important. An extension of the traditional GDD concept—often referred to as GDD+—adds more practical value for forage crop planning by accounting for crop-specific growth responses and real-world field conditions.

What Are Growing Degree Days (GDD)?

GDD measure heat accumulation over time to estimate plant growth and development. The basic idea is that plants grow only when temperatures exceed a certain base threshold. For many cool-season forage crops, such as alfalfa or perennial grasses, this base temperature is commonly set at 0°C or 5°C (32°F or 41°F), depending on the crop and region.

The standard GDD formula is:

GDD = (Daily maximum temperature + Daily minimum temperature) / 2 − Base temperature

Daily GDD values are summed across the season to estimate crop progress. While this approach works well, it assumes ideal conditions and uniform crop response, which is not always realistic in forage systems.

What Is GDD+?

GDD+ builds on the traditional GDD model by integrating additional biological and environmental factors that influence forage growth. While definitions can vary by region or advisory system, GDD+ generally adjusts heat-unit accumulation to better reflect actual forage productivity, not just temperature.

GDD+ may include:

• Adjustments for upper temperature limits (reducing growth when it is too hot)
• Crop-specific growth curves (e.g., alfalfa vs. orchardgrass)
• Consideration of photoperiod (day length) effects
• Corrections for moisture stress or delayed spring green-up
• Alignment with observable growth stages, not just calendar dates

The goal of GDD+ is to link temperature accumulation more closely to harvest readiness and forage quality.

Why GDD+ Matters in Forage Crop Planning

Forage crops differ from grain crops in that they are harvested multiple times and quality is just as important as yield. GDD+ helps producers make better decisions in several key areas:

1. Optimizing Cutting Timing

Forages rapidly lose quality as they mature. Crude protein declines and fiber increases as plants move from vegetative to reproductive stages. GDD+ allows producers to time harvests based on physiological maturity rather than fixed dates, helping maintain high digestibility and energy content.

For example, alfalfa may reach optimal first-cut quality at a certain GDD+ threshold, regardless of whether spring is early or late.

2. Improving Yield Predictions

Traditional GDD may overestimate growth during heat stress or underestimate growth during cool but sunny periods. GDD+ corrects these issues, leading to more accurate yield forecasts. This helps with feed budgeting, storage planning, and marketing decisions.

3. Supporting Grazing Management

In pasture systems, GDD+ can guide when to begin grazing, rotate livestock, or allow rest periods. By tracking accumulated GDD+, managers can avoid overgrazing immature stands or grazing too late when forage quality has already declined.

4. Adapting to Climate Variability

As weather patterns become more variable, relying on calendar-based decisions becomes riskier. GDD+ provides a flexible framework that responds to actual growing conditions, allowing forage producers to adapt more effectively to warmer springs, heat waves, or uneven seasonal growth.

Practical Use of GDD+ on the Farm

Many extension services and agricultural decision-support tools now provide GDD+ tracking through online dashboards or mobile apps. These tools often combine local weather data with crop growth models to give real-time recommendations for cutting or grazing.

To use GDD+ effectively, producers should:

• Know the base and upper temperature thresholds for their forage species
• Track GDD+ consistently throughout the season
• Pair GDD+ data with field observations, such as plant height and bud or head development
• Adjust management decisions based on both model output and on-farm conditions

Conclusion

GDD+ represents a more refined and practical approach to forage crop planning than traditional heat-unit models alone. By better reflecting how forage crops actually grow under variable conditions, GDD+ helps producers improve harvest timing, maintain forage quality, and manage risk. When combined with field knowledge and good agronomic practices, GDD+ becomes a powerful tool for sustainable and profitable forage production.