FROM THE FARM REPORT: WHEN IS THE RIGHT TIME TO HARVEST?

In my last Farm Report article, I mentioned having a strong interest in forage quality, mainly due to a research project that I completed last summer with Alltech Inc. and Poulin Grain Inc. This project was presented this past June at the American Dairy Science Association annual conference. I know it is late in the year now to be discussing managing grass silage, as everyone is in the midst of another busy season of corn harvest, but a key factor of what I learned last summer is that what you put in the bunk at the end of the day has a large impact on cow performance. Ensuring that forages are cut at an optimal time directly impacts the nutritional quality of the forage. The traditional method for determining when to cut grass silage is to cut first cut at late boot stage before the plant heads out, and then subsequent cuts every 28-30 days after. This requires having to go out into the field and determine the correct time to cut forages based on the growth stage.

Is there an easier way? The use of growing degree days (GDD) has become popular within recent years in predicting the phenological stages of plant maturity, more specifically within determining the optimal time for corn harvest, not necessarily for grasses. GDD refers to the number of heat units within a day. This is where the project “Growing degree days as a predictor of pre-harvest quality for Northeastern grass forages” comes in. As a team, we collected 373 grass silage samples at the time of ensiling from 29 farms in New England and New York. Data included the date of harvest, field location and the cut interval. For this project, we calculated GDD using the Cornell Climate Smart Farming Calculator, using a base temperature of 0°C. For first cut GDD accumulation began on March 1st of that year, and for subsequent cuts, the last cut date was used.

For first cut samples, strong correlations were found between GDD and acid detergent fiber (ADF; r = 0.67), undegraded NDF at 240 h (uNDFom240; r = 0.81, crude protein (CP; r = -0.67), NDF digestibility (D) at 12 h (NDFD12; r = -0.74), NDFD at 30 h (NDFD30; r = -0.74), milk per ton (r = -0.62) and relative forage quality (r = -0.70 ). Therefore, as GDD accumulates, ADF and uNDFom240 tend to increase, while CP, NDFD12, NDFD30, milk per ton and relative forage quality tend to decrease. This suggests that the longer you wait to cut, the more GDD is accumulated, the poorer quality silage. Moderate correlations were found between GDD and organic matter corrected amylase neutral detergent fiber (aNDFom; r = 0.42) and ether extract ( r = -0.55).

For second cut, moderate correlations were found for GDD and ADF (r = 0.43), uNDFom240 (r = 0.48) and NDFD12 (r = -0.44). We collected third cut samples and found no significant correlations.

The use of GDD for first cut appears to be the most critical, since it had the best relationship with forage quality. Thereafter, assuming farms stick to the recommended harvest cut window, GDD is less predictive of forage quality. These findings are similar to previous research and what we already know about the relationship of plant maturity and forage quality. As forage matures, NDF increases due to the physiological changes that occur within the plant. This includes the development of xylem tissue, accumulation of cellulose, and increase in lignification. As this occurs, the digestibility of NDF declines; this decline could be over 40 percentage units (% of NDF). From a milk production perspective, we know that for every 1% increase in NDFD there is an increase in dry matter intake of almost 0.40 lb and an increase in milk production of about 0.50 lb at 4% fat-corrected milk. Therefore, every unit of NDFD that we possibly lose from the accumulation of GDD, or plant maturity is about a 0.50 lb loss in milk. The bottom line is that what we put in the bunk, and the timing of harvest truly matters. From our findings, the target GDD for grass silage was approximately 900-1,000 GDD based on the stage of plant maturity. Is there possibly a better way to predict the optimal time to cut forages, with the increasing use of GDD and climate smart farming practices, there might be.

— Victoria Blaney

vblaney@whminer.com

* References available upon request.