Characterizing cereal rye biomass and allometric relationships across a range of fall available nitrogen rates in the eastern United States

Authors: Mirsky, Steven; SPARGO, JOHN - Pennsylvania State University; CURRAN, WILLIAM - Pennsylvania State University; REBERG-HORTON, S. CHRIS - North Carolina State University; RYAN, MATT - Cornell University; Schomberg, Harry; ACKROYD, VICTORIA - University Of Maryland

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2017
Publication Date: 4/20/2017
Citation: Mirsky, S.B., Spargo, J.T., Curran, W.S., Reberg-Horton, S., Ryan, M., Schomberg, H.H., Ackroyd, V.J. 2017. Characterizing cereal rye biomass and allometric relationships across a range of fall available nitrogen rates in the eastern United States. Agronomy Journal. 109:1520-1531.

Interpretive Summary: Cereal rye is the most ubiquitous cover crop species in the landscape of the Eastern US due to its cold tolerance, which allows it to be planted after corn or soybean harvest when other cover crops would fail to establish. Cereal rye provides a variety of ecosystem services including decreased soil erosion, decreased N leaching and runoff, and improved soil health. The provision of these ecosystem services is dependent on large cereal rye biomass production, which in turn varies depending on climate and available residual soil N. Further, it would be very useful for researchers and farmers to be able to estimate final cereal rye biomass production based on early measurements (i.e. tiller counts, early season biomass, or sensor technology), in order to make the best possible management decisions based on cropping system goals and constraints. We conducted a three-year experiment at sites along a latitudinal gradient (PA, MD, and NC) where we applied fall nitrogen (N) at several different rates to mimic possible variations in residual soil N in different climates. We measured cereal rye tiller production, biomass, tissue N content, and normalized difference vegetation index (NDVI) at two early and one late growth stages with the intent of evaluating late season biomass and N accumulation in connection with early season measurements. Cereal rye biomass and, to a lesser extent, N accumulation varied across site years. Nitrogen application increased biomass, averaging 2853, 4844, and 9739 kg ha-1, respectively, for the early and late growth stages across site-years. Cereal rye demonstrated a stronger N accumulation response to fall N levels at early growth than at the late one. Early-season biomass accounted for the greatest amount of variation in the models and was a better predictor of late season biomass than shoot density and NDVI. Nonetheless, our models had a predictive ability ranging from 34-60%. This study illustrates the difficulty in predicting late season cereal rye biomass and N content based on early season measurements and demonstrates the need for further research into this topic. Our results also make clear the impact of varied residual N fertility levels on cereal rye biomass, which may help farmers to make cover crop management decisions based on field conditions prior to, and during, cereal rye growth.

Technical Abstract: Cereal rye (Secale cereale L.) is the most commonly grown cover crop in the Eastern US due to its winter hardiness, adaptability to a wide array of soil and environmental conditions, and many potential benefits. These benefits hinge in large part on biomass production, which varies according to multiple factors, especially climate and available soil N. To test the effects of climate and soil fertility on cereal rye growth and development, an experiment was initiated in the fall of 2009 and replicated for three years along a latitudinal gradient (PA, MD, and NC) where fall N fertilizer was applied at five or six rates. Reponse measurements included cereal rye tiller production, biomass, tissue N content, and normalized difference vegetation index (NDVI) at growth stages (GS) 25, 30, and 60. Cereal rye biomass and, to a lesser extent, N accumulation varied across site years. Nitrogen application increased biomass, averaging 2853, 4844, and 9739 kg ha-1, respectively, for GS25, GS30 and GS60 across site-years. Cereal rye demonstrated a stronger N accumulation response to fall N levels at GS25 and GS30 than at GS60. Early-season biomass (GS25) accounted for the greatest amount of variation in the models and was a better predictor of late season biomass (GS60) than shoot density and NDVI. Variance attributed solely to early-season (GS25 and GS30) biomass constituted 38.5-65.2% of the total model variance. Unsurprisingly, GS25 biomass was nearly as good of a predictor when it was the sole parameter in a model as when it was included in a more complex model (r2 of 0.592 and 0.508 for biomass and total N, respectively, at GS60 in the linear regression model compared to an adjusted R2 of 0.526 and 0.440 in the multiple regression model). Nonetheless, our models had a predictive ability ranging from 34-60%. This study illustrates the difficulty in predicting late season cereal rye biomass and N content based on early season measurements.