Brenda Tubaña, Hector Fajardo and Daniel Forestieri
Cover cropping provides a host of benefits that with years of repeated practice can pay off with high crop yields, reduced input costs and healthier ecosystems. Cover crops are grown not for short-term economic gain but for the benefits they provide for the subsequent main crops. Cover crops create biomass that protects bare soils against runoff and erosion, smothers weeds, helps control diseases and improves soil fertility. Thus, the benefits from cover crops rely heavily on biomass.
The nutrient composition and the amount of cover-crop biomass are essential information for evaluating soil fertility benefit from this practice. Cover crops absorb and store nutrients in the biomass. Upon termination in early spring, cover-crop biomass left on the ground undergoes decomposition, wherein nutrients are released back to the soil. Other organic materials from biomass accumulate and, with time, help in building up organic matter, a component of the soil commonly used as a metric for soil health.
A study was initiated in 2019 at the LSU AgCenter Sugar Research Station to evaluate the feasibility of projecting winter cover crops biomass yield and nutrient turnover using remote sensor technology. Winter cover crops are planted in fall and terminated in early spring of the following year. Normal growth and biomass accumulation can be disrupted in years with hard and wet winters. As a result, the benefits from cover crops are also reduced. The early detection of the potential benefits from cover crops, in terms of biomass ground coverage and nutrient turnover using sensor-based estimates, can provide valuable information for field planning. As such, suitable management practices for the next main crop can be put in place in anticipation of any changes in cover crop returns. On large-scale and long-term use, sensor readings and images can be used for mapping cover crop growth patterns and detecting stress in the field.
A mixed species of winter cover crops consisting of legumes (hairy vetch, crimson clover, balansa clover) and brassica (tillage radish, rapeseed) were broadcast and drill-seeded after sugarcane was planted in fall 2019. The biomass accumulated by cover crops later in fall was enough to cover almost the entire newly planted field that would have been otherwise left bare for several weeks (Photo 1). Before termination in early spring 2020, sensor readings and cover crop biomass clippings were collected from one-meter-square areas in the field (Photo 2). Sensor readings, or normalized difference vegetation index (NDVI), were taken using a GreenSeeker sensor (Photo 3) and a camera attached to a drone (Photo 4) before biomass clippings were sampled. Strong associations between sensor readings and cover crop dry biomass were recorded. GreenSeeker NDVI established a relatively better association with biomass nitrogen content than the drone-and-camera NDVI. These indicate that sensor-based estimates, especially from GreenSeeker, can be established for early projection of aboveground biomass coverage and nutrient recovery of cover crops. Because the nutrient recovery is also derived from dry biomass, recovery rate of all plant-essential nutrients other than nitrogen can also be predicted.
Fast and real-time information on cover crop growth and biomass accumulation in a landscape can be acquired using remote sensors. Thus far, the results from this study have shown that both GreenSeeker sensor and images taken by a camera attached to a drone can provide good estimates of cover crop dry biomass and nitrogen recovery rate.
Brenda Tubaña holds the Jack E. and Henrietta Jones Professorship in the School of Plant, Environmental and Soil Sciences. Hector Fajardo and Daniel Forestieri are graduate students in the school.
(This article appears in the fall 2020 issue of Louisiana Agriculture.)
Photo 1. Cover crops cover bare soil in early winter at the LSU AgCenter Sugar Research Station in St. Gabriel. Photo by Hector Fajardo
Photo 2. Graduate student Jose Mite collects biomass clippings of cover crops from a 1-meter-square sampling area before termination in early spring. Sensor readings using GreenSeeker and drone-and-camera were taken before biomass sampling. Photo by Daniel Forestieri
Photo 3. Graduate student Hector Fajardo takes GreenSeeker-NDVI readings from the 1-meter-square sampling area. Several sampling areas were set up in the field. Photo by Barbara Campos
Photo 4. Visiting scholar Elias Escobar holds a drone camera while graduate students Daniel Forestieri, left, and Hector Fajardo help gather data in a field. The camera is being calibrated before launching it to take NDVI readings. Photo by Dominique Galam