Translating Grazing: Calculating Nitrogen Credits from Cool-Season Integrated Crop Livestock Systems

This study will evaluate the impact of grazing management of an ICL on nitrogen stocks, inputs, and mineralization. We propose to capitalize on an ongoing experiment, implemented in 2017 at the University of Florida’s North Florida Research and Education Center in Marianna, FL. We propose to build upon current activities with additional crop and soil monitoring efforts in order to further describe nitrogen stocks, inputs, and mineralization in the experiment.

Treatments and experimental design

The annual ICL rotation in the experiment supports cotton in years 1 and 2, and peanut on year 3 of the rotation. All plots (other than control) receive small grain cover crops during the cool season, plus N fertilization as described below. The treatments are organized in a randomized, complete block design with three replications and consist of a range of grazing intensities on winter cover crops (Table 1).

Table 1. Marianna, FL Field Experiment Treatments

Each experimental plot measures 6 x 17 m and is individually fenced. Cool-season cover crops are established in the fall of each year, no-till drilled to 56 kg/ha each of oat and rye. Nitrogen is applied three weeks after planting (34 kg N/ha) and after the first grazing event (56 kg N/ha) on high fertilization plots. Other nutrients are applied, based upon the UF-IFAS soil test recommendations. Grazing commences when herbage biomass reaches 1,120 kg DM/ha above targeted residual biomass and grazing events occur every 2 weeks thereafter. Plots are mob-stocked with 2-4 yearling steers, withdrawn from the paddocks when residual stubble mass reaches target residual biomass. Grazing time for each plot is recorded in order to estimate grazing days and stocking rate for each treatment. Both pre- and post-grazing biomass are estimated using the double-sampling procedure every 14 days during the grazing season. All crops are managed under irrigation using a central pivot system. Cash crops are planted approximately by mid-April. Crop production and harvest index are determined by harvesting a strip 1 x 2.5 m within each plot.

Response Parameters

Objective 1

Nitrogen stocks and inputs will be measured from soil and crops. Soils will be collected from each plot at three depths (0-5, 5-15, 15-30 cm) at the end of each grazing season (mid-April) and analyzed for total nitrogen, 1M KCl extractable NO₃-N, and NH₄-N. Cover crop contributions to nitrogen stocks and inputs will be measured in below and aboveground biomass. Root biomass and growth dynamics will be measured in three samplings using a 10.8×20 cm soil corer before initiating grazing, approximately six weeks into the grazing season, and at the end of the grazing period. Simultaneously, ingrowth of cover crop roots will be measured using wire and mesh 7.5×30 cm cylinder root ingrowth cores as described by Cooley et al. (2019). Two separate sets of these ingrowth cores will be deployed serially for approximately six weeks buried in the soil at initiation of grazing, removed six weeks later and another set deployed, then retrieved at the end of the grazing season. Roots from each of these assessments will be washed from their respective soil sample, dried, and weighed. Composited dried root samples from all depths within a plot will be analyzed for total nitrogen. Aboveground biomass production will be measured by the forage monitoring efforts previously described. Forage samples from each plot will be characterized on a variety of chemical parameters including nitrogen using NIRS (near-infrared spectroscopy).

Objective 2

Relative nitrogen mineralization rates (release of plant available nitrogen) will be measured during the transition from cover crop to row crop phases of the system to compare relative availability of mineralized nitrogen from grazed cover crops using the ion-exchange membrane technique (Qian & Schoenau, 2005). Strips of dimensions 2.5x10 cm of anion and cation exchange membranes will be used. Four pairs of strips (anion and cation) will be placed inserted into the soil in each plot. Ion-exchange membrane strips will be installed starting at cover crop termination and be retrieved and replaced every four weeks until week 12. Upon completion of each interval strips will be removed from soil, cleaned and NO₃-N and NH₄-N will be extracted from them with 1M KCl. Resulting extractants will be colorimetrically analyzed for NO₃-N and NH₄-N.

Relative mineral nitrogen availability measured by ion-exchange membranes will be compared against modeled nitrogen credits using the University of Georgia cover crop nitrogen availability calculator (Woodruff et al., 2018). This comparison will serve to potentially verify the utility of this model for grazed cover crops where the animal component likely alters the nitrogen mineralization dynamics and to gauge if any tradeoffs in nitrogen availability result from different levels of grazing intensity.