Ag. Order 4.0 Regulation Resources

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UPDATE (1/18/2024). The current status of cover crop nitrogen scavenging credits in the Ag. Order regulation described below and the targets and limits (shown in the figure above) are uncertain due to (1) pending litigation on parts of the regulation, and (2) input from a future expert panel. In this letter (8/11/2023) to the State Water Resources Control Board, I explained why I strongly support cover crop nitrogen scavenging credits for growers as an important component of addressing the 'wicked problem' (see below) of nitrogen pollution of ground water. Those credits caused a huge increase in cover crop interest and adoption in the region over the past few years. Despite the uncertainty of the regulation, I continue to do field trials to help growers integrate cover crops in their rotations, with the assumption and hope that the cover crop nitrogen scavenging credits will be allowed in future revisions of the Ag Order regulation. Stay tuned. 

'Wicked problems' generally lack clear solutions because each problem is linked to other problems, and the nature and characterization of each cannot be isolated (Rittel & Webber 1973)’ cited in Game et al. 2013. Here's a short video where Keith Grint describes 'tame' and 'wicked problems'.  

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The Central Coast Regional Water Quality Control Board adopted Ag. Order 4.0 on April 15, 2021. 

This blog post by UCCE farm advisors Richard Smith & Michael Cahn provides a good overview of the implications of Ag. Order 4.0 on nitrogen management.

Photo below shows Ag. Order planting date trials by USDA-ARS and UCCE, Funded by the California Leafy Greens Research Board.  Here's a link to a PDF of our first peer-reviewed paper from this research. (Brennan E.B., R.F. Smith. 2023. Predicting cereal cover crop biomass using shoot length in California vegetable systems. Agricultural & Environmental Letters 8:e20099.)  Here's link to a paper I wrote several years ago that talks about the challenges and importance of cover cropping in the central coast region of California (Brennan E.B. 2017. Can we grow organic or conventional vegetables sustainably without cover crops? HortTechnology 27:151-161.)

Ag Order 4.0 provides incentives for cover cropping.  Check out these Videos to learn more:

• Cover crop nitrogen credits for farms in California’s central coast, Ag Order 4.0 (10:23)
•  A Simple Method to Help Farmers Estimate Cereal Cover Crop Shoot Biomass (8:02)

 

Below are links to 2 Google sheet CALCULATORS the I made to allow growers and others to quickly predict the oven dry shoot biomass, C:N ratio, nitrogen uptake, and nitrogen scavenging credit for Merced rye and Pacheco Triticale cover crops. To use these you'll need a Google account and will need to first save a copy of the calculators to your Google drive. Once you have a copy of the calculator in your Google drive you can use it.   

-Merced Rye Cover Crop Nitrogen Scavenging Credit Calculator 

-Pacheco Triticale Cover Crop Nitrogen Scavenging Credit Calculator 

 

Feekes Developmental Growth Stage Resources The Feekes scale is widely used in the U.S. to track the developmental stages of cereal crop. It can also be used to track cover crop development and as this video describes, can help growers know then their cover crops have reached the C:N ratio requirement of Ag. Order 4.0  

• Broeske et al. 2020. Visual Guide to Winter Wheat. Development and Growth Staging, Univ. Wisc. (72 page guide)
• Lollato R. 2018. Wheat Growth and Development, Kansas State Univ.  (1 page guide)

Other Ag. Order related videos

Advocating for Cover Crop Nitrogen Credits -Ag Order 4.0 Adoption hearing, Central Coast Water Board (9:31)

Historical Win for Farmers, Cover Crops & Ground Water Protection in California’s Central Coast (3:03:56)

  

Misc. publications on cover crop effects on reducing nitrate leaching

Jackson L.E., L.J. Wyland, L.J. Stivers. 1993. Winter cover crops to minimize nitrate losses in intensive lettuce production. Journal of Agricultural Science 121:55-62.

Heinrich A., R. Smith, M. Cahn. 2014. Winter-killed Cereal Rye Cover Crop Influence on Nitrate Leaching in Intensive Vegetable Production Systems. HortTechnology 24:502-510.

Thapa R., S.B. Mirsky, K.L. Tully. 2018. Cover Crops Reduce Nitrate Leaching in Agroecosystems: A Global Meta-Analysis. Journal of Environmental Quality 47:1400-1411.

Tonitto C., M.B. David, L.E. Drinkwater. 2006. Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics. Agriculture, Ecosystems & Environment 112:58-72.

Wyland L.J., L.E. Jackson, W.E. Chaney, K. Klonsky, S.T. Koike, B. Kimple. 1996. Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs. Agriculture, Ecosystems & Environment 59:1-17.

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