It wasn’t an earthquake, but it may result in the single largest change in California dairy farming since adoption of the bulk tank.

Two separate regulatory processes saw daylight in June. The first, adoption of a Nitrate Management Zone Implementation Plan, occurred on June 3. The second regulatory process, a draft remand of 2013 Waste Discharge Requirements General Order for Existing Milk Cow Dairies (GO), was issued June 15. Both impact dairies in the Central Valley of California, which represents roughly 90% of California’s milkshed.

A 35-year plan

The Central Valley Regional Water Quality Control Board (RB5) adopted the Modesto Nitrate Management Zone Implementation Plan (MZIP). This is the first of many MZIPs to be adopted by RB5.

For dairies, this Plan starts a 35-year timer. The MZIP granted an exception to dairies, bovine operations (not rangeland), and those enrolled in the Irrigated Lands Regulatory Program. The 35-year period exempts these businesses from not causing or contributing to exceedances of the water quality objective. Since the “water quality objective” is drinking water, that means any water making its way to groundwater needs to have less than 10 milligrams per liter (mg/L) of nitrate nitrogen. Every five years, both individual landowners and their management zones will provide documentation to confirm if the 35 years are needed, or if nitrate leaching will be reduced sufficiently before the deadline.

Other nuances exist. Between now and then, dairy producers and the Central Valley Dairy Representative Monitoring Program (CVDRMP) have much work to do. The goal at the end of the 35 years is to have achieved no further degradation of the groundwater from nitrate. This is not a “wait 35 years to start” situation. This is a “start now and work diligently for the next 35 years” situation.

The 35-year window provides time to identify and implement improvements needed to manage surplus nitrogen at each farm and then appropriately handle the remaining nitrogen. Specific deliverables are called for over the timeline.

The Modesto Management Zone will continue to test residential drinking water and provide safe water to residents with nitrate concentrations above the drinking water standard. A detailed timeline is provided with the MZIP.

Examples of MZIP activities in the first three years include the use of a new data management system for annual reports (a web-based portal) and data collection to allow analyses at different scales, development of site-specific Irrigation and Nutrient Management Plans, and reporting of compliance with the Irrigation and Nutrient Management Plans annually thereafter.

At 10 years, each dairy must verify it is on track to achieve zero nitrogen surplus by the 35-year mark and show progress at regular intervals. This becomes difficult for facilities with intermittent fallowing of land due to Sustainable Ground Water Management Act restrictions (due to lack of irrigation water).

For now, the focus is on meeting the no further degradation goal in 35 years. However, the local management zone board of directors will review nitrate loading information and identify if they need to modify associated fees based on nitrate loading. Maintaining seats at each MZIP table is essential for dairy representation as decisions are made in the future.

The other shoe

Additional requirements for all Central Valley dairies will kick in when the State Water Resources Control Board adopts its revised draft order anticipated in September. RB5 will address the additional requirements as it modifies its dairy regulatory processes. The 125-page document is quite detailed.

The new regulatory framework for nitrogen discharges proposes an interim and a final regulatory step to protect groundwater quality. It defines a nitrogen discharge limit of 10 mg/L of nitrate nitrogen. The framework includes the nitrogen discharge limit; four components related to land application of dairy manure; three components related to manure retention ponds; and one component related to the provision of alternative water supplies.

The state is dictating the whole-farm nitrogen accounting component and the alternative water supply component. Some discretion is left to RB5 regarding the land application and manure retention pond components. The latter implementation requirements may be those suggested by the state or those identified and defended by RB5. The draft order is in a 45-day public comment period.

Here are a few statements from the draft remand:

• “At their core, waste discharge requirements are prospective regulatory tools that establish enforceable requirements and conditions on ongoing or future discharges.”
• “The Nitrogen Discharge Limit applies to all dairy manure management practices.”
• “The most important component of the new regulatory framework for nitrogen discharges is enforceable final numeric land application rates that directly correlate to the Nitrogen Discharge Limit.”

The draft refers to an application rate that fits into a two-part requirement: “The first part of our conceptual proposed implementation requirement is a concentration-based groundwater loading limit that is expressed as the maximum annual pounds of nitrogen that is allowed to leach below the root zone per acre of land application area, divided by the number of acre feet of water that leaches below the root zone per acre of land application area . . . The second part of our conceptual proposed implementation requirement is a multi-year land application rate formula that will be used to determine how much nitrogen can be applied to dairy cropland consistent with the groundwater loading limit to meet the nitrogen discharge limit.”

Note that a process or technology that primarily handles nitrogen and does not redistribute salts (such as potassium, sodium, and chlorine) isn’t necessarily helpful, since application of brine water to fields in a dry climate leads to buildup of salt in soils. RB5 has no mechanism for the discharge of brine water to land. Also of concern is high potassium water, which will result in high potassium forage and potentially contribute to animal health issues.

Additional changes in the remand include needing to determine separation from the bottom of liquid manure storage structures to first encountered groundwater, and dealing with cattle included in the previous regulatory accounting process (mature cows on hand in 2005) and current mature cow number if these are not the same.

This is happening in California, but it is a wake-up call for all in agriculture. Regardless of where you live, be sure you or your clients understand the implications of current and potential regulatory changes as you consider facility modifications and long-term contractual obligations associated with manure management. Assemble a knowledgeable team before making decisions. Include different voices as you discuss options. The future of animal operations is on the line, and informed decisions are critical.

The authors are a livestock waste management specialist, a dairy advisor, and an agronomy and nutrient management advisor, respectively, with the University of California Agriculture and Natural Resources program.

This article appeared in the May 2026 issue of Journal of Nutrient Management on pages 10 and 11.

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