Sampling harvested forage and plant tissue for nutrient budgets

Water & nutrients · By the Milkshed Ledger editorial team · Published 2026-11-04

An old black and white photograph of a flooded alfalfa check being irrigated, seen between the ears of a horse — Irrigating alfalfa by flooding a check, Jackson Ranch, Kern County. Photo: Carleton E. Watkins, Library of Congress, public domain.

The nitrogen that leaves a dairy's fields in the harvested crop is half of the equation that its nutrient management plan is built on. The other half — nitrogen applied — can be measured from manure, wastewater, fertiliser and irrigation water. Removal can only be known by weighing what comes off each field and sampling it for nutrients. That is why the Central Valley dairy order requires plant tissue sampling at every harvest, and why the way samples are taken matters so much.

What the order asks for

The Monitoring and Reporting Program of Order R5-2013-0122 requires, for each land application area and each harvest:

a record of the total weight (tons) of harvested material removed and its percent moisture;
laboratory analyses of the harvested material for total nitrogen, total phosphorus and total potassium, expressed on a dry-weight basis, and for fixed solids (ash) and moisture.

A mid-season tissue test for total nitrogen is also described, for use when a grower wants to add fertiliser beyond 1.4 times the nitrogen expected to be removed, to show that the crop needs it.

Nitrogen removed per field is then calculated as tons harvested × dry-matter fraction × nitrogen concentration in the dry matter. An error in any of the three numbers carries straight into the application-to-removal ratio reported for that field. The State Water Board's September 2026 order asks the regional board to develop more accurate methods of measuring nitrogen applied and removed, so the reliability of these measurements is likely to receive more attention, not less.

Principles of a representative sample

A laboratory analyses a few grams. Those grams have to stand for hundreds of tons of crop. Three principles make that possible:

Many small portions, mixed. A composite of many subsamples taken across the whole harvest of a field is far more reliable than one large grab.
Taken systematically, not by eye. Subsamples taken at regular intervals — every so many loads, every so many bales — avoid the natural tendency to pick the best-looking or most convenient material.
Protected from change. Moisture is lost quickly from chopped forage in the sun, and nitrogen can be lost from wet samples that heat up. Samples are sealed in airtight bags, kept cool and delivered to the laboratory promptly.

Corn silage and other chopped forage

For crops harvested with a forage chopper, the practical approach is to take subsamples from the trucks or wagons as they arrive at the silage pile or bunker, before the material is packed. A handful from several points in a load, from a set proportion of loads across the field's harvest, goes into a clean bucket with a lid. The bucket is mixed thoroughly — on a clean tarp if necessary — and reduced by quartering to the amount the laboratory requires, which is then sealed in a bag with the air pressed out and chilled.

Weights come from truck scales where available or from load counts multiplied by a measured average load weight. Because a single field may be chopped over more than one day, it helps to label each subsample with the field and date and to keep a running count of loads per field.

Winter forage

Many Central Valley dairies grow a winter cereal or mixed forage between summer corn crops. The same principles apply. If the crop is chopped green, sample loads as above; if it is cut, wilted and chopped from windrows, sample at chopping rather than from the standing crop, because moisture changes greatly during wilting and the analysis must match the material that was weighed.

Alfalfa and baled hay

Alfalfa is cut several times a year in the Central Valley, and each cutting is a separate harvest with its own weight and nutrient content: protein, and therefore nitrogen, varies with the stage of growth and the season. For baled hay, the accepted method is a coring probe — a sharpened tube driven into the end of the bale with a drill or brace. A sample is built from cores of a good number of bales chosen at random across the lot from one field and cutting; extension and forage-testing guidance commonly calls for about twenty. Grabbing loose leaves or flakes is not equivalent: leaves are much richer in nitrogen than stems, and are the first part of the plant to fall off.

Alfalfa is a legume and fixes nitrogen from the air, which affects how its removal is treated in a nutrient budget. Nutrient management specialists and the dairy's plan set out how alfalfa fields are accounted for.

Pasture

Grazed pasture has no truck to weigh. Removal is estimated from forage measurements before and after grazing, with clipped samples taken from marked areas and analysed like any other plant tissue. The protocol should be described in the dairy's sampling plan so that it is applied the same way every time.

At the laboratory

Use a laboratory that reports the analyses the order requires, on a dry-weight basis, and keep the chain-of-custody form with each result. Looking at results across years and fields is a quick check: a corn silage sample whose nitrogen content is far above or below the farm's own history is worth questioning before it goes into the annual report.

Keeping the paperwork straight

Each sample should be traceable to a field, a crop, a cutting or harvest date and a weight. A simple harvest log — field, date, loads or bales, total tons, sample ID — kept alongside laboratory results is the easiest way to produce the harvest section of the annual report and the tissue sampling records described in our guide to the compliance binder.

Sources

Central Valley Regional Water Quality Control Board, Order R5-2013-0122, Monitoring and Reporting Program, Table 2 (nutrient monitoring, plant tissue) and Attachment C (nutrient management plan).
State Water Resources Control Board, Order WQ 2026-0028, 15 September 2026, sections III.A.5 and III.J.
University of California Agriculture and Natural Resources, UC Alfalfa and Forage Program.