USDA is a federation of agencies, and the meat market data that buyers read comes from four of them. Knowing which agency publishes which report is more than trivia: each agency has a different methodology, a different revision history, and a different feel for the market.
AMS (Agricultural Marketing Service)
AMS is the agency most working buyers encounter first. AMS runs the Livestock Mandatory Reporting program, which obligates large packers to report negotiated sales of boxed beef, pork carcasses, and live cattle to USDA on a daily basis. The Market News division of AMS aggregates those reports and publishes them under the LM_ and NW_ prefixes. The most-cited reports are LM_XB403 for the afternoon boxed beef cutout, LM_PK602 for the afternoon pork carcass cutout, the LM_CT series for negotiated cattle (a family of regional and summary reports rather than a single code), and NW_LS421 for imported trim. AMS also publishes the institutional meat purchase specifications (IMPS) that define every standard cut.
AMS reports are transactional. They report what cleared, not what was forecast or surveyed. That makes them the most useful daily read for a buyer trying to reconcile their own bid against the market.
MARS (Market Analysis and Reporting Service, the API layer)
MARS is the structured data layer behind AMS reports. The same data that appears on a published PDF is available through the MARS API at marsapi.ams.usda.gov for a free key. For most buyers, MARS is invisible: it is the underlying source for every dashboard, broker quote service, and trade desk analytics platform that ingests USDA data programmatically. When a software product (like the live dashboards on this site) shows USDA data, the data is almost always coming through MARS rather than being scraped from PDF.
MARS coverage is comprehensive for current data and reasonably complete back several years, depending on the report. Some legacy reports were migrated onto MARS after the underlying data was already being published in PDF form, which means the MARS history for those reports starts later than the PDF history.
ERS (Economic Research Service)
ERS is the analytical and forecasting arm of USDA. ERS publishes the Livestock, Dairy, and Poultry Outlook each month, which carries production and price forecasts for the major proteins. ERS also publishes the Livestock & Meat Domestic Data set, which includes long-history price series for whole-bird poultry, beef, and pork going back decades, far longer than AMS daily reports cover. ERS data is usually monthly or annual frequency rather than daily, and it is the source of choice when a buyer needs long-term context (multi-decade price trend, production share by region).
ERS analytical reports are written in plain English and are useful as a cross-check on a buyer's own market read. The Livestock, Dairy, and Poultry Outlook is one of the better single documents to read each month for context on where the U.S. meat market sits in the broader supply-demand picture.
NASS (National Agricultural Statistics Service)
NASS runs the surveys. Cattle (the semi-annual inventory report), Quarterly Hogs and Pigs, Cattle on Feed, Livestock Slaughter, Cold Storage, and the various poults and chicks production reports are all NASS products. The surveys typically draw from a sample of producers and feedyards, with the results scaled to a national estimate. Cattle on Feed is monthly. Hogs and Pigs is quarterly. The semi-annual Cattle inventory is January and July.
NASS data is forecast-relevant rather than transaction-relevant: it is what producers said they were doing, not what the market cleared at. The surveys carry confidence intervals (Cattle on Feed placements typically come within plus-or-minus 2 percent of the trade pre-report estimate), and revisions sometimes come in subsequent reports as additional data arrives.
How to tell which agency owns a report
The report code is the cleanest tell. LM_ and NW_ prefixes are AMS Market News (transactional). The Livestock, Dairy, and Poultry Outlook and the Meat Price Spreads data set are ERS (analytical). Anything called "Cattle on Feed", "Hogs and Pigs", "Cattle inventory", "Livestock Slaughter", or "Cold Storage" is NASS (survey). When in doubt, the agency name appears at the top of every USDA report, and a quick read of the masthead tells you the methodology before you read the numbers.