Would Texas keep access to U.S. agricultural markets?

Yes. Trade between Texas and the United States continues after independence, governed by agreement the way trade between neighboring countries always is, and both sides have overwhelming reasons to keep the food and fiber flowing.

The buyers and the supply chains do not move

The grocers, processors, feedlots, and food companies that buy Texas beef, cotton, grain, and produce do not relocate because Texas changes its flag. The demand is where it is, the supply is where it is, and the trucks, rail lines, and ports that connect them are where they are. The most basic fact of this market is that the United States buys what Texas grows and Texas grows what the United States needs. That relationship does not switch off.

Trade between neighbors runs on agreements, and the tools already exist

After independence, commerce between Texas and the United States is governed by a trade agreement, the same as commerce between any two trading partners. Texas would pursue tariff-free terms through a customs union or a free trade agreement, and at a minimum can trade under the World Trade Organization schedules the United States has already agreed to. The live FAQ answer on tariffs covers this mechanism in detail. The point here is simple: there is a well-worn, ready-made set of tools for keeping agricultural trade open, and Texas would use them.

Continued access runs in both directions, because the dependence is mutual

Texas does not only sell to the United States. It buys too, and the two agricultural economies are deeply interwoven. That mutual dependence is exactly what makes open trade the obvious outcome. Cutting off Texas agriculture would raise costs and disrupt supply chains on both sides of the border for no gain. Neighbors who need each other trade with each other, and they write the agreements that keep it smooth.

Texas trades with the whole world, not just Washington

Keeping U.S. market access does not mean depending on it. Texas is the top exporting state in the union and sells agricultural products around the globe. As an independent nation, Texas would negotiate its own access to markets everywhere, on terms written for Texas, instead of being bound by federal trade deals that have repeatedly favored other sectors at agriculture's expense. U.S. access continues, and the rest of the world opens wider.

The bottom line

Texas keeps access to U.S. agricultural markets through the ordinary trade agreements neighbors use, the supply chains stay intact because the dependence runs both ways, and an independent Texas gains the power to open new markets worldwide on its own terms.