Grain feeding increases intramuscular fat content in cattle and results in meat with reduced perceived toughness.

Study: Identifying Premium-Quality Beef in the United States—A Comparison of Beef Palatability from Grain-Finished Young and Mature Beef Cattle with Varying Marbling Scores

Cows fed grain develop more fat inside their meat, and people find that meat softer and tastier. Even older cows made tender meat if their meat had lots of fat streaks.

Study: Effects of Feeding High-Moisture Corn on Meat Performance, Meat Quality, Muscle Metabolism, and Gut Microbiota in Kazakh Rams

Feeding rams more high-moisture corn made their meat 56% fattier inside the muscle and 55% easier to chew, which means the meat was less tough. This supports the idea that grain-fed animals have more marbling and softer meat.

Study: Grain Feeding Improves Yak Meat Tenderness and Lipid Deposition: Meat Quality, Amino Acid and Fatty Acid Profiles, Metabolomics, and Transcriptomics

When cattle are fed grain instead of grass, their meat gets more fat inside the muscle and becomes easier to chew. This study showed that grain-fed yaks had almost twice as much fat in their meat and much less resistance when cut, meaning the meat was noticeably more tender.

Study: Adipogenic/lipogenic gene expression and fatty acid composition in chuck, loin, and round muscles in response to grain feeding of Yanbian Yellow cattle.

Cattle fed grain developed more fat inside their muscles over time, and the type of fat got healthier—more of the good kind that makes meat tender. So yes, grain-fed beef is likely less tough.