Cattle raised for beef in the United States are typically fed either grain or grass, with varying effects on production—a faster growth rate is often observed among cows raised on grain, while grass-fed beef is often associated with health-promoting fatty acids. These diet-driven outcomes have been linked to variation in how the different feed types affect the community of bacteria living in the gut. And now, researchers have found they can influence the gut viral community as well. Leveraging a technique called viral metagenomic sequencing, the team was able to comprehensively characterize viruses in fecal samples from 6 grain-fed and 6 grass-fed Angus beef cattle, finding most were bacteriophages—viruses that exclusively infect and reproduce within bacteria. The data also showed clear differentiation in viral community composition between the two feed types, with grain-fed cattle harboring approximately one-third fewer gut viral genomes than those fed grass and different dominant viral groups. Further analysis also showed grain-fed cattle weighed more than grass-fed cows over the course of the experiment. These findings provide new insight into how feed type influences the gastrointestinal viral community in cows, ultimately affecting their growth, and could pave the way toward new microbe-targeting strategies to boost cattle production.