Play behavior in farm animals is a definitive indicator of positive welfare states — animals that play are animals that are physiologically comfortable and emotionally positive.
Play as a welfare indicator has the significant advantage of being immediately observable without equipment or training. A field of lambs gamboling in spring sunshine communicates excellent welfare instantly and viscerally. A piggery where young pigs never play communicates chronic welfare problems just as clearly. Incorporating play observation into routine farm assessments changes how producers and inspectors perceive welfare — shifting from deficit detection (looking for lameness, injury, disease) to positive welfare evaluation (looking for evidence of good lives). This shift in perspective motivates different management responses and creates farms that prioritize what animals need to thrive rather than just what prevents obvious harm.