What is the most likely cause of droopy ears and ataxia in a group of cows that escaped to eat lush grass?

The most likely cause of droopy ears and ataxia in cows that have escaped to eat lush grass is primary photosensitization. This condition typically occurs when animals graze on certain types of lush, green pastures that contain high levels of chlorophyll, which can lead to the accumulation of photodynamic agents in the liver. These agents become active when exposed to sunlight, causing damage to the skin and the liver, resulting in both visible symptoms such as droopiness of the ears and neurological signs such as ataxia.

Primary photosensitization particularly affects animals that have underlying liver issues that impair their ability to detoxify these substances. The lush grass can also contain certain plants that exacerbate this condition. Therefore, recognizing the link between the cows' diet and the development of these symptoms is crucial for identifying primary photosensitization as the root cause in this scenario.