Horses are by far the most sensitive to Black Walnut-primarily if it is used as shavings in bedding for stalls. With as little as 20% horses will start to display clinical signs of toxicity within hours. Just for your reference, early clinical signs of black walnut toxicity would be depression, limb edema, stiffness, and warm hooves. If exposure continues, horses may colic, experience extreme swelling of the neck and chest, elevated heart and respiratory rates, and high body temperature. While it does not seen that sheep are sensitive to Black Walnut bedding, I would be very careful of them eating the husks of fallen nuts as they can start to become toxic as they decay. Penicillium mold also affects the decomposing husk and produces a neurotoxin called Penitrem A, which is toxic to livestock and can be fatal to dogs.                                                                                                     I do not see any research on sheep or horses being affected by grass growing around Black Walnut trees. Researching plants that are affected by Black Walnut roots or jugalone-I do not see any grasses listed. Plants that are very sensitive to jugalone include those in the nightshade family-and that's OK because you do not want those in your pasture anyways as they are toxic to horses and livestock. If you were to observe plants being affected by jugalone you would see yellowing or discoloration of the leaves as well as twisted growth. Are there other plants around the trees that you are concerned about?                                                              From your comments, those trees sound like it is preferable to leave them as they are large and provide good shade. I believe that you would be safe to leave them as long as you are able to rake up leaves/nuts/hulls and avoid using bark or chips as mulch or animal bedding.