Hello, thanks for reaching out to ask extension. I am based in western Oregon hence will base on that but most of these apply everywhere. 

Resources that you might find relevant

• Avoiding Poisonous Plants in Pasture and Hay – https://extension.oregonstate.edu/crop-production/pastures-forages/avoiding-poisonous-plants-pasture-hay
• Weed, Poisonous Plant, and Pest Resources for Small Farms – https://extension.oregonstate.edu/collection/weed-poisonous-plant-pest-resources-small-farms
• Native Plant Gardening Collection – https://extension.oregonstate.edu/collection/native-plant-gardening
• Native Plant Picks for Bees – https://extension.oregonstate.edu/catalog/pub/em-9363-native-plant-picks-bees

Safe Wildflowers for Oregon Pastures

Oregon Sunshine (Eriophyllum lanatum) – Drought-tolerant and great for pollinators.
Common Yarrow (Achillea millefolium) – Non-toxic and beneficial for soil health.
Purple Coneflower (Echinacea purpurea) – Hardy, attracts bees and butterflies.
Showy Fleabane (Erigeron speciosus) – Low-maintenance and excellent for grazing areas.
Nodding Onion (Allium cernuum) – Edible for humans, safe for livestock.
Self-Heal (Prunella vulgaris) – Medicinal herb that grows well in pastures.
Douglas Aster (Symphyotrichum subspicatum) – Thrives in moist conditions, good for erosion control.
Goldenrod (Solidago spp.) – Pollinator-friendly and non-toxic to livestock.
California Poppy (Eschscholzia californica) – Low-water needs, great for Oregon’s dry summers.
Western Blue Flax (Linum lewisii) – Drought-tolerant and safe for livestock.

Wildflowers to AVOID in Oregon Pastures

Larkspur (Delphinium spp.) – Extremely toxic to cattle and sheep.
Lupine (Lupinus spp.) – Some species are toxic, especially to pregnant livestock.
Buttercup (Ranunculus spp.) – Causes mouth irritation and digestive distress.
Foxglove (Digitalis purpurea) – Highly poisonous, affects the heart.
Nightshade (Solanum spp.) – Toxic to the nervous system.
Water Hemlock (Cicuta spp.) – One of the most toxic plants in North America.
Tansy Ragwort (Senecio jacobaea) – Causes fatal liver damage in livestock.

Planting Tips for Oregon Pastures

• Choose Native Wildflowers – They require less maintenance and support local biodiversity.
• Mix with Grasses – Plant alongside pasture-friendly grasses to maintain forage quality.
• Avoid Wet Areas for Some Species – Certain wildflowers, like Eriophyllum lanatum, prefer well-drained soils.
• Monitor for Toxic Weeds – Some invasive toxic plants (like Tansy Ragwort) can outcompete wildflowers if not managed.