Will certain type of grass kill your cow if they eat it?

No. All species of grasses are not poisonous to cattle.

However, there are certain anti-quality factors associated with different varieties of grasses, but this is more to do with the environmental conditions these grasses were subject to that may create situations where certain grasses are regarded as "poisonous" to cattle. For instance, Grass Tetany is a magnesium deficiency illness that occurs in grasses that are lush in spring and have high amounts of potassium but low amounts of magnesium, which can make cattle (lactating dams and young stock) sick and even die. Nitrate toxicity is an anti-quality factor that occurs when grasses are stressed and experience sudden drought or sudden growth after drought conditions. Prussic Acid poisoning (which is a form of cyanide poisoning) occurs in various warm-season grasses such as Star Grass after experience lush growth after a drought. Fescue toxicity is an intolerance to the ergot fungus found in Tall Fescue plants, causing gangrene in livestock that become easily affected and have not developed immunity to this fungus. There are claims that Johnson grass is poisonous to cattle, but no real evidence has been laid on the table that this is true. There are reports from producers that have grazed/fed their cattle this type of grass without any adverse effects.