Sweet clover poisoning, an insidious hemorrhagic disease, is seen in animals that consume toxic quantities of spoiled sweet clover (Melilotus officinalis and M. alba) hay or silage. During the process of spoiling, the coumarins in sweet clover are converted to toxic dicumarol, a potent vitamin K antagonist and anticoagulant. However, grazing fresh, undamaged yellow or white sweet clover normally is a safe practice. 

Andrew Pegman BVSc MANZCVS

like Andrew already carried out: the growing plant is no danger for wildlife, and wildlife usually has no access to spoiled silages.

Best thanks for your answers, which make me more shure and evidence based !

But we can find dicoumarol (low concentrations ng/g) in blood of livestock or wildlife exposed to Melilotus spp consumptions. In fact, we found toxic concentrations only when hays are molded (Aspergillus or others fungi actions) to have an AVK actions like rodenticides. Dicoumarol is rapidely cleared by liver. Herbivore's intoxications need high level of dicoumarol to inhibit VKORC1 and long time expositions. In Canada, Melilotus has been used as hay or silage but only  selected varieties with low concentrations in coumarins have been cultivated.

To complete the answer of Gilbert Gault, you can find here some publications on the sweet clover disease.

Article Sweetclover Production and Agronomy

Article Vitamin K treatment of sweet clover poisoning in calves

Article Studies on the Hemorrhagic Sweet Clover Disease Iv. the Isol...

Article Studies on the hemorrhagic sweet clover disease. VI. The syn...

 Thank you for your good hints! It looks like to be evident that Sweet Clover is only poisonous in the conex of problematic agricultural plant production and live stock feeding.

It usually occurred in animals as a results of dicoumarol contents and used as rodents poison. Signs of poisoning may not appear for up to 3 weeks after feeding moldy sweet clover hay, and depends on the quantity of dicoumarol consumed. Poisoning is likely to occur when dicoumarol levels exceed 10 mg/kg of hay. Hay containing dicoumarol levels of 10-20 mg/kg of feed can be fed for 100 days before poisoning develops. Feeds containing 60-70 mg/kg of feed can cause poisoning in as little as 21 days. Prothrombin times above 40 seconds are suggestive of decreased clotting ability. Blood prothrombin, activated partial thromboplastin times, and clotting times are markedly increased from their normals of 9 to 12 sec, 30 to 45 sec, and 3 to 15 min, respectively. There are varieties of sweet clover that contain very low levels of coumarin and, therefore, hay from these varieties is safe, even if moldy. To prevent the risk of moldy sweet clover poisoning, sweet clover hay or haylage should not be fed for at least 3 weeks before parturition or elective surgery.

During the process of spoiling, the coumarins in sweet clover are converted to toxic dicumarol, a potent vitamin K antagonist and anticoagulant. Any method of hay storage that allows molding of sweet clover promotes the likelihood of formation of dicumarol in the hay. When toxic hay or silage is consumed for several weeks, dicumarol alters proenzymes required for synthesis of prothrombin, resulting in hypoprothrombinemia (by preventing formation of the active enzyme). It probably also interferes with synthesis of factor VII and other vitamin K–dependent coagulation factors. Dicumarol concentrations of 20–30 mg/kg of hay ingested throughout several weeks are usually required to cause poisoning in cattle. However, all species of animals are susceptible, mostly occurred in cattle and, to a limited extent, sheep, pigs, and horses. The time between consumption of toxic sweet clover and appearance of clinical disease varies greatly and depends on the dicumarol content of the particular sweet clover variety being fed, age of the animals, and amount of feed consumed. If the dicumarol content of the ration is low or variable, animals may consume it for months before signs of disease appear. Hay containing dicoumarol levels of 10-20 mg/kg of feed can be fed for 100 days before poisoning develops. Feeds containing 60-70 mg/kg of feed can cause poisoning in 21 days.

The first indication of dicumarol poisoning may be the death of one or more animals. In affected animals, the first signs may be stiffness and lameness, due to bleeding into the muscles and joints. Cardiovascular signs ( severe hemorrhaging will cause anemia : pale mucous membranes, rapid heart rate and death), respiratory System (epistaxis), renal System (hematuria), reproductive System (excessive hemorrhaging during calving),ocular System (hemorrhaging into the anterior chamber of the eye). Death is generally caused by massive hemorrhage or bleeding. Prothrombin times above 40 seconds are suggestive of decreased clotting ability. Blood prothrombin, activated partial thromboplastin times, and clotting times are markedly increased.

References:

Radostits QM, Searcy GP, and Mitchall KG. Moldy sweet clover poisoning in cattle. Can Vet J 1980; 21(5): 155-158.

Baumann CA, Field JB, Overman RS and Link KP. Studies on the hemorrhagic sweet clover disease. J Biol Chem 1942; 146: 7-14.

Stegelmeier BL. Poisonous Plant Research Laboratory, USDA-ARS. https://www. msdvetmanual.com/toxicology/sweet-clover-poisoning/overview-of-sweet-clover-poisoning

Puschner B, Galey FD, Holstege DH and Palazoglu M. Sweet clover poisoning in dairy cattle in California. J Am Vet Med Assoc 1998; 212(6): 857-859.