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EC number: 205-766-1 | CAS number: 150-68-5
Thirteen-week studies were conducted to evaluate the toxicity associated with repeated ingestion of monuron and to determine the concentrations to be used in the 2-year studies.
Groups of 10 F344/N rats of each sex were fed diets containing 0, 750, 1500, 3000, 6000, or 12000 ppm monuron. All diets were available ad libitum. Formulated diets were first offered to each group on September 25, 1978. Necropsies were performed between December 26 and December 29, 1978.
Animals were checked two times per day, moribund animals were killed and necropsies were performed. Feed consumption was measured once per week. One day before the animals were killed, formulated diets were replaced with control feed. Individual animal weights were recorded once per week. At the end of the 13 week studies, survivors were killed. A necropsy was performed on all animals, except those excessively autolyzed or cannibalized.
Eight of 10 male rats and 9/10 female rats fed diets containing 12000 ppm monuron died during the first 2 weeks of the studies. Five of 10 male rats and 6/10 female rats fed diets containing 6000 ppm monuron died during the first 4 weeks of the studies. The final weights of the rats were inversely related to the concentration of monuron in the feed. Female rats fed 750-12000 ppm ate less than did the controls.
Discoloration and mottling of the lungs, smoothing and thinning of the stomach, and enlargement and discoloration of the adrenal glands were noted at necropsy and appeared to be compound related.
A generalized atrophy of lymphocytic and hematopoietic tissues was observed in male and female rats fed diets containing 12000 ppm monuron. These changes included lymphoid depletion and congestion of the splenic white pulp, lymphoid depletion with overall reduction in size of the thymus, myeloid depletion of bone marrow, and lymphoid depletion of B-and T-cell areas of lymph nodes causing a marked reduction in the size of all examined lymph nodes. Similar but less severe changes were seen in male and female rats fed diets containing 6000 ppm monuron; changes seen in the lymphocytic and hematopoietic tissues of rats fed monuron at lower concentrations were considered to be slight or equivocal.
Because of weight gain depression and histopathologic changes observed at higher dose levels, doses selected for the 2-year studies in rats were 750 and 1500 ppm monuron in feed.
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