Dietary nitrate loads on cows in dairy farms near Lake Dian, Kunming City, Yunnan Province, China.

Abstract

In a cropland area near Lake Dian, large amount of the crop wastes, i.e. vegetable and flower residues, were fed to cows in dairy farms. Since several vegetables and flowers tend to accumulate nitrate, animals kept such situations were suspected to ingest large amount of nitrate. The objective of this study was to quantify the dietary nitrate loads on dairy cows in the area. Questionnaire surveys on feedstuffs fed to cows were conducted in the dairy farms from October 2009 to September 2010. Nitrate contents in the feedstuffs and methemoglobin concentrations in the blood of cows were measured. The feeding amounts of crop wastes from vegetables and flowers were 28.4 kg to 50.0 kg per head per day on a fresh weight basis. The contents of nitrate nitrogen in the residues of rape blossoms, parsley, stem lettuce and carnations were 0.19%DM to 0.37%DM, which were potentially toxic levels for ruminants. Those in the residues of celery and broccoli were 1.36%DM and 0.59%DM respectively, which were toxic levels. However, the nitrate nitrogen contents in the total diets (sum of feed ingredients) were 0.15%DM to 0.19%DM, which were below the allowable limit. The ratios of methemoglobin to the total hemoglobin in the blood were less than 10%, indicating that the dairy cows did not fall into acute nitrate poisoning.