Nitrate/Nitrite Poisoning of Cattle in Brachiaria brizantha cv. Marandu Pasture in the Semiarid Region of Northern Minas Gerais, Brazil

Abstract

Background: Nitrate/nitrite poisoning is an important cause of economic losses in ruminant livestock. Under environmental conditions such as prolonged drought followed by intense rainfall, several grasses may accumulate toxic nitrate concentrations. Although outbreaks have been reported in different Brazilian regions, cases involving Brachiaria brizantha cv. Marandu in the semiarid region of northern Minas Gerais remain poorly documented. Therefore, the objective of this report was to describe the epidemiological, clinical, pathological and toxicological findings of spontaneous nitrate/nitrite poisoning in cattle associated with Brachiaria brizantha cv. Marandu.

Case: A 36-month-old Nellore male, recently introduced into a herd in the semiarid region of northern Minas Gerais, Brazil, was maintained on a pasture predominantly composed of Brachiaria brizantha cv. Marandu. Shortly after the dietary transition, the animal developed progressive clinical signs compatible with toxicosis, including hypersalivation, anorexia, apathy, prostration, and persistent lowered head posture. Clinical evolution was marked by worsening weakness and permanent recumbency, culminating in death 5 days after the onset of signs. A complete necropsy was performed approximately 1 h post mortem. Gross findings included dark brown blood suggestive of methemoglobinemia and diffuse blackened discoloration of the spleen and liver. Tissue samples from liver, spleen, and kidney were collected and fixed in buffered formalin for histopathological evaluation. Additionally, pasture samples consumed by the animal were submitted to diphenylamine testing to detect nitrate accumulation. Histopathology revealed no significant microscopic lesions, consistent with the functional pathogenesis of nitrate/nitrite intoxication. In contrast, the diphenylamine test yielded a positive result, confirming high nitrate concentrations in the forage. Diagnosis was established through the integration of clinical history, necropsy findings, absence of histological changes, and laboratory confirmation.

Discussion: The poisoning was likely associated with environmental and management factors acting simultaneously. The transition from prolonged drought to intense rainfall, typical of the semiarid climate, probably promoted rapid pasture regrowth and nitrate accumulation in plant tissues. Furthermore, the recent introduction of the animal, without gradual dietary adaptation, may have compromised ruminal microbial capacity to efficiently reduce nitrate to ammonia, allowing toxic nitrite buildup. Once absorbed, nitrite oxidizes hemoglobin iron, forming methemoglobin and impairing oxygen transport, resulting in severe systemic hypoxia. Unlike several Brazilian reports linked to excessive nitrogen fertilization or manure application, this case occurred in the absence of pasture fertilization, highlighting the isolated impact of climatic conditions and dietary transition. To the authors' knowledge, this is the 1^st report of spontaneous nitrate/nitrite poisoning associated with Brachiaria brizantha cv. Marandu ingestion in cattle from the semiarid region of northern Minas Gerais, Brazil. These findings expand the epidemiological knowledge of nitrate poisoning under semiarid conditions and emphasize the need for preventive management strategies in forage-based production systems. The case reinforces the importance of gradual adaptation of newly acquired animals and increased surveillance of pastures during climatic transition periods that predispose to nitrate accumulation and toxic outbreaks.

Keywords: cattle, poisoning, toxic plants, nitrate/nitrite, pasture, outbreaks, economic losses.