Margaret Stalker, Clark Sinclair, Felipe Reggeti, Nick Schrier, Kalie Bernardo
Animal Health Laboratory, University of Guelph, Guelph, ON (Stalker, Reggeti, Schrier); Tavistock Veterinarians, Tavistock, ON (Sinclair, Bernardo).
AHL Newsletter 2019;23(3):9.
Three 12-15 mo-old Holstein heifers were found dead overnight in 2 adjacent pens in a freestall barn. A recent feed change to sorghum silage had occurred one week ago, and 60 heifers were being fed the silage as the main source of roughage along with a small amount of corn and access to a protein lick tank. Feed intake was considered normal. No clinical signs were observed the previous evening. The animals were found dead in lateral recumbency in the morning. One heifer was sent to the AHL for postmortem examination. The animal was in good body condition, oral mucous membranes were slightly pale, and ocular mucous membranes were slightly dusky. No significant gross abnormalities were found on postmortem examination. The rumen was full of silage, with a small gas cap, and rumen pH was 6.5-7.0. Based on the history of multiple animals affected following a recent feed change to sorghum silage, the silage was removed and replaced with corn silage. A silage sample, as well as postmortem ocular fluid from the heifer, was submitted to the AHL Toxicology laboratory for nitrate analysis.
The sorghum silage nitrate analysis was 40,000 mg/kg dry weight (40,000 ppm or 4%). In general, feed with <5,000 ppm nitrate is considered safe to feed under all conditions, 5,000-9,999 ppm is considered safe for non-pregnant animals but should be limited to 50% of the ration for bred cows, 10,000-17,000 ppm should be limited to 35-40% of the total dry matter in the ration and should not be fed to pregnant animals; feeds >17,000 ppm nitrate are considered to be toxic and should not be fed to cattle.1 Ocular fluid nitrate analysis was 52 mg/l. Nitrate values in excess of 10 ppm are suspicious for, and in excess of 20 ppm are considered indicative of toxicity2.
Fast-growing forage crops such as sorghum may accumulate toxic levels of nitrates under stressful growing conditions, for example periods of dry weather followed by a rain, or after heavy applications of nitrogen fertilizers. Although ensiling can reduce nitrate levels by 25-65%, the crop must be at the correct moisture level for proper fermentation to occur. Baleage is generally too dry to ferment completely, and nitrate levels are stable in dry hay.3
Nitrates in forage are reduced to nitrite in the rumen, then quickly converted by rumen bacteria to ammonia, which is absorbed and excreted into urine. High levels of nitrate ingestion and subsequent nitrite production overwhelm conversion to ammonia, and nitrite is absorbed directly, oxidizing hemoglobin to methemoglobin (MHb) which is incapable of oxygen transport. Clinical signs of hypoxia can be subtle, and include increased respiratory rate, apprehension, weakness, depression and recumbency. Pregnant animals may abort. If MHb reaches 75-80%, animals begin to die. If the accumulation is not lethal, MHb is slowly reduced back to Hb over a 12-24 hour period. At autopsy, few distinctive lesions are evident. Blood and tissues may be dark or “chocolate brown” colored, but this is not a consistent feature. Forage testing of composite samples is the only method to evaluate risk for nitrate toxicosis, prior to feeding. AHL
References:
1. Niles G. As livestock producers dip into low-quality forages, beware nitrate. Colorado State University VDL http://csu-cvmbs.colostate.edu/vdl/Pages/livestock-producers-dip-into-low-quality-forages-beware-nitrate.aspx
2. Burrows and Tyrl. Toxic Plants of North America, 2nd ed. Wiley-Blackwell, 2013.
3. Crop Report – August 1, 2019, OMAFRA Field Crop News http://fieldcropnews.com/2019/08/crop-report-august-1-2019/ .