Cow Calf Corner: Beef herd expansion status; Prussic acid and nitrate poisoning are concerns after a light frost

October 31, 2016
Beef herd expansion status
Derrell S. Peel, Oklahoma State University Extension Livestock Marketing Specialist
The unexpectedly rapid and harsh adjustment in feeder cattle prices in 2016 has raised many questions about the status of herd expansion late in the year and beyond.  Have changes in producer expectations altered herd expansion in 2016 and, more importantly, for 2017 and beyond? Begin with a review of the story so far.  On Jan. 1, 2015, the inventory of replacement heifers was a record 20.8 percent of the beef cow inventory.  Beef cow slaughter in 2015 was a record low level of 7.6 percent of the beef cow herd inventory.  The combination of large replacement heifer inventories and low cow slaughter facilitated the 3.5 percent year-over-year jump in beef cow inventories in 2015.
On Jan. 1, 2016, the inventory of beef replacement heifers was 20.7 percent of the herd inventory; nearly as large a percentage as the record 2015 level. With large replacement heifer inventories available, the beef cow herd was poised to continue strong herd expansion in 2016.  USDA did not provide a July Cattle report so no mid-year update of herd expansion was available.  Quarterly cattle-on-feed inventories show that heifers in feedlots increased year over year in April and have been higher by a consistent amount of roughly 4.5 percent year over year in the July and October quarters as well.
The October 1 heifer-on-feed inventory was still 8.5 percent below the previous 5-year average for that date.  Heifer slaughter was below year earlier levels into early June and has shown year-over-year increases so far in the second half of the year.  Weekly heifer slaughter has averaged 11.7 percent year-over-year increases since July.  The result is year-to-date heifer slaughter that is up 2.5 percent over 2015 and with continued year-over-year increases for the remainder of the year is projected to finish with an annual total up roughly 3.5 percent year over year.
Beef cow slaughter started 2016 with the low levels from 2015 but quickly changed to year-over-year increases by the end of the first quarter.  The second and third quarters showed even stronger year-over-year increases resulting in a year-to-date increase in beef cow slaughter of 12.1 percent compared to last year.
Strong beef cow culling through the rest of the year is projected to bring the 2016 annual beef cow slaughter to a roughly 13 percent year-over-year increase.  More beef cow slaughter is expected because 1) last year’s net culling was unsustainably low and 2) the 1 million head increase in cow numbers last year inevitably means more cow culling. However, the projected rate of 2016 beef cow slaughter would represent a net beef herd culling rate of less than 8.5 percent of the herd, well below the average level of nearly 10 percent.  In other words, beef cow culling has not returned to normal levels and the 2016 beef cow slaughter level is consistent with continued modest herd expansion this year.
The rate of beef cow slaughter and modest year-over-year increases in feedlot heifer inventories since April do not yet indicate herd liquidation but may point to little or no additional herd expansion in 2017.  The ratio of steer to heifer slaughter increases during herd expansion and typically peaks and begins declining several months to more than a year ahead of the cyclical peak in cow inventories.  The ratio of steer to heifer slaughter (12-month moving average) peaked most recently in July 2016 (at the highest level since February 1975) and declined slightly in August and September.  This most likely suggests a herd inventory peak at the end of 2017, but will depend on how fast the ratio changes in the coming months.
Prussic acid and nitrate poisoning are concerns after a light frost
Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist
Although late October has been very warm and “summer-like,” the average first frost date for much of the Southern Plains is here.  Soon a cold front will bring near-freezing to sub-freezing nighttime temperatures
It was discovered in the early 1900s that under certain conditions sorghums are capable of releasing hydrocyanic acid or commonly called prussic acid.  Prussic acid when ingested by cattle, is quickly absorbed into the blood stream, and blocks the animal’s cells from utilizing oxygen.  Thus the animal dies from asphyxiation at the cellular level.  Animals affected by prussic acid poisoning exhibit a characteristic bright red blood just prior to and during death.  Lush young regrowth of sorghum-family plants are prone to accumulate prussic acid especially when the plants are stressed such as drought or freeze damage. Light frosts that stress the plant but do not kill it are often associated with prussic acid poisonings.
Producers should avoid grazing fields with sorghum type plants following a light frost.  The risk of prussic acid poisoning will be reduced, if grazing is delayed until at least 1 week after a “killing freeze.”  As the plants die and the cell walls rupture, the hydrocyanic acid is released as a gas, and the amount is greatly reduced in the plants.  One can never be absolutely certain that a field of forage sorghum is 100 percent safe to graze.
Cattle that must be grazed on forage sorghum pastures during this time of year should be fed another type of hay before turning in on the field, and should be watched closely for the first few hours after turn in.  If signs of labored breathing, such as would be found in asphyxiation, are noted, cattle should be removed immediately.  Call your local veterinarian for immediate help for those animals that are affected.  Be certain to read OSU Fact Sheet PSS-2904 “Prussic Acid Poisoning” before turning cattle to potentially dangerous fields.
Frosts also stress the plant before a hard freeze kills it.  Plant stress from frosts will impair the normal metabolism of the plant.  Therefore the plant continues to take up nitrates from the soil, but is inefficient at converting the nitrates to protein.  Therefore nitrate accumulations may reach dangerous levels.  Testing the forage before grazing or cutting for hay will provide important knowledge about the safety or danger in the forage.  Visit with an OSU County Extension office about testing procedures and read OSU Fact Sheet PSS-2903 “Nitrate Toxicity in Livestock.”
Cow-Calf Corner is a weekly electronic newsletter by the Oklahoma Cooperative Extension Agency.