Evaluation of the Relationship ofTime of Day to Forage Nitrate Levels

Evaluation of the Relationship of Time of Day to Forage Nitrate Levels LeValley, * R.C. 1 ; Pettijohn, M.B. 2 ; Selk, G.E. 3 ; Gallaway, M.R .4 ; Highfill, G.A. 5 ; New, M.G. 6 ; Zhang, H. 7 1. Extension Area Livestock Specialist, Oklahoma State University, Duncan, OK 73533 2. Extension Educator, Agriculture, Oklahoma State University, Grady County, Chickasha, OK 73018 3. Extension Animal Reproduction Specialist, Oklahoma State University, Stillwater, OK 74078 4. Extension Educator, Oklahoma State University, Stephens County, Duncan, OK 73533 5. Extension Area Livestock Specialist, Oklahoma State University, Enid, OK 73701 6. Extension Educator, Oklahoma State University, Comanche County, Lawton, OK 73501 7. Professor, Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078 Abstract Forage sorghums are used by Oklahoma cattle producers for summer grazing or harvested for hay. While they can be very productive and high quality, they can also accumulate toxic levels of nitrate when stressed. Based on the assumption that the plant continues soil nitrate uptake during nighttime hours, followed by accelerated conversion of the nitrate to protein during daylight hours, Extension recommendations have been to wait until afternoon to cut forage sorghum for hay if anticipated nitrate levels are marginally high. To evaluate the significance of the change in nitrate concentration in forage sorghums during the day, samples were collected at two hour intervals from 8:00 am to 6:00 pm. A cooperator’s field was divided into quadrants. Two were sampled on day one and the remaining two quadrants sampled on day two. Three random samples, consisting of ten stems each, were taken from each quadrant at the specified interval, resulting in 18 samples per quadrant. The samples were analyzed at the OSU Soil, Water and Forage Analytical Laboratory (SWFAL) to determine the level of nitrates, (ppm NO3). Results were analyzed using SAS analysis of variance, with time of day, day, and interactions, as the potential sources of the variation in nitrates. There was no significant variation due to time of day; however there was a difference between days. The study will be repeated in the summer of 2008 to gain additional data.

Two questions need to be explored in order to provide the best possible management recommendations to producers:

Will field research data support current recommendation?

If there is a reduction in nitrate levels during the day, is it significant enough to warrant waiting until later in the day to harvest forage?

Nitrate Toxicity in Ruminants Nitrate(NO 3 ) is converted to Nitrite (NO 2 ) in the rumen. Nitrite is absorbed through the rumen wall into blood stream. Nitrite converts Hemoglobin to Methemoglobin = reduced Oxygen transport. Oxygen supply to tissue decreases and asphyxiation occurs.

Field trial procedure

Divide cooperator’s field into 4 sub-fields

Sample each every other hour, (8 A.M., 10 A.M., 12 P.M., 2 P.M., 4 P.M., 6 P.M.)

Three random samples, each consisting of 10 stems, cut at 4” height

Samples cut into 6” pieces and mixed, grab samples taken for lab analysis

Fields 1 & 2 sampled on 9/17/07

Fields 3 & 4 sampled on 9/18/07

Location: Grady Co. (near Amber)

Samples submitted to OSU SWFAL for Nitrate analysis

Samples from fields 1 & 2 were also evaluated with Cardy Nitrate Meter

Results In this trial, there was no significant relationship between time of day and nitrate levels in forage, there was however a difference in days. The study will be repeated in the summer of 2008 to gain additional data.