Spring Nitrogen Fertilization of Pastures

Author: Sam Angima

Publish Date: Winter 2008

Nitrogen is an essential element necessary for plants to grow especially for forages. Nitrogen is important in so many physiological pathways such as chlorophyll, protein synthesis, photosynthesis, utilization of sunlight, nutrient uptake, vitamin production, amino acid synthesis, and together with phosphorus in energy systems. If forages do not have enough nitrogen, they may appear stunted with very slow growth especially in the cool spring time and most important; they develop less tillers therefore you end up with less hay or pasture. So it is very important to apply nitrogen to forage like perennial rye grass, fescue, and orchard grass during winter or early spring. Remember that small amounts of nitrogen will increase early growth but when nitrogen is depleted, the resultant hay or pasture will be low in proteins (measured as total digestible nutrients – TDN). But how and when should you apply nitrogen?

I highly recommend a soil test right now since there are many different soils in Oregon. Any-one figure for one soil series will be misleading for other areas. Timing wise, we would use T-Sum 200 guidelines that use cumulative temperature measurements necessary for grass growth to begin on the basis of plant physiology. The T stands for temperature and 200 stands for accumulated heat units for consecutive days beginning January 1 of each calendar year until a total of 200 is reached. Now, a heat unit (or growing degree day), is the average of the high and low temperature for the day in degrees Celsius (oC). The formula for calculating heat units is: (maximum oC + minimum oC) ÷ 2. For example if the high for January 1st 2008 was 12 oC and low was 4 oC, the number of heat units then for that day would be (12+4)÷2 = 8. If temperature is less than 0 oC then use 0 in this formula. Most of us take our temperature in degrees Fahrenheit (oF). In this case convert oF to oC by using this formula: [(oF-32) x 0.556 = oC]. For example if your temperature is 40 oF, then corresponding temperature in Celsius is (40-32) x 0.556 = 4.45 oC.

If you take daily heat units this way for your site and add them up from January 1, 2008, then when you accumulate 200 heat units it is time to start applying nitrogen. . If you do not take your own temperatures, the OSU calculator gives you current cumulative temperatures around the state for your area. For details on how to individually calculate T-Sum 200, get the OSU guide EM 8852. Scroll to page 4-5 and follow the procedure outlined. This publication goes into details on how to manage your pasture to maximize on your inputs and changes in weather as the year progresses.

It is important to also think of the environment when you are ready to apply nitrogen. If the forecast calls for rain, it will be better to wait until the storms have passed before applying nitrogen to reduce chances of leaching and runoff. Nitrogen is very mobile in the soil system and will tend to leach before plants can use it. Also if it is dry, you can loose nitrogen to the atmosphere, so waiting a few more days to get some wetness will help the fertilizer to dissolve and become available to plant roots.

After applying nitrogen at T-Sum 200, it takes anywhere between 1-2 weeks for plants to start responding giving a flush of growth. At this point, nutrients stored in the root system are made available for tiller development and growth. I would recommend applying only a quarter to half of the recommended nitrogen at this time because of the possibility of leaching as plant growth is still restricted. Depending on your location, you may then apply the second half of nitrogen 60-75 days later to reinvigorate and flush up all tillers that have so far developed depending on your stocking rate and whether the pasture is irrigated. This also coincides with warmer weather when root growth is at its maximum. The other benefit of applying nitrogen at T-Sum 200 is that you can put your herd to graze the pasture up to three weeks earlier than traditional methods.

If you were not able to have a soil test done, then typically, 60 lb N/acre per application is recommended for rotational grazing systems. You can also determine how much nitrogen to apply by clipping to measure forage growth and by testing for crude protein. It makes economic sense to apply only the amount of nitrogen removed as protein in the forage.