Nitrogen Grows Grass: A look at how much N grasses really need

Like Ontario, Finland's nitrogen fertility recommendations for grasses are based off research from the 1960s and '70s. Since grass varieties and production practices have changed over time, Termonen and colleagues decided to revisit the nitrogen responses of timothy and meadow fescue to determine if the existing recommendations are still appropriate.

Plots were established at two locations in 2014. Between 2015 and 2017, three cuts per year were harvested from each plot. The total annual nitrogen rates were 0, 150, 200, 250, 300, 350, 400, and 450 kg N/ha. These rates were split-applied so that 44%, 36%, and 20% of the total N was applied to first, second, and third cut, respectively. P and K were applied so as not to be limiting; micronutrient deficiencies were addressed in the establishment year. The cut height was 7 cm.

Unsurprisingly, higher nitrogen rates resulted in higher grass yields. Even at 450 kg N/ha, yields were still increasing with added nitrogen. The maximum N rate recommended in Finland is currently 250 kg N/ha annually, but the work by Termonen and colleagues suggests agronomic benefits of N application up to about 390 kg/ha annually. When the researchers compared their yield results to those from older work, they found that grass yields at 250 kg N/ha in the mid-20^th century were 70% of yields at the same N rate in the recent trial (8 000 kg DM/ha vs 11 500 kg DM/ha).

They attribute higher yields and N demand in contemporary grasses to three things: improved crop genetics, climate change, and more intensive crop management. Breeding efforts have improved grass yield potential in northern Europe by about 0.4%-0.5% per year, which would account for about half of the difference in yields observed by this trial compared to Finnish work in the 1960s and '70s. Climate change has caused milder winters and a longer, warmer growing season that enables more grass production. Since the original nitrogen fertility work was conducted, Finnish farmers have transitioned from a 2-cut system to a 3-cut system. These things together may explain the recent higher grass yields, N demand, and N use efficiency.

Crude protein content increased with higher N application and peaked at 18% on a DM basis. The authors did not specify at what stage of maturity the grasses were harvested. When compared to historic data, it seems that older varieties were directing more nitrogen to protein formation, while the recent study found more N going to biomass production (yield). N rate had the greatest effect on third cut crude protein content, and the smallest effect on the protein levels of first cut.

Higher N rates increased the proportion of stems in second and third cut timothy, which lowered the digestibility. Meadow fescue did not change its leaf:stem ratio over subsequent cuts as N rates increased. This may be due to the different growth habits of these species; timothy is a jointing grass while meadow fescue is non-jointing. The decline in digestibility as N rates increased was more detrimental to overall quality than the increase in crude protein. As always, this shows there is a trade-off between yield and quality in forage production.

Nitrate-nitrogen (NO₃-N) levels in the forage were below 2 000 ppm (DM basis) at nitrogen rates less than 350 kg/ha per year. NO₃-N concentrations above 2 200 ppm can cause acute nitrate poisoning in cattle. Nitrogen applications of 250 kg N/ha annually kept NO₃-N levels below 1 000 ppm, which is generally considered safe for non-pregnant livestock.

Termonen and colleagues' work shows that grass responses to nitrogen have changed over the last 60 years. A combination of improved grass genetics, enhanced production methods, and changing growing conditions contribute to higher N demands and N-use efficiencies. The way grasses use nutrients and how we define overall quality has also changed somewhat.

This study also highlights the trade-off producers face when it comes to providing nitrogen for forage grasses. While the cost of establishing a mixed stand is much less than that of applying nitrogen, even a good stand of white clover - the most generous legume with its nitrogen - can only provide up to 200 kg N/ha. High grass yields require more nitrogen than legumes can supply. Choosing between high yields and low input costs is ultimately an individual decision based on land values, pasture terrain, forage requirements, and the operator's preferred production methods.

Source: Termonen, M., P. Korhonen, S. Kykkanen, A. Karkonen, M. Toivakka, R. Kauppila, and P. Virkajarvi. 2020. Effects of nitrogen application rate on productivity, nutritive value, and winter tolerance of timothy and meadow fescue cultivars. Grass and Forage Science. 75(1):111-126.