Get reliable science

We evaluated the effects of nitrogen (N) fertilization on species composition of grazed annual rangeland in Northern California. Treatments included rates of 0, 30 (low) and 60 (high) pounds of actual N per acre, in the form of ammonium sulfate, combined with seasonal timings of fall and early spring applications in year 1 and 0, 30 and 60 pounds N per acre in year 2. Treatments were applied during the 2012/2013 and 2013/2014 growing seasons at three different locations and then monitored the spring of 2015. Each experimental site was grazed, though small differences in grazing strategy existed. Analyzed with all three sites combined, the two consecutive years of early-spring applications at 60 pounds N, combined with grazing, reduced medusahead (Taeniatherum caput-medusae) basal cover from 23 to 6% (P=0.01). No significant difference in medusahead existed between any other fertilizer treatments, although the year 1 + year 2 N applications resulted in lower medusahead cover of 10% compared to the application in year 1 only of 16% (P=0.006). The average of all low and all high N applications lowered medusahead cover from 14% and 13%, respectively, from 23% with zero N applied (P=0.02), indicating that on average the lower rate can provide some benefit in reducing a medusahead population. Interestingly, fertilizer treatments did not have significant effects on basal cover of any other grass and forb species, only on medusahead. A strategy of enticing cattle to graze medusahead patches through N fertilization may help reduce small medusahead infestations, and further studies would be needed to determine limits to the proportional area of a field that should fertilized to sustain a negative impact on medusahead.