Phosphorus Requirements of Alfalfa‐Smooth Bromegrass‐Orchardgrass and Reed Canarygrass Pastures under Two Grazing Pressures1

Abstract

AbstractMost recommendations for P fertilization of pastures are based on data from small plots from which the herbage is clipped and removed. This may overestimate the sward growth responses to fertilizer P because of the absence of trampling and nutrient cycling by grazing animals. In a beef‐steer grazing study conducted on Aquic Argiudoll‐Typic Argiaquoll soils in Iowa we determined the effect of fertilizer P applied to two pasture types under two grazing pressures. Pasture types were: (1) ‘Pioneer 520’ alfalfa (Medicago sativa L.)‐‘Baylor’ smooth bromegrass (Bromus inermis Leyss.)‐‘Napier’ orchardgrass (Dactylis glomerata L.), and (2) ‘Rise’ reed canarygrass (Phalaris arundinacea L.). The alfalfa‐grass pastures received no N fertilizer, whereas reed canarygrass pastures received 180 kg ha−1 year−1 of N. The 1.32‐ha pasture units were divided into three 0.44‐ha rotational grazing strips, upon which either 0, 20, or 40 kg ha−1 of P was topdressed in the spring of 1979. In 1980, three small plots were designated in each of the 0.44‐ha rotation strips to which the same three P treatments were randomly assigned. The grazing pressures were at 8 and 11 kg of available dry matter/steer‐day. No significant effects resulting from grazing pressure were detected for dry matter yield, plant P, or soil‐test P values. Herbage dry matter yield in 1979 was not influenced by P rate; however, in 1980, both pasture types exhibited a positive linear response to P where no P had been applied in 1979. In rotation strips that had received no P fertilizer in 1979, alfalfa‐grass dry matter yields in 1980 exhibited a quadratic relationship to soil‐test P levels, i.e., yield optimum at 25 ppm of soil‐test P (Bray 1). Reed canarygrass dry matter yields in 1980 exhibited a linear relationship to soil‐test P levels with an apparent leveling off of yield above soil‐test P of 20 ppm. We concluded that yield response to P topdressing is improbable at soil‐test P levels at or above 22 to 25 ppm (Bray 1) for alfalfa‐grass and 15 to 20 ppm for reed canarygrass pastures.