Dairy Manure Type, Application Rate, and Frequency Impact Plants and Soils

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The impacts of dairy rations on manure chemistry, manure N mineralization in soils, and crop N uptake have been evaluated after single manure applications. No information is available on these impacts when manure is applied to soils at different rates and frequencies. Manure from lactating dairy cows (Bos taurus) fed three diets differing in crude protein (CP) content were applied to a Plano (fine‐silty, mixed, superactive, mesic Typic Argiudolls) and a Rosholt (coarse‐loamy, mixed, superactive, frigid Haplic Glossudalfs) soil at two application rates (225 or 450 kg total N ha−1) and three application frequencies (once, twice, or thrice). Oat (Avena sativa L.), sorghum [Sorghum bicolor (L.) Moench] and sorghum ratoon were grown in succession during a 170‐d period. Plant responses to manure from different CP diets were much less than responses due to soil type and manure application rate and frequency. Manure N uptake efficiency by plants was greatest for manure derived from a low‐CP diet than manure derived from either the medium‐ or high‐CP diets. Manure application rate and frequency increased NO3 levels in oat and sorghum shoots, which if used as forage could have detrimental impacts on dairy cow health. No significant differences were measured in second‐crop (sorghum) yield or N uptake due to a previous manure application at either N application rate. Third‐crop (sorghum ratoon) yield and N uptake were significantly increased, however, due to previous manure applications, but only in pots that received two manure applications at the high rate. Longer term, repeated applications of dairy manure derived from different diets at lower rates could provide more pronounced impacts on plants and soils than those observed during this relatively short‐term greenhouse study.