Strategies to optimize nitrogen efficiency when fertilizing with pig slurries in dryland agricultural systems.

In dryland agricultural systems, pig slurry (PS) is usually applied to cereal crops only at sowing, and slurries accumulate for the rest of the year in pits. In this context, a four-year experiment was established in order to evaluate the feasibility of PS applications at the barley or wheat tillering stage. The main treatments were PS either applied at sowing (25 Mg ha −1 ) or not, but they alternated after a two-year period. Both were annually combined with eight side-dressing treatments at cereal tillering: mineral N as NH 4 NO 3 (M; 60 or 120 kg N ha −1 yr −1 ), PS from fattening pigs (PSf; 17, 30, 54 Mg ha −1 yr −1 ), PS from sows (PSs; 25, 45, 81 Mg ha −1 yr −1 ) and a treatment without N. The combined fertilization treatments were 18 plus a control (no N applied). In the context of crop rotation, the biennial alternation of PS applied at sowing allowed the control of soil nitrate increments, while PS side-dressing improved N recovery compared with a unique application at sowing. The highest yields (>3.6 Mg ha −1 yr −1 ) were obtained with an annual average (4-yr) N rate close to 173 kg N ha −1 (±40 kg N ha −1 ). The best overall strategies corresponded to PSs side-dressings of 50–90 kg N ha −1 . These PSs rates also recorded the highest values on the five calculated N-efficiency indexes, which were higher than or similar to results from M side-dressings or those recorded in the literature. These similarities (M vs. PSs) were also shown by the reduction of unaccounted-for N inside the overall N balance. Thus, split PS application during the crop cycle is a sound fertilization option in dryland systems. [ABSTRACT FROM AUTHOR]