Long-term yield patterns in barley-based cropping systems in Northern Syria. 3. Barley monocropping

Results from monocropped barley treatments in long-term rotation (RTN) and continuous barley (CB) trials at two sites were examined for fertilizer effects on yield means and long-term yield trends. In RTN trials, mean responses to fertilizer (N:P2O5) applied annually at rates of 20:60 at the drier and 40:60 at the wetter site were almost double those from biennial application, confirming the need for annual fertilization in monocropped barley systems. In CB trials, with N and P applied annually in nine factorial combinations, at rates up to 120:90, responses to each nutrient were curvilinear and dependent on the presence of the other nutrient. Trend analysis showed a decline in grain yields over time where NP fertilizer had not been applied or applied only at low annual rates (< 60:45 or < 60:90, according to site); but straw yields, at worst, remained approximately stable in the absence of fertilizer and generally increased strongly with higher NP rates. Uncertainties in the interpretation of trend-analysis results indicate the need for methodology improvements, to include (i) additional single-value parameters of the growth environment, to improve the model's ability to account for seasonal variability, and (ii) a more flexible, non-linear function for time. It was concluded that barley monocropping is not necessarily non-sustainable in the medium term, provided adequate annual fertilization is maintained; but risks of pest or disease build-up, in addition to the superiority of legume-barley systems in biomass and crude protein output demonstrated in preceding papers, undoubtedly favour the introduction of some forage legumes into long-term barley sequences. The most practicable (and acceptable) systems may well be ones in which continuous barley is interrupted by a legume, or even a bare fallow, every third or fourth year. Above all, farmers should be helped to experiment with different forage legumes, barley-legume sequences, and modes of legume harvest, to optimize outputs in relation to their evolving and individual enterprise needs.