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[ratio ]P2O5) applied annually at rates of 20[ratio ]60 at the drier and 40[ratio ]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[ratio ]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[ratio ]45 or < 60[ratio ]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.