How does defoliation management impact on yield, canopy forming processes and light interception of lucerne (Medicago sativa L.) crops

The frequency of defoliation is the major management tool that modulates shoot yield and the accumulation of C and N root reserves in lucerne crops. A fully irrigated, 2-year-old lucerne (Medicago sativa L.) crop was grown at Lincoln University (43°38'S and 172°28'E) and subjected to four defoliation treatments. These involved the combination of two grazing frequencies (28 or 42 days) applied before and/or after mid-summer. Annual shoot dry matter (DM) yield ranged from 12 to 23 t/ha. These differences were largely explained by the amount of intercepted photosynthetically active radiation (PAR(i)) using a conservative conversion efficiency of 1.6 g DM/MJ PAR(i). Part of the reduced PAR(i) in the frequently defoliated treatments was caused by the shorter regrowth period that impeded crop canopy closure to the critical leaf area index (LAI(crit)) of 3.6. Canopy architecture was unaffected by treatments and a single extinction coefficient for diffuse PAR(i) (k(d)) of 0.81 was found for 'Grasslands Kaituna' lucerne. The pool of endogenous nitrogen (N) in taproots was reduced by frequent defoliations. This explained differences in leaf area expansion rate (LAER), which decreased from 0.016 m²/(m² °C day) at 60 kg N/ha to 0.011 m²/(m² °C day) at 20 kg N/ha. The pool of soluble sugars was also positively associated with LAER but the concentrations of carbohydrates and N reserves and the pool of taproot starch were poorly related to LAER. The slower LAER in the frequently defoliated treatments was mostly caused by the smaller area of primary and axillary leaves, particularly above the 6th node position on the main-stem. Developmental processes were less affected by defoliation frequency. For example, the phyllochron was similar in all treatments at 34 °C day (base temperature of 5 °C) per primary leaf during spring/summer but increased in autumn and ranged between 44 and 60 °C day. Branching and senescence started after the appearance of the 4th main-stem node, and both were unaffected by defoliation frequency. These results suggest that the expansion of individual leaves, both primary and axillary, was the most plastic component of canopy formation, particularly after the appearance of the 6th primary leaf. Future mechanistic modelling of lucerne crops may incorporate the management or environmental responses of LAER that control PAR(i) and impact on shoot DM yields.