Background <p><i>Leptochloa crinita</i> is a native forage grass from arid and semi-arid regions of the Americas, typically used for range grazing. Forage biomass productivity (FBP) and drought tolerance are important traits exhibiting broad intraspecific variation. To investigate possible morphophysiological mechanisms underlying FBP, we monitored biomass production, both total and partitioned into different above-ground and underground plant parts, total (LA) and specific leaf area (SLA), stomatal conductance (g_s), and SPAD index in a genetically-diverse <i>L. crinita</i> germplasm collection of 21 accessions during two years of field experiments under unrestricted water availability.</p> Results <p>Broad and significant variation (<i>p</i> &lt; 0.0001) was consistently found among the accessions for total dry matter (TDM), roots dry matter (RDM), dry matter of leaves (LDM), stems (SDM), and panicles (PDM), dry matter of all the aerial plant parts combined (ADM). LA, SLA, g_s, and CI. ADM, which directly estimates FBP, varied 3.3–4.2 folds. Nine of the 11 morphophysiological variables analyzed were significantly and positively correlated with FBP, in both years, being the strongest associations with SDM (<i>r</i> = 0.98), LDM (<i>r</i> = 0.73–0.89), LA (<i>r</i> = 0.77–0.88), and g_s (<i>r</i> = 0.91). Analysis of individual and correlated variables indicated that greater FBP was mainly driven by a stronger photoassimilates partitioning to above-ground organs, a faster and overall greater LA development, and a prolonged functionally-active photosynthetic apparatus, as suggested by the ability of high-yielding accessions to maintain high g_s and SPAD index levels for longer periods. In contrast, low-yielding accessions invested, proportionally, more biomass in roots, even under favorable water availability, reflecting a conservative resource-use strategy for these accessions. The fact that ADM and percentual biomass allocation in roots was strongly and negatively correlated (<i>r</i>= -0.75 to -0.86, <i>p</i> &lt; 0.001) suggests a general trade-off between FBP and drought tolerance in this species.</p> Conclusions <p>This work revealed broad genetic diversity for FBP in <i>L. crinita</i> under non-limiting water availability conditions, and identified high-yielding accessions, of relevance for forage production in irrigated arid regions. Mechanistically, high FBP was associated with increased photoassimilates partitioning to above-ground organs, a greater and faster LA development, and an active photosynthetic apparatus for longer periods. The FBP-correlated traits identified herein will be of selection value for <i>L. crinita</i> breeding programs.</p>