Crop phenology drives wheat forage yield and quality under Mediterranean conditions regardless of stem structure

Wheat ( Triticum aestivum .L) is a widely cultivated dual-purpose crop for forage and grain in Mediterranean agro-ecosystems. Stem structural traits contribute to grain yield stability under terminal heat and drought stress. However, there is limited information regarding the association and potential trade-offs of stem traits with forage productivity and quality. We hypothesized that stem structural traits (diameter and solidness) might influence forage productivity and quality, potentially leading to trade-offs between structural biomass and forage quality (fiber digestibility). Therefore, this study focuses on evaluating a set of twelve advanced wheat lines and two commercial checks for stem diameter, stem solidness, and forage productivity and quality at the milk-dough stage over two seasons under rain-fed conditions. We observed considerable genotypic variation for days to heading (74-132 days), plant height (90 -135 cm), dry matter (DM) yield (992 -1536 g m-2) and forage quality traits [acid detergent fiber (ADF, 35-44%), neutral detergent fiber (NDF, 51-66%), acid detergent lignin (ADL, 4.3-6.1%), DM digestibility (DMd 48h, 66-80%), and neutral detergent fiber digestibility (NDFd 48h, 50-64%)]. With the exception of plant height, stem structural traits showed no significant association with forage yield or digestibility. In addition, no trade-offs were detected between stem solidness and diameter and DM yield or digestibility (NDFd). In contrast, crop phenology was the primary driver of forage productivity and quality. Acid detergent fiber, reduced digestibility, confirming its role as key element dictating forage quality. We identified advanced spring wheat lines (YF5H, YF9S, RF27H, and RF28S) that combine higher DM yield, NDFd, and DMd with reduced ADL content. These findings highlight late-heading lines as a promising breeding strategy for enhancing forage productivity without compromising digestibility, while further emphasizing that lignification rather than stem structure per se is the primary constraint limiting forage quality.