Maintaining grain yield of Th. intermedium across stand age through constant spike fertility and spike density: Understanding its response to various agronomic managements

Thinopyrum intermedium subsp. intermedium (Host) Barkworth & D.R. Dewey is a perennial grass proposed as a dual-use crop for both forage and grain. Being in the nascent stage of domestication, its grain yield potential is still low compared to annual counterparts. The understanding of Th. intermedium development and the resulting grain yield in field is limited along with its response to agronomic management. To identify the interrelations between development traits and their influence on grain yield, various crop measurements were evaluated during four growing seasons in field grown Th. intermedium conducted under various autumn defoliation operations and nitrogen (N) fertilizations. Under sufficient N treatments (i.e., 100 kg N ha−1), grain yield remained constant over the four years of the experiment with a mean of 1 t ha−1 resulting from a constant spike fertility and a spike density level above 400 spikes m−2. However, significant mortality and inhibition of reproductive growth of tillers can occur when number of tillers is too high and/or resources too scarce (e.g., unfertilized plots or water deficiencies). In addition, excessive aboveground production at the beginning of the reproductive phase can be detrimental to the final grain yield through the negative influence of tiller density and aboveground biomass (DM) on the yield per spike. The highest aboveground production was observed during the second year with 1415 tillers m−2 at the beginning of the reproductive phase and 16 t of DM ha−1 at grain harvest. Although grain yield response to N fertilization was positively associated to spike density, excessive aboveground biomass could be enhanced by N fertilization. The fertilization of 50 kg ha−1 in autumn combined with a fertilization of 50 kg ha−1 in early-spring could sustain tiller fertility without hampering grain production. In autumn, plant regrowth was low with the highest value of 1.3 t of DM ha−1 observed in the first year. Autumn defoliation could be used to maintain the yield per spike in case of excessive biomass production by the reduction of the DM and the final tiller density coupled with the increase of the grain weight the next year. Finally, shredding post-harvest crop residues at ground level to promote light penetration at the bases of plants may have a positive influence on the tiller fertility. Therefore, under fertile soil conditions (i.e., deep soils and sufficient N fertilization) combined with an optimal stand establishment (i.e., from 500 to 1000 tillers m−2 at the beginning of the growing season) we demonstrated that grain yield potential can be maintained as the crop ages. In the future, breeding should raise resource allocation to the grain by increasing yield per spike and avoid overproduction of new tillers through tillering or rhizome propagation without compromising the vigor of regrowth and its environmental performance.