Regrowth of prairie grass (Bromus willdenowii Kunth) and perennial ryegrass (Lolium perenne L.) in response to temperature and defoliation

This glasshouse study was undertaken to determine the effect of temperature and defoliation on the regrowth of prairie grass (Bromus willdenowii Kunth cv. Matua) in comparison with perennial ryegrass (Lolium perenne L. cv. Dobson). Individual plants of prairie grass and perennial ryegrass were grown in 2 mini-swards with one half as prairie grass and the other as ryegrass. From H0 (13 weeks after sowing) to the completion of the experiment at H3, one sward was maintained at a day/night temperature of 18/10˚C and the other at 25/15˚C. From H0 to H1, prairie grass was defoliated 4 times at the time taken to regrow 1 new leaf per tiller (1-leaf stage), 2 times at the 2-leaf stage, or once at the 4-leaf stage to 60, 90, or 120 mm stubble height. Similarly, ryegrass was defoliated 3 times at the 1- leaf stage, once at the 1-leaf stage then once at the 2-leaf stage, or once at the 3-leaf stage to 30, 60, or 90 mm stubble height. Plants were subsequently harvested at H1, H2, and H3, being the commencement, mid-point, and completion of the full regrowth cycle (4- and 3-leaf stage for prairie grass and ryegrass, respectively). Prairie grass was more adversely affected by frequent defoliation than ryegrass. The combination of high temperature and frequent defoliation reduced growth rates by 66 v. 54%, stubble dry matter (DM) by 50 v. 11%, root DM by 62 v. 45%, and stubble water-soluble carbohydrate (WSC) by 52 v. 21% for prairie grass and ryegrass, respectively. In contrast, ryegrass was more affected by defoliation height than prairie grass, particularly at the higher temperature. Close defoliation and high temperature reduced growth rate by 35 v. 25%, root DM by 18 v. 0%, and stubble WSC by 84% v. 36% for ryegrass and prairie grass, respectively. The number of tillers per plant was reduced by close defoliation, more so at the high temperature in ryegrass but not in prairie grass. Defoliating prairie grass to 90 mm stubble height at the 4-leaf per tiller stage compared with the 1-leaf per tiller led to maximum restoration of stubble WSC reserves as well as maximising leaf and root growth. The higher stubble WSC and greater root DM of prairie grass, together with its ability to maintain growth rates and tillering under high temperature, are attributes which explain why prairie grass appears to be more productive and persistent than ryegrass in a subtropical environment.