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. Do bson). Individual plants of prairie grass and perennial ryegrass were grown in 2 m ini-swards with one half as prairie grass and the other as ryegrass. From H -0 (13 weeks after sowing) to the completion of the experiment at H-3, one sward was maintained at a day/night temperature of 18/10 degrees C and the other at 25/15 degrees C. From H-0 to H-1, prairie grass was defoliated 4 t imes at the time taken to regrow 1 new leaf per tiller (1-leaf stage), 2 ti mes at the 2-leaf stage, or once at the 4-leaf stage to 60, 90, or 120 mm s tubble height. Similarly, ryegrass was defoliated 3 times at the 1-leaf sta ge, 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 h arvested at H-1, H-2, and H-3, being the commencement, mid-point, and compl etion 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 ryeg rass. 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 p rairie 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 t emperature reduced growth rate by 35 v. 25%, root DM by 18 v. 0%, and stubb le WSC by 84% v. 36% for ryegrass and prairie grass, respectively. The numb er of tillers per plant was reduced by close defoliation, more so at the hi gh 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 st ubble WSC reserves as well as maximising leaf and root growth. The higher s tubble WSC and greater root DM of prairie grass, together with its ability to maintain growth rates and tillering under high temperature, are attribut es which explain why prairie grass appears to be more productive and persis tent than ryegrass in a subtropical environment.