The influence of regrowth time on the forage quality of prairie grass (Bromus willdenowii Kunth. cv. Matua), perennial ryegrass (Lolium perenne L. cv. Dobson) and tall fescue (Festuca arundinacea Schreb. cv. Dovey) was determined under non-limiting soil nutrient and moisture growth conditions. In a glasshouse, individual plants of each species were arranged in separate mini-swards and were defoliated at 6, 10 and 14 weeks after sowing to a stubble height of 60 mm for perennial ryegrass and tall fescue and 90 mm for prairie grass. Following defoliation at 14 weeks, selected individual plants were cut to the previous stubble height as each new leaf per tiller was fully expanded, to provide leaf material for nutrient analysis, until prairie grass, perennial ryegrass and tall fescue had attained 6–8, 5 and 3 leaves/tiller, respectively.

The concentration of leaf phosphorus (P) decreased from 6.6 to 5.9 g/kg dry matter (DM) in prairie grass, increased from 5.9 to 6.9 g/kg DM in perennial ryegrass, and initially increased to 8.8 g/kg DM and then decreased to 8.4 g/kg DM in tall fescue. The mean potassium (K) content in perennial ryegrass was 29.6 g/kg DM and was not significantly affected by duration of regrowth, whereas K content in prairie grass and tall fescue fell from 51.7 to 43.6 g/kg DM and from 55.5 to 47.9 g/kg DM, respectively, after the first leaf per tiller formed. Calcium levels increased with regrowth in all species and at the completion of regrowth were 5.8, 3.8 and 3.4 g/kg DM in prairie grass, perennial ryegrass and tall fescue, respectively. The magnesium (Mg) and sodium (Na) content of perennial ryegrass showed no change throughout the regrowth period and had measured values of 2.5 and 2.8 g/kg DM, respectively. For tall fescue, the concentration of leaf Mg decreased from 0.30 to 0.24 g/kg DM, whereas the Na concentration increased from 1.2 to 2.1 g/kg DM. The Mg content of prairie grass remained constant at 2.0 g/kg DM, whereas the Na content increased from 2.7 to 4.3 (g/kg DM).

While the crude protein content of all grasses declined over the regrowth period, values remained over 200 g/kg DM, well above the recommended content for lactating cows. The leaf water-soluble carbohydrate (WSC) of prairie grass and perennial ryegrass increased over the regrowth period from 29.7 to 43.9 g/kg DM and from 25.9 to 72.5 g/kg DM, respectively, whereas tall fescue showed no change at 55.6 g/kg DM. The change in in vitro organic matter digestibility (OMD) with age was 125 and 44 (g/kg DM) for tall fescue and perennial ryegrass, respectively. The OMD of prairie grass decreased following the onset of stem elongation at the 5-leaves/tiller stage of regrowth from 824 to 756 g/kg DM.

In this glasshouse study, the pattern of change in K and Ca content was the same as observed in the field but the absolute content, including that of Na, was greatly elevated, particularly in prairie grass. In terms of nutrient content capability, N, P and K were readily taken up by these C₃ grasses, while the uptake of Mg and Na appear to reflect genetic differences between species. The differences in forage quality as determined under optimal growth conditions in this study, as compared with field grown forage, are presumed to indicate possible soil nutrient deficiencies in field situations.