Pasture growth rates in winter, in many regions are low, and this often sets the limit on the stocking rates possible on those pastures. This research was done to examine some of the reasons for the differences in productivity between grasses In winter. Eight perennial and two annual grasses were compared over three years in a field experiment, where mean daily air temperatures in winter averaged 5°C. Species separated into five groups. The largest yields, over winter from plots cut either in autumn or early winter, were obtained from Kangaroo Valley perennial ryegrass (Loliurn perenne) and Wimmera annual ryegrass (L. rigidurn). Their yields were significantly greater than from oats (Avena byzantina) and prairie grass (Bromus catharticus), which in turn were greater than mountain rye (Secale montanum) and Sirosa phalaris (Phularis aquatica); the next group were CPI 69358 perennial ryegrass, currie cocksfoot (Dactylis glomerata) and Australian Commercial phalaris, and least was CPI 68318 cocksfoot. Greater forage yield, within years, was obtained from grasses that switched into reproductive development early in winter rather than from a high leaf area index in swards prior to winter. Differences in yield between years were, though, related to differences in leaf area. The average sward net assimilation rates over winter were greater for ryegrass cultivars, and oats, than for the other species. Plots uncut from autumn had greater yields throughout winter but lower net assimilation rates. It was concluded that improved pasture yields in winter can be obtained by selecting or recommending grasses that commence reproductive development early. This effect would apply in most environments where temperate grasses and cereals are grown.