The influence of soil wetness on utilized output from grassland on commercial farms

Records of grassland productivity have been used for 93 farms in 1975–76 (a dry year) and 117 farms in 1977–78 (a wetter year). The samples were selected to contrast well-drained and poorly/ badly drained farms and were further subdivided into dairy and beef. The number of farms in each of the four subsamples for each year was between 19 and 32. In addition the number of days on which the soil was at meteorological field capacity (field capacity days) on each farm was calculated.
The mean utilized metabolizable energy (UME) output within the dairy sample was 45 GJ ha⁻¹ on both well-drained and poorly/badly drained farms. Within the beef sample the output was 41 GJ ha⁻¹ on well-drained farms and 37 GJ ha⁻¹ on the poorly/badly drained farms.
The correlation between fertilizer N and UME output was stronger on well-drained farms than on the poorly/badly drained farms in the wetter year (r = 0.69 v. 0.16 on dairy farms; r = 0.56 v. − 0.12 on beef farms). In the drier year the converse was found (r=0.15 v . 0.49 on dairy farms; 0.13 v. 0.44 on beef farms). The effect of field capacity days on output was inconsistent; only within the dairy sample in the wetter year did increased wetness appear to reduce output.
It is suggested that soil wetness may have only a small effect on utilized output from grassland on a whole-farm basis because (i) most farms have at least some well-drained land, (ii) much of the utilization damage occurs in relatively short periods in spring and autumn and (iii) despite having utilization problems, badly drained land is capable of growing large quantities of grass.