The effect of stocking rates on white clover morphology and yield

An assessment of the effect of stocking rate on white clover morphology and yield was made in a field experiment conducted at Lincoln College, Canterbury, from October 1983 to August 1984. Irrigated white clover/ryegrass pasture was rotationally grazed at low, 22.5, and high, 27.5, ewes ha⁻¹. Fifteen 0.1 m² quadrats were selected from each paddock to cover the range of clover content. Measurements were taken immediately, and three days after grazing; and after 32 days of regrowth during spring, summer, autumn and after 66 days in winter. Total residual herbage mass, botanical composition and the components of yield of white clover and ryegrass were recorded for each quadrat. The number of clover growing points, leaves, stolons and ryegrass tillers and clover leaf size, lamina and petiole dry weight, stolon length and tiller dry weight were measured from a bulked sample of four cores of 72 cm² from the outside of each quadrat. Multiple regression analysis was used to select clover and ryegrass variables from the residual herbage which best allowed prediction of subsequent clover yield. Stocking rates produced swards with different residual herbage mass, with a mean over-all season of 890 and 460 kg ha⁻¹ ha in the low and high stocking rates respectively. There were also marked changes in the botanical composition and morphology of the swards as a result of different residual herbage mass. The low stocking rate was ryegrass-dominant (60%) and with 20% clover. The high stocked treatment had 54% grass and more clover (46%). After regrowth, the high stocking treatment had much less dead material (5%) than the low (13%). In the residual herbage mass of the low stocked paddock clover had large leaves (0.50 - 1.30 cm²) and few growing points (1805 m⁻²) compared with the high which had twice as many leaves of small size (0.26 - 0.46 cm²) and many growing points (3905 m⁻²). There was also a seasonal trend in clover yield components. Leaf number reached a peak in December with 4230 and 7590 leaves m⁻² and growing points peaked in February with 2710 and 6390 m⁻². Furthermore, above-ground stolon dry weight peaked in late spring/early summer with 50 and 55 gm⁻², and below-ground in autumn and winter with 30 and 37 gm⁻² in the low and high stocking rate respectively. Net herbage production was 8110 and 6300 kg ha⁻¹ in the low and high stocking rate respectively. Although white clover content expressed as a percentage was higher in the high stocking rate, clover yield was similar in both stocking rates. Clover yield was increased in summer and autumn by 13 and 35% respectively in the high stocking rate, compared with the low. White clover yield was increased when there were many leaves or many growing points left after grazing. In summer, stolon length was the most important predictor of clover yield in the low stocking rate (r² = 0.71) and leaf number (r² = 0.78) in the high stocking rate. Petiole dry weight was the best predictor of yield in winter in the low stocking rate (r² = 0.80). Leaf number, growing points and leaf size were the most important variables in the residual herbage mass for prediction of subsequent clover yield. These variables are suggested as indices of the severity of grazing.