From forest to pasture without weeds: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Dry matter production, botanical composition and seedling size of a ryegrass/ subterranean clover pasture sown on 1 April out of Pinus radiata were measured over three harvest dates (28 July, 17 August, 7 September) in Darfield (43° B'S, 172° 8'E). Pasture was grown at 4 lime rates (0, 2.5, 5, 10 T lime/ha) and 2 nitrogen rates (0 and 400 kg/ha). Lime application ameliorated initially high aluminium levels from 1.7 to 0.4 me/100g at 2.5 T lime/ha. Neither pasture production nor botanical composition responded to lime applications. The application of 400 kg nitrogen/ha increased total dry matter production and growth rates per unit of thermal time. From 17 August to 7 September, nitrogen increased production from 6.5 kg DM/°Cd to 17.5 kg/°Cd. Gorse seedling establishment was 43 seedlings/m² where no nitrogen was used but reduced to 13/ m2 by September with the application of 400 kgN/ha. Height, extension rate and phyllochron of broom and gorse seedlings grown in five rates of lime (0, 2.5, 5, 10, 20 T/ha) were measured in a pot experiment at the Lincoln University glasshouse. Average height, extension rate and phyllochron of both species were unaffected by lime rate. The rate of leaf development increased at 1650°Cd for both species, suggesting a larger seedling phyllochron to this point, followed by an adult phyllochron. Secondary leaf production was initiated in broom seedlings at 1895°Cd when grown without lime. Plants grown with lime lacked this axillary development. A further pot experiment investigated the effects of molybdenum and boron on the growth characteristics of seedling gorse and broom. There was no difference in the growth of gorse seedlings grown in 2.5 T lime/ha, plus or minus 140g/ha of molybdenum or plus or minus 0.5 T/ha boron. The growth of broom seedlings was not affected by molybdenum but the final height was 33 mm taller when grown with boron. The dry matter production of the tallest broom seedlings was 2.91 g/plant when grown with boron, compared to 1.11 g/plant when grown without boron. Dry matter production of broom seedlings grown at five rates of lime (0, 2.5, 5, 10, 20 T lime/ha) plus or minus 30 kg pasture seed/ha, were measured in an additional pot experiment. Pasture competition suppressed the growth of broom seedlings. Shoot dry matter of the tallest broom seedlings was 1 g/plant (50%) lower with competition. The current experiments showed that weed control is vital in the conversion of forestry. The use of fertiliser and lime to increase pasture competition wilI suppress the establishment of major woody weeds. Areas for further study are suggested.