Grazing management of oversown tussock grassland in the South Island high country

Abstract

Continuous stocking, alternating (two paddock) and rotational (six paddock) grazing practices were compared at low (1.9), medium (3.0), and high (4.1 SU/ha/year) stocking rates for an over sown tussock grassland at Tara Hills High Country Research Station, North Otago. The experiment was on steep land YGE soil on a shady aspect ranging from 760 to 930m in altitude and receiving a 530mm annual rainfall. Effects on pasture, distribution of soil nutrients and productivity of Merino wether hoggets were examined from 1978 for six consecutive pasture growing seasons (November to April).

Herbage mass (kg DM/ha) and net herbage accumulation (kg DM/ha) were assessed independently for inter-tussock and tussock by a comparative yield method using relocatable assessment points along permanent transects. The calibration adopted was justified and shown not to bias dry matter estimates.

After six year's, percentage cover of standing vegetation, assessed by point analysis, was similar for all treatments (79%). White clover increased in cover in wet years and decreased in dry years, but was not affected by grazing management. Cocksfoot decreased at higher stocking rates, especially under continuous stocking. Ryegrass increased at the high stocking rate, but an overall 60% decline in cover was attributed to abnormally cold winter temperatures in 1982.

By the third year, net inter-tussock herbage accumulation was greatest for alternating and rotational grazing at the high stocking rate, but this effect did not recur following the 1982 winter. As each grazing season progressed alternating and rotational grazing maintained greater herbage mass levels than continuous stocking.

At medium and high stocking rates a spell from grazing proved essential for survival of cocksfoot. While an increase in stocking rate reduced the crown area of cocksfoot plants, it improved regrowth per unit crown area. Precise management is required to maintain as well as utilise this grass.

Increased pasture utilisation improved herbage digestibility, mainly by maintaining pasture in the vegetative form. With the exception of sodium, herbage macro- and micro-nutrient levels were sufficient for animal requirements. Blue tussock demonstrated resilience to grazing, and the digestibility of the regrowth was superior to that ungrazed.

By the sixth year, alternating grazing at the medium stocking rate gave maximum liveweight gain/ha. An interaction was demonstrated, with advantage to alternating and rotational grazing over continuous stocking strengthening with increasing stocking rate. Live weight gain for alternating and rotational grazing was similar, and it is suggested the built-in buffering ability of blue tussock was responsible.

Wool growth/ha increased linearly with increased stocking rate, with an advantage to alternating and rotational grazing evident at high stocking rates. Fibre diameter was 1.5 micron less at the high than at the low stocking rate.

Mean inter-tussock herbage utilisation per grazing for low, medium, and high stocking rates was 31.7, 74.1 and 83.6% respectively. Grazing practice did not greatly affect overall utilisation. Live weight gain (g/animal/day) was shown to be a function of herbage allowance (kg DM/anima1/day) and different relationships for continuous stocking and rotational grazing were identified. These were used to explain the stocking rate/grazing practice interaction evident in animal performance, and reasons are proposed why many grazing experiments have failed to show advantages to rotational grazing over continuous stocking.

After six years soil sulphur was greater under high than low stocking. Soil su1phur potassium and phosphorus were generally greatest for areas where residual herbage mass was least. Transfer to night stocking camps was evident, especially for urine-carried nutrients. Increased stocking rate and intensified grazing practice both reduced the C/N ratio, suggesting a more efficient use of nitrogen by pasture plants.

The application of the principal findings to the grazing management of improved tussock grasslands is briefly discussed in re1ation to conditions of climate, livestock, suitable pasture cu1tivars, tertain and runholder motivation.