The potential for browse of a weedy shrub legume : a case study of broom (Cytisus scoparius L.)

Abstract

The potential of broom, a noxious shrubby legume, for browse was evaluated in three glasshouse and three field experiments between December 1978 and February 1980, under various soil and climatic conditions.

Broom seed is predominantly hard. However, scarification by soaking for 3.0 to 5.5 hours gave 75 per cent or more germination of normal seedling compared with 50 per cent from hand chipping and only 2.5 per cent from untreated controls. Seedlings from scarified seeds were more vigorous than those from untreated controls.

The effect of sowing depth on the emergence and seedling vigour of broom was measured under glasshouse conditions, using unsterilized sand. Surface sowing of seed gave the lowest estab1ishment of less than 25 per cent, while seed sown deeply at 5 cm, also had a reduced establishment. More than 85 per cent establishment and a significant increase in seedling vigour as measured by height, number of trifoliate leaves and dry weight was obtained when seeds were sown at 1 to 4 cm below the soil surface.

The effect of l00, 50.5, 27.3 and 7.8 per cent daylight on the growth, development, nodulation, chemical composition and palatability of broom to sheep was studied from December 1979 to February 1980. Low light intensity reduced branch number, stem diameter, dry matter accumulation and concentration and nodule number. The water soluble carbohydrate and acid detergent fibre in broom forage also fell at low light levels. However, the proportion of leaf in the forage, the nitrogen and ash concentration and palatability to sheep all increased under low light intensity.

Broom was grown as a pure stand and in combination with ryegrass at six levels of P in pots in a heated glasshouse. Broom alone grew significantly better and had a higher yield, P and N concentration and uptake than broom grown with ryegrass.

Growth and yield of broom were reduced by vigorous pasture species ('Grasslands Nui' perennial ryegrass and Woogenellup subterranean clover) in the field on a Waimakariri very stony sandy loam. The broom seedlings were further suppressed when they were cut 1 to 2 cm above ground level, and the pasture was topdressed with N fertilizer. Up to 60 per cent of broom plants died under intense competition. However, a small but significant response to N was observed from broom grown alone. Further, broom grown alone was tolerant of early summer drought and survival and growth was good.

In the final experiment, a naturalised stand of 3 to 5 year old broom growing on a Barrhill fine sandy loam at Courtenay was cut to 10, 20, 30 and 40 cm above the ground. Regrowth, yield and chemical composition were evaluated at 4, 6 and 8 weekly intervals between December 1978 and May 1980.

From initial cutting to June 1979, regrowth of broom was poor, but growth improved markedly during the 1979/1980 season. It is postulated that vigorous regrowth after pruning only occurs after a rest period.

The annual yield of broom forage was substantially different among cutting heights and intervals. Yields of 299, 351 and 382 g m⁻² yr⁻¹ for 4, 6 and 8 weekly cuts were obtained. At cutting heights 10, 20, 30 and 40 cm the yield were 250, 338, 368 and 420 g m⁻²yr⁻¹. Longer cutting intervals and increased cutting heights adversely reduced the leaf proportion in the forage.

The N concentration in broom forage ranged from 3.5 to 4.4 per cent, and was influenced by time of year and cutting treatments. Fibre concentration increased with longer cutting interval, but not with height of cut.

Cutting stimulated vegetative growth and reduced flower formation and apparently prevented pod set.

The N concentration of the grass with the broom at 2.4-2.8 per cent was comparable to that in grass grown with herbaceous pasture legumes or supplied with N fertilizer.

The significance of the results for broom as browse, and possible methods of controlling its spread through pasture and grazing management are discussed.