Defining the subterranean clover zone on Mt Grand, Central Otago

Abstract

Historically sub clover has been widely used for pasture improvement in summer dry pastures in both South and North Islands. However its use has declined with more focus on white clover since the 1970's. In recent years there has been resurgence in its use due to the release of new cultivars from Australia coinciding with a series of dry summers in New Zealand in the late 1990's. These dry summers highlighted some of the weaknesses of white clover in extended drought and were the catalyst for not only the renewed interest in sub clover use but also research into production and persistence of sub clover cultivars and their grazing management.

The Lincoln University Mt Grand high country station in Central Otago where this study was based is typical of farms in that area with a history of sub clover in early years of hill country pasture improvement but over the last 30 or so year's only white clover has been oversown.

This project recorded where early sub clover introductions have persisted, where white clover has established and the area covered by volunteer annual clovers that make up the total clover herbage on offer to grazing animals.

Four experiments were conducted both across the farm and in small plots from November 1999 to December 2003. The aim was to determine the sub clover zone using cover % to estimate botanical composition, exclusion cages to measure herbage production and temperature data loggers and rain gauges to characterize climatic conditions.

Climate stations recorded air and soil temperatures at 5 sites, four on sunny aspect at 450, 620, 750, 910 m a.s.1. and one south facing site at 630 m a.s.1. The effect of altitude and aspect on temperature and rainfall was compared and thermal time (ºCd) calculated. Thermal time for the spring growth period (mid Aug-Dec) was about 1400 °C.d at 450N and declined at 100 °C.d per 100 m elevation (r² = 0.99) to be < 800°C.d at 910N. Lower accumulated thermal time with increasing altitude may restrict full lifecycles of annual clover (germination to mature seed). Rainfall differed little with altitude or aspect making temperature and associated evapotranspiration the drivers of botanical composition. Interestingly the 630S site with its southerly aspect had thermal time values between those for the 620N and 750N sites.

A preliminary botanical survey showed that sub clover and volunteer annuals were the predominant clovers on offer on the sunny faces. Herbage production from exclusion cages placed on favourable sunny face sites confirmed this with 25% of November/December herbage production of 4000 kg/DM/ha coming from sub clover cultivar 'Mt Barker'.

More detailed botanical composition analysis through visual cover scoring was done over two growing seasons in 2002 and 2003, next to the five climate stations. The contrast in botanical composition between north and south faces was dramatic. White clover was between 12 and 40% of cover on the south face compared to less than 5% on sunny faces with annual clovers the dominant on sunny faces. Adventive clovers cluster (T glomeratum), striated (T stratum), haresfoot (T arvense) and suckling (T dubium) made significant contributions to sunny face herbage production. Suckling clover was the only annual clover recorded on the south face.

It was concluded that white clover had occupied all suitable niches and further spreading of its seed need not continue. Instead earlier flowering cultivars of sub clover should be introduced to drier steeper faces currently dominated by cluster and striated clovers.