Modifying merino grazing and pasture composition in the high country by salt application

Abstract

Sodium is an essential element for animal production, with >0.07% Na required in herbage DM for lactating ewes. Na however, is non essential for plant production. With Na concentrations decreasing with increasing distance from the coast, inland farming areas of New Zealand, including much of the high country are often sodium deficient for grazing livestock. In extensive grazing systems pasture quality and legume content of steep shady faces is often low due to poor pasture utilization. This project tested whether the application of salt (NaCl) could be used to attract sheep and enhance grazing, and so improve pasture utilisation, botanical composition and oversowing success of steep shady faces.

Five experiments were conducted on small plots (up to 30x8 m) between December 2003 and May 2005. The aim was to determine the effects of salt, fertilizer and seed application on botanical composition and seedling establishment. All experiments were conducted on a predominantly steep (20-38°), south facing slope (600 m.a.s.l.) at Mt Grand Station, with a mean annual rainfall of 713 mm. Mt Grand Station is a sodium deficient (<0.03%), inland, high country property in Central Otago, New Zealand.

In Experiment 1, the effect of salt (0 and 150 kg NaCl ha⁻¹) was compared with urea (0 and 100 kg N ha⁻¹) and sulphur superphosphate (0 and 95 kg S ha⁻¹, and 40 kg P ha⁻¹). The effect of salt was greater than urea or sulphur superphosphate with lactating merino ewes strongly attracted to salt within 48 hours of application. The application of salt decreased the percentage of grasses, legumes, weeds and litter in the pasture and increased the percentage of bare ground. The application of urea increased the percentage of grasses while sulphur superphosphate increased the percentage of legumes.

In Experiments 2 and 3, the effects of salt rates from 0 to 300 kg NaCl ha⁻¹ on botanical composition were examined. Increasing salt rate resulted in an increase in the percentage of bare ground and a decrease in the percentage of legume, litter and weeds. Salt applied at greater than 100 kg NaCl ha⁻¹ resulted in bare ground greater than 50%, with total grasses decreasing below 30% and total legume below 5%. Effects persisted for up to 16 months after application.

In Experiment 4, the effect of salt application on the success of oversowing was examined by oversowing salt (100 kg NaCl ha⁻¹) and seed (Trifloium ambiguum 'KTA 202' 10 kg ha⁻¹, Lotus pedunculatus 'Sunrise' 2 kg ha⁻¹ and Plantago lanceolata 'Tonic' 2 kg ha⁻¹ applied together). When seed was oversown with salt, seedling establishment of plantain was increased from 1.17 to 6.16 seedlings m⁻², while Caucasian clover and Lotus pedunculatus failed to establish. The increase in establishment of plantain reflected increased bare ground, and a decrease in the vegetative competition associated with intense grazing. Sheep continued to return to plots where salt was applied, keeping the pasture mass low, for at least six months after salt application.

In Experiment 5, salt (0 and 100 kg NaCl ha⁻¹) was applied to cocksfoot dominant pasture in the absence of grazing. This experiment examined the change in soil and herbage Na concentration over time following the application of salt. The initial Ministry of Agriculture and Forestry Quick Test (MAF QT) for Na was increased from 3 to 13 and the cocksfoot herbage from <0.03% Na to 0.24% Na within 14 days, being above adequate for animal production.

These small plot studies demonstrate that the application of salt at 100 kg NaCl ha⁻¹ may be a potential tool to improve feeding quality and production on steep southerly aspects through increased grazing intensity, seedling establishment from oversown seed. The application of salt on a farm scale, to modify the spatial grazing patterns of stock, drawing animals from the sunny to shady faces needs to be examined. This would improve pasture utilization of the shady faces and allow annual legumes on the sunny faces to reseed.