Effects of gibberellic acid application on pasture growth and composition: A thesis submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours

Abstract

Autumn is a critical time of the year on pasture-based farming systems as pasture growth is usually limiting at this time of the year. Often nitrogen (N) is applied at this time of the year to increase dry matter yield and feed supply. However high levels of N in the pasture can lead to high N loss via animals, and alternatives to boost pasture growth need consideration. This trial compared the response of simple (perennial ryegrass white clover and tall fescue white clover) and diverse pastures (including chicory, plantain and legumes) to a single application of GA, N or a combination in autumn. Gibberellic acid (GA) and nitrogen (N) were applied in the form of ProGibb and Urea at a rate of 60g ProGibb/ha and 50kg N/ha and their effect on pasture yield, pasture composition and nutritive value were measured over four weeks. GA application resulted in an average increase in dry matter production of 353kgDM/ha, and N application resulted in a small average increase of 129kgDM/ha. The response of pasture height to GA was evident after week one in the GA treated pastures but took four weeks to emerge for N treated pasture, only occurring in the last week of the trial. Simple perennial ryegrass white clover pastures responded the greatest to GA application, while simple tall fescue and white clover and the diverse pastures had a much smaller response. GA was also found to have a limited effect on botanical composition and pasture nutritive value for any pasture type. The cost of production was lower for GA compared with N ($0.09/kg ProGibb applied and $0.68/kg Urea applied respectively).lt was found that GA is a cost effective solution to increase pasture production in the autumn. As this occurred with no increase in pasture N content, GA may be proposed as tool to increase dry matter production with low environmental impact.