The effects of gibberellic acid and time of grazing on nitrogen partitioning in dairy cows grazing perennial ryegrass pastures: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Time of grazing during the day and gibberellic acid application were assessed as potential management strategies for manipulating forage composition with the aim of improving milk solids production and reducing nitrogen (N) losses in urine. Two replicate groups, each containing Friesian x Jersey cows (8 groups, 40 cows total) were allocated fresh pasture following either morning (AM) or afternoon (PM) milking. Pastures were treated with either urea (N, 50kgN/ha) or gibberellic acid (GA, 8g gibberellic acid/ha). The trial was conducted over a nine day period which included a five day adjustment period. Each group had the same pasture allowance of (15kgDM/cow/day) above a residual of 1500kgDM/ha and pastures had similar metabolisable energy (ME) content (12.3±1.5MJME/kgDM). GA and N pastures had similar dry matter production (2909±215kgDM/ha) and similar dry matter intakes ( 1 l.4±0.9kgDM/cow/day). Nitrogen content of GA pasture (3.0% DM) was significantly lower than N fertilised pasture (3.2% DM) and white clover in the GA pasture (18.7% in sward) was significantly higher than in N fertilised pasture (7.4% in sward). In all treatment pastures clover (4.4% DM) had higher N content than grass (2.82% DM). Water soluble carbohydrate (WSC) was 18.6 %DM in the afternoon treatment pastures (GPM and NPM) and significantly higher than 17.6 %DM in the morning treatment pastures (GAM and NAM). In all treatment pastures clover (12.3% DM) had lower WSC content than grass (19.5% DM). The ratio of WSC: CP was significantly affected by time of day and fertiliser treatment. GA treated pasture and PM pastures both had significantly higher WSC: CP values than N or AM pastures, respectively. Both neutral detergent fibre (NDF) and acid detergent fibre (ADF) were lower in AM pasture than in PM pasture.