Effect of replacing nitrogen fertiliser with gibberellic acid on response of perennial ryegrass and white clover pasture : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

New Zealand dairy farms are no longer allowed to use high rates of nitrogen (N)-based fertiliser to achieve maximum pasture production due to the associated losses of N through nitrate leaching and nitrous oxide emissions. Gibberellic acid (GA), a naturally occuring phytohormone, has shown in short term studies, to increase, then reduce pasture yield when applied exogenously. The purpose of this research was to compare the long term effects of substituting N fertiliser with GA on a perennial ryegrass (Loliume perenne, PRG) and white clover (Trifolium repens, WC) pasture. A twelve-month, cut and carry plot trial explored dry matter (DM) production, botancial composition, ryegrass tiller size and density and forage nutritive value. To test these results, several treatment regimens were compared in a grazing trial with dairy cows which explored milk production and N excretion. Using a randomised block design with ten treatment regimens and four blocks, GA was applied to PRG and WC pasture plots to evaluate the timing and frequency of GA use in spring and autumn. Gibberellic acid replaced N fertiliser (once, twice or three times) in combinations of early (August and February), delayed (September and March) or late (October and April) applications. The control treatment was N fertiliser applied at 250 kg N/ha/y, the positive control was GA applied with every application of N fertiliser (80 g GA/ha/y + 250 kg N/ha/y) and the untreated control had no N fertiliser or GA. The first of ten applications occurred in August 2012 and the last in May 2013. Measurements included DM yield, botanical separation, tiller counts and nutrient content. Annual herbage yield was 14.6, 18.3 and 18.3t DM/ha/y (P<0.001) for the three controls (untreated, GA+N fertiliser and N fertiliser, respectively). By replacing N applications with GA, herbage yield declined showing a positive relationship between annual N fertiliser application and DM yield. There was no effect of GA on annual yield, nor was there any evidence of annual yield reductions associated with frequent GA application under low (100 kg N/ha/y) or moderate (250 kg N/ha/y) N fertiliser. Applying N early and delaying GA application improved the N response in spring (P<0.001), but not in autumn (P=0.28). The N response was greater for treatments which received less N fertiliser (i.e. double GA applications, 34.6 and 42.3 verses 19.2 kg DM/kg N with N fertiliser), and which received the N early (i.e. the GA was applied late). In spring, both N fertiliser and GA application saw increases in PRG (69.2 and 64.9%, respectively) and decreases in WC content (26.8 and 32.4%, respectively) compared with the untreated control (49.0 and 47.5% for PRG and WC, respectively). When expressed as yield, under cut and carry conditions, WC growth (948±129 and 1547±3036 kg DM WC/ha, respectively in spring and autumn) was relatively unresponsive to either N fertiliser or GA application. While PRG, through leaf length and numerical increases in tiller density responded positively to N fertiliser (2541 kg DM/ha) and GA (2186±190 t DM/ha), yielding significantly (P<0.001) higher than the untreated control in spring (1099 kg DM/ha). Tiller density, after three applications of GA was numerically higher (5938 and 6450 tillers/m2, respectively) than the untreated control (4900 and 5900 tiller/m2, respectively) but lower compared with N fertiliser (7038 and 7663 tillers/m2, respectively). Treatment of pastures with GA had only small effects on nutrient content. The crude protein (CP) content was most affected by changes in the application of GA and N fertiliser. While other nutritive components were less sensitive. In early spring CP of the pasture was 20.3±1.0% after the application of GA, similar to the untreated control (21.0%) and significantly (P<0.001) lower than N fertiliser (23.6%). Subtle changes in fibre content, with the application of GA may have been caused by small increases in sheath length in early spring (P=0.02), which are more fibrous and less digestible. However, metabolisable energy (ME) content (12.2±0.2MJME/kg DM) was not especially sensitive to the application of N fertiliser or GA in this study. A grazing study carried out to compare the use of GA or N in autumn (adopting the delayed treatment) found no effect of treatment on herbage pre-grazing pasture mass (2058±6.9 kg DM/ha, P=0.20), WC (3.6±0.86%, P=0.53), PRG (88.0±2.33%, P=0.24) or CP content (17.8±1.60%, P=0.31). All milk yield components: 14.6±0.40 L/day, 1.4±0.09 kg MS/cow/day, 5.4±0.33% fat and 4.4±0.21% protein were similar among treatment groups (P=0.71, 0.14, 0.16 and 0.06, respectively). Although faecal-N content of cows on GA pasture (3.72% N) was higher (P<0.001) than cows on N fertilised pasture (3.45% N) there was no treatment effect (P>0.05) on urinary-N content (0.4±0.02% N) or N excretion in milk (101±4.4 g N/cow/d). The lack of response to GA or N fertiliser highlighted the importance of timing of growth promotants for late lactation. In this study it was found that, the amount of N applied was more important for DM production than how much GA was applied. In spring delayed or late applications of GA, in place of N fertiliser was important to avoid N deficiency. While in autumn GA was able to be used in place of N fertiliser without affecting DM production. However growing times required in autumn may limit the effectiveness of GA on DM production and, consequently animal production.