Effects of nitrogen fertiliser and regrowth interval on herbage dry matter yield, chemical composition and nitrogen solubility of alternative pasture forages in irrigated Canterbury conditions : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The New Zealand agricultural sector is challenged with finding approaches to reduce the loss of nitrogen (N) through nitrate leaching, without the loss of farm production and profitabilty. One way to do this is to use different forages, regrowth intervals and N fertiliser rates to manipulate herbage N concentration and plant nitrogen use efficiency (NUE). The objective of this thesis was to quantify, for a range of pasture forages, herbage dry matter (DM) yield, chemical composition, N solubility and herbage N fertiliser requirements in response to different rates of N fertiliser, and regrowth interval in irrigated pastures in Canterbury. The thesis reports on four experiments. The objective of experiment 1 was to quantify the effect of N fertiliser rate on herbage DM yield and chemical composition, of two herb species (plantain and chicory) three legume species (lucerne, red clover and white clover) and seven grass species (prairie grass, Italian ryegrass, tall fescue, tetraploid perennial ryegrass, diploid perennial ryegrass, high sugar perennial ryegrass and cocksfoot) at optimum defoliation time over a two year period. In this study, conducted on the Canterbury Plains, New Zealand, the twelve species were grown at six N fertiliser rates ranging from 0 to 450 kg N/ha/year and managed under irrigation and cutting management. Herbage DM yield and N concentration were measured at each harvest over two years (1 December 2014 to 30 November 2016). Defoliation management was on average, every 29 days in grasses and herbs with a total of 9 harvests per year, and 38 days in legumes with a total of 7 harvests per year. Forages were not harvested over the winter (June, July). As N fertiliser rate increased from 0 to 450 kg N/ha/year, annual herbage production increased linearly, from 5016 to 13812 kg DM/ha/year in grasses and from 6844 to 11481 kg DM/ha/year in herbs. In contrast, annual DM yield for legumes was unaffected by N fertiliser rate and ranged from 10596 to 10936 kg DM/ha. Additionally, there were contrasting (P<0.001) responses in herbage N concentration between species. At all N fertiliser rates, annual herbage N concentration was highest in legumes (4.3 % N), intermediate in herbs (3.1 % N), and lowest in grasses (2.7 % N). The herbage N concentration of legumes were unaffected by increasing N fertiliser rate (4.4 % N at 0 kg N/ha/yr to 4.5 % N at 450 kg N/ha/yr), whereas in grasses and herbs it increased. However, the increase in N concentration in the grasses and herbs as N fertiliser rate increased, was small (2.6 % N at 0 kg N/ha/yr to 3.2 % N at 450 kg N/ha/yr, P<0.001). Thus, it is concluded for plantain, chicory, prairie grass, Italian ryegrass, tall fescue, tetraploid perennial ryegrass, diploid perennial ryegrass, high sugar perennial ryegrass and cocksfoot, the reduction in use of N fertiliser as a mitigation strategy to decrease herbage N intake may be ineffective. In addition, the results suggest that there were no benefits in using herbs instead of grasses for reducing N intake in livestock in an irrigated Canterbury environment. The objective of experiment 2, was to quantify the effect of N fertiliser rate on herbage DM yield and chemical composition of plantain, chicory, red clover, white clover, diploid perennial ryegrass and cocksfoot at 4 regrowth intervals (1, 2, 3 and 4 weeks) in spring and autumn. In this study, conducted on the Canterbury Plains, New Zealand, the six species were grown at three N fertiliser treatments; nil (0 kg N/ha/year), medium (180 kg N/ha/year) and high (450 kg N/ha/year), and managed under irrigation and cutting management. The effect of regrowth interval and fertiliser rate on herbage DM yield, plant N concentration, water soluble carbohydrate: crude protein (WSC: CP) ratio and digestible organic matter in the DM (DOMD) were measured each week over a 4-week regrowth period, in autumn 2015 and spring 2016. As regrowth interval increased, herbage DM yield increased (from 179 kg DM/ha to 922 kg DM/ha in the autumn, and from 169 kg DM/ha to 1772 kg DM/ha in the spring, P<0.05). Averaged over the N fertiliser rates, herbage DM yield was greatest in plantain (1544 kg DM/ha/yr) and lowest in diploid perennial ryegrass (1148 kg DM/ha/yr). N response rates were highest in perennial ryegrass (31.5 kg DM/kg N applied) and plantain (32.2 kg DM/kg N applied, P<0.05). Herbage N concentration of grasses and herbs in autumn was high (>3.2% N, averaged over the N fertiliser rates), compared to spring (2.5 % N, averaged over the N fertiliser rates) and increased with N fertiliser rate (3.1 N % at nil N fertiliser to 3.9 N % at high N fertiliser, P<0.001), but decreased with regrowth interval (3.8 N % at week 1 to 2.9 N % at week 4, P<0.001). The difference in herbage N concentration between N fertiliser rates was notably higher at the beginning of the regrowth interval, compared to after 4 weeks of regrowth (12.4 % difference between nil and high N fertiliser at 1-week regrowth, and 10.2 % difference between nil and high N fertiliser). Thus, manipulating regrowth interval may reduce herbage N intake more so than N fertiliser application. Overall, delaying grazing to 4 weeks, under a moderate N regime, did not reduce herbage quality but reduced herbage N concentration in autumn and spring. These results suggest plantain is a suitable alternative to perennial ryegrass to reduce N losses without impeding farm production because of its high N response rates, high herbage DM yield and low herbage N concentrations, compared to the other species. The objective of experiment 3 was to determine the herbage N concentration and N fertiliser requirements in diploid perennial ryegrass, high sugar perennial ryegrass grass, Italian ryegrass, cocksfoot, prairie grass, chicory and plantain at maximum herbage DM yield under glasshouse conditions over a six-month summer and autumn period. In this study, the seven forages were grown over 19 weeks under seven N fertiliser rates, ranging from 0 – 40 g N/m2 (0 – 400 kg DM/ha) total N applied. Accumulated herbage DM yield was highest and N concentration lowest in prairie grass (456 g DM/m, 1.9 % N) and plantain (451 g DM/m2, 1.9 % N), averaged over the N fertiliser rates. At the highest (40 g N/m2) N fertiliser rate, response rates were lower (20.9 g DM/g N applied) than the low (17.5 g N/m2) N fertiliser rates (27.4 g DM/g N applied). This suggests that the utilisation of N for higher herbage DM yields was not as effective when high amounts of N fertiliser was applied. This was shown in the soil N concentration (soil mineralisable N) which was highest at the high N fertiliser rate (8.6 g/m2 at the 40 g N/m2 N fertiliser rate vs 7.0 g N/m2 at the 17.5 g N/m2 N fertiliser rate). Herbage N concentration increased when applications of 10 g N/m2 or above were applied to forages, although large differences were not shown until the highest N rate was applied (2.0 % N vs 2.5% N). This result indicates the difficulty in altering herbage N concentration in plants at the final harvest, as found in the previous chapters. Overall, the high herbage DM yields and low N concentrations in prairie grass and plantain found in this trial indicates they could be used to reduce herbage N intake without impeding on farm production. The objective of experiment 4 was to quantify the effect on N fertiliser rate on chemical composition and N solubility in plantain, chicory, lucerne, white clover, diploid perennial ryegrass and cocksfoot at optimum defoliation time in the summer and autumn. This study examined the six forages under two N fertiliser rates (180 kg N/ha/year and 450 kg N/ha/year) using herbage collected in summer and autumn. Grasses were the highest (P<0.01) in neutral detergent insoluble N (NDIN) (380 mg/g N) and lowest in non-protein N (NPN) (111 mg/g N). Herbs contained higher (P<0.0.1) amounts of insoluble N (836 mg/g N) that was slower to degrade in the rumen compared to legumes (766 mg/g N) and grasses (511 mg/g N). Legumes were shown to be higher in NPN (180 mg/g N), than grasses (111 mg/g N) and herbs (76 mg/g N) which is easily converted to urea and urinary N. As a result, legumes may contribute to higher urinary losses in pastoral systems. The addition of high N fertiliser applications increased NPN in all species from 97 mg/g N to 148 mg/g N. Results from this trial suggest herbs contain N that is more available to be utilised by the animal for growth and production, reducing the amount of N lost as urinary N. In addition, high N fertiliser rates may lead to higher N intakes. Overall, the use of Italian ryegrass, prairie grass, tall fescue, perennial ryegrass, high sugar ryegrass and plantain, with lower N fertiliser rates and 4-week regrowth intervals could be used to reduce herbage N intake. However, the use of legumes in traditional ryegrass-white clover pastures may contribute to higher N losses in the system. Moreover, the herbage DM yield, N concentration and N solubility results from this PhD have indicated the use of alternative pasture forages may be incorporated into farm systems without hindering farm production or profitability.