Agronomic and physiological performance of four perennial grasses under summer dry conditions in New Zealand

Agronomic and physiological performance of four perennial grasses under summer dry conditions in New Zealand The main aim of this research was to quantify the effects of different levels of moisture and nitrogen (N) supply on dryland brome, cocksfoot, perennial ryegrass and tall fescue pastures grown under summer dry conditions. This was assessed from establishment (2014/15) to the end of the second year (2015/16) in two field experiments. Moisture and temperature are the major factors that affect germination. To quantify their combined effects, a laboratory experiment was used to calculate hydrothermal time requirements. Increasing temperature from 25 to 30 and 35 °C caused a rapid decline in germination rate of cocksfoot and tall fescue even when moisture was not limited (0 MPa). Applying moisture stress, especially when temperature was higher than 20 °C resulted in a more rapid decline in cocksfoot germination rate and final germination percentage than in perennial ryegrass, tall fescue and brome. To explore the effects of different soil moisture levels on pasture production in the field, two experiments were established in October 2014 at Ladbrooks and Ashley Dene, which differed in plant available water content (PAWC). The Wakanui soil at Ladbrooks has a 41-55% higher PAWC than the stony Lismore silt soil at Ashley Dene. A Latin square design was established at both sites with four grass species and four replicates. During establishment period, from October 2014 to June 2015, total dry matter (DM) production at Ladbrooks was 4.34 t ha-1 compared with 1.93 t ha-1 at Ashley Dene, with no differences among species. In the second year from July 2015 to July 2016, the experiments included a nitrogen (N) treatment. At Ladbrooks 900 kg N ha-1 and at Ashley Dene 500 kg N ha-1 were applied. In 2015/16, mean annual DM production was 19.8 t ha-1 for +N and 9.20 t ha-1 for −N treatments at Ladbrooks. At this location, there were no differences for DM production among the species. In 2015/16 at Ashley Dene, the +N cocksfoot treatment DM was 6.0 t ha-1 higher than the average of 3.2 t ha-1 for the other ±N species. This difference at Ashley Dene was explained by the interaction between species and N after 85 mm of rainfall in mid-January of 2016, when only the cocksfoot responded to the applied N. The impact of seasonal temperatures on DM yield was quantified using thermal time accumulated above a base temperature of 3 °C. In the 2015/16 period, the relationship between accumulated thermal time and DM was separated into two phases at Ladbrooks and into five phases at Ashley Dene. These phases indicated decreased or increased growth due to the onset of moisture stress or rainfall. At Ladbrooks, the break points were not different among species, but were later in +N compared with –N treatments. For +N pastures linear growth in Phase 1 lasted until 6/3/2016, after 2355 ± 44.6 °Cd. For −N treatments Phase 1 stopped after only 2161 °Cd, on 19/2/2016. The temperature adjusted linear growth rate was differed among species and N treatments. For cocksfoot and perennial ryegrass it was 6.94 kg DM °Cd-1 ha-1 which was higher than brome and tall fescue (6.00 kg DM °Cd-1 ha-1). At Ashley Dene, the additional yield from +N cocksfoot resulted in a higher linear growth rate in Phase 3, after the 85 mm of rainfall in January 2016. Plant available water content was 254 mm for +N and 195 mm for −N pastures at Ladbrooks compared with a mean of 101 mm at Ashley Dene. In the 2015/16 period, mean total water use was 823 mm for +N pastures which was higher than the −N pastures (777 mm) at Ladbrooks. There were no differences among species. Mean water use efficiency for +N pastures was 20.4 kg ha-1mm-1 which was higher than −N pastures (8.36 kg ha-1mm-1). There were no differences among the species. In 2015/16, total water use at Ashley Dene was 364 mm and was not different among species. However, water use efficiency for cocksfoot was 16.6 kg ha-1mm-1 which was higher than other species. The higher production of + pastures at Ladbrooks was explained by higher radiation interception and also radiation use efficiency (RUE). The lower yield at Ashley Dene compared with Ladbrooks was due to the reduction in the amount of photosynthetically active radiation intercepted by the canopies as well as a lower RUE. Higher radiation interception of +N cocksfoot on 20/1/2016 and 21/4/2016 was the main explanation for its higher DM production. Overall, the range of dry matter production yield from the different soil moisture and N conditions could be quantified by calculation of growth rates in relation to thermal time, soil water extraction patterns, estimation of light interception and radiation use efficiency, explain the physiological basis of yield differences.