Response of intensively grazed ryegrass dairy pastures to fertiliser phosphorus and potassium

Abstract

Intensively grazed, rain-fed dairy pastures on the predominantly sandy soils in the high rainfall ([800 mm annual average) Mediterranean-type climate of south-western Australia comprise [90% ryegrass (annual ryegrass, Lolium rigidum Gaud. and Italian ryegrass, L. multiflorum Lam.). To maximise pasture use for milk production, the pastures are rotationally grazed by starting grazing when ryegrass plants have 3 leaves per tiller, and fertiliser nitrogen (N) and sulfur (S), in the ratio of 3-4 N and 1S, need to be applied after each grazing for profitable pasture dry matter (DM) production. In addition, farmers usually also apply low levels of phosphorus (P) and potassium (K) fertiliser to these pastures after each grazing, despite Colwell soil test P usually being well above critical values for pasture production, and fertilizer K being only required for clover in the traditional clover (Trifolium subterraneum L.) ryegrass pastures of the region. In field experiments undertaken May 2006-June 2010 on intensively grazed ryegrass dairy pastures in the region, no significant ryegrass DM responses to applied fertiliser P or K were obtained, regardless of level or method of P or K application. When no P was applied, soil test P declined gradually, by between 4.4 and 7.1 mg/kg per year, and remained above the critical value for the soils at 2 sites, but declined below the critical value for soil at a third site. Critical soil test P is located near the maximum yield plateau in the flat part of the relationship between yield and soil test P, particularly when, as appropriate for dairy production, the critical value is for 95% of the maximum pasture DM yield. Consequently, when no P is applied and soil test P decreases, significant pasture DM yield decreases will only occur when soil test P approaches the steeper part of the relationship, which can take some time. In addition, as occurs on farms, faeces deposited by cows while grazing supplied P to pasture even when no fertiliser P was applied. Soil K testing proved unreliable for indicating the need for fertiliser K applications to pasture in the next growing season because many soil samples collected within and between urine patches contained elevated levels of K deposited by cows while grazing. We conclude fertiliser P should only be applied to intensively grazed ryegrass dairy pastures when soil testing indicates it is required. Further research is required to assess if plant K testing is an alternative, but urine patches may also pose a problem for plant testing.

Does balanced phosphorus fertilisation sustain high herbage yields and phosphorus contents in alternately grazed and mown pastures?

“…The observed response of DM yield to P fertilisation of our experiment was more or less in line with results reported for multiple-year experiments in grazed grassland. Reported responses of DM yield range from 0 up to 110 kg DM kg -1 P (Bolland et al 2011;Bolland and Guthridge 2007;Davison et al 1997), responses of P yield were not mentioned.…”

Determining the critical plant test potassium concentration for annual and Italian ryegrass on dairy pastures in south‐western Australia

“…Pasture‐based dairy production systems in Western Australia (WA) differ from most other dairy areas in Australasia due to the Mediterranean climate and sandy soil type in the region. The majority of farms operate a dryland production system where the typical rain‐fed pasture growing season lasts from approximately May to November, with negligible growth occurring in the warm, dry period between December and April (Bolland, Guthridge, & Blincow, ; McDonnell, Staines, Edmunds, & Morris, ). Due to these environmental conditions, dairy pastures have historically comprised a mix of annual ryegrass ( Lolium rigidum Gaud.)…”

“…and Italian ryegrass ( L. multiflorum Lam. ), together with subterranean clover ( Trifolium subterraneum L.), with these species completing their growth cycles and setting seed before the onset of moisture stress at the end of the growing season (Bolland et al., ; Callow, Michell, Baker, & Hough, ). Dairy production systems in the region have intensified significantly over the last two decades (Bolland & Guthridge, ), with use of N fertilizer becoming commonplace and farmers adopting the 3‐leaf grazing management principles outlined by Fulkerson and Donaghy ().…”

Changes in soil phosphorus pools of grasslands following 17 yrs of balanced application of manure and fertilizer

“…Data from experiments on permanent grassland, which is not grazed, generally show stable P-test values at P surpluses close to equilibrium (Schils & Snijders, 2004;Stroia et al, Changes in soil P pools at balanced fertilizer application 9 2007; Messiga et al, 2015), or even a slight increase in P-test values (Gallet et al, 2003;Malhi et al, 2003). On the other hand, data from permanent grazed grassland often show a decline in P-test values at P surpluses close to equilibrium (Condron & Goh, 1989;Davison et al, 1997;Bolland et al, 2011). Data from the grazed Winchmore long-term trial in New Zealand (Condron & Goh, 1989) showed a decline in both readily available and more residual bound inorganic P pools at a slight P deficit [zero P application in combination with small output of animal products (<1.4 kg P/ha)].…”