Effect of defoliation by grazing or shoot removal on the root growth of field-grown wheat (Triticum aestivum L.)

Abstract

Dual-purpose crops for grazing and grain production can be highly profitable, provided grazing does not cause significant loss of grain yield. In many plants, defoliation causes a transient reduction in the allocation of resources to stem and root growth and remobilisation of soluble resources to re-establish leaf area rapidly. In Australia, the usual autumn and winter period of defoliation for grazed crops, May–July, coincides with a phase of near-linear root depth penetration in ungrazed crops, and the crop recovery period after grazing occurs during stem elongation, when grain number and yield potential are determined. However, few studies have investigated the potential impact of crop defoliation through grazing on root growth of wheat in the field. We investigated the effect of defoliation by grazing or shoot removal on the root growth of wheat crops in four field experiments in south-eastern Australia in which the timing, frequency and intensity of defoliation varied. Despite significant impacts of defoliation on aboveground biomass (50–90% reduction) and grain yield (10–43% reduction) in all experiments, we found little evidence of effects on the rate of root penetration or final rooting depth. A notable exception was observed in one experiment when defoliation commenced very early (four-leaf stage, Zadoks growth stage Z14) in a repeatedly defoliated crop, reducing rooting depth from 1.65 to 1.35 m. The only other measured impact on roots was in an early-sown winter wheat crop grazed by sheep for 3 months (6 June–3 September), in which root length density was reduced by ~50% in surface layers above 1.0 m depth, but there was no impact on maximum root depth or root length density at 1.0–2.0 m depth. Our results suggest that grazing has little impact on the rooting depth of wheat unless it occurs very early and repeatedly, when plants are allocating significant resources to establish the primary roots. However, there may be some reduction in the density of roots in surface layers during recovery after long-term grazing, presumably associated with reduced proliferation of the nodal root system. We conclude that most significant yield penalties due to grazing relate to impacts on the assimilation of aboveground resources, rather than to reduced water or nutrient acquisition by roots.

Evaluating Dry Matter Production and Grain Yield of Dual-Purpose Winter Wheat Using Field Experiment and Modelling

“…This is mostly the case in seasons where soil moisture is the limiting factor. However, in seasons with a full soil moisture profile, early sown crops explore more soil water owing to a deeper rooting system [14,15]. This is in agreement with the results of the field experiment of this study (Table 3), which was sown early (mid-March).…”

“…In seasons where there is sufficient soil moisture from an early start of the autumn rain, stored soil moisture from the fallow period or irrigation, early sown dual-purpose cereals into mild-warm soil and air temperatures offer a high dry matter quantity and quality [2]. Early sown crops develop a deeper rooting system to explore soil moisture and develop a high canopy, which increases radiation interception [14,15]. In Southern Australia, using the crop simulation model APSIM [16], Moore [17] found that establishment opportunities for winter cultivars, 15 mm or more of rain over a three-day period during April and May, occurred between 58% and 98% of the sites and years combinations considered.…”

Root system-based limits to agricultural productivity and efficiency: the farming systems context

“…Rasmussen et al (2015) found that soil N levels in spring were generally low, in the range of 1-5 kg N ha -1 per 10 cm soil layer, while others have found slightly higher values; for example, Beaudoin et al (2005) found levels between 5 and 10 kg N ha -1 . In both Denmark and Australia, the level of N available to wheat within deeper layers has been found to vary between 1 and 15 kg N ha -1 per 10 cm soil (Thorup- Kirkegaard et al, 2015). In the high soil N examples, between 100 and 200 kg of N or more per hectare is found in the sub-soil below 0Á50 m depth.…”

Dual-purpose cropping – capitalising on potential grain crop grazing to enhance mixed-farming profitability

“…Previous research has shown that root growth of canola ceases immediately after defoliation, and taproot starch reserves are remobilised to support new leaf area, leading to gradual loss of fine roots (McCormick et al 2012(McCormick et al , 2013. In testing whether similar effects occur in cereal crops, Kirkegaard et al (2015) conducted extensive field experiments and found little evidence that grazing influences root penetration or final rooting depth, except when defoliation occurs regularly and in early plant development. Their work supports earlier conclusions of Harrison et al (2011cHarrison et al ( , 2012b that yield reductions of grazed winter wheat crops are primarily due to green tissue removal, reduced light interception and photosynthesis, rather than to a reduction in water or nutrient acquisition by roots.…”