Take home messages

• Commercially available barley varieties showed head loss percentages exceeding 5%, highlighting the need for growers to prioritise resilient genotypes with improved structural traits, such as shorter peduncles and optimised plant height, in their farming systems.
• Dwarf genotypes with shorter peduncles showed reduced head loss and should be prioritised in breeding and variety selection for areas prone to wind and other environmental stresses.
• Genotypes were identified that combine taller stature with short peduncles, which have potential for both reduced head loss and improved weed competitiveness, making them ideal candidates for dual-purpose breeding objectives.

Background

Barley (Hordeum vulgare), one of the earliest domesticated crops, plays a vital role in global agriculture, ranking as the fourth most-produced cereal worldwide. It is valued for its versatility, serving as livestock feed, a food source, and a critical ingredient in the brewing industry. In Australia, barley is the second largest cereal crop, with annual production averaging 9–10 million metric tonnes. A substantial portion of Australian barley is exported, particularly prized in Asian markets for its high malting quality. However, climatic challenges, including high winds, extreme temperatures, and delayed harvests, threaten barley production, particularly through head loss – the pre-harvest detachment of the grain-bearing head, which leads to direct yield losses and grower profitability (PIRSA 2021, McCallum et al. 2022).

Head loss is a complex trait influenced by both environmental and genetic factors. Environmental stresses, such as wind and temperature extremes, weaken the peduncle (the stem segment supporting the head), leading to breakage. Structural traits like peduncle length, thickness, and flexibility play a critical role in determining susceptibility. Yield losses from head loss can range from 10% to 30%, translating into substantial economic impacts for growers and the broader barley industry. Addressing this issue through genetic improvements, such as selecting for resilient peduncle traits and optimising spike architecture, is essential to ensure sustainable production in the face of climate change.

Method

A total of 138 barley genotypes, including 45 commercially released varieties and advanced breeding lines developed by Intergrain Pty Ltd, were evaluated at two experimental sites: Esperance, Western Australia, and Inverleigh, Victoria. The trials were established in a randomised block, paired plot design with two replications. Each site contained 576 plots, with each plot measuring 140cm in width and 380cm in length. Plots consisted of five rows spaced 28.5cm apart. Sowing was carried out on 2 June 2022 at Esperance (WA) and 19 June 2022 at Inverleigh (Vic).

Head loss percentage was assessed manually by counting the total number of tillers and spike heads retained within a 0.5m² quadrant in each plot. A secondary method of estimating head loss was based on yield comparison: yield was recorded after the first harvest (28 November 2022) and again after a delayed second harvest (29 January 2023). The difference between these two yields provided an indirect estimation of head loss. However, due to inefficiencies in the single plot harvester, this method was excluded from further analysis. Additional data collection included recording plant height prior to the first harvest. From each genotype, 15–20 tillers were collected by sampling three random spots within a plot. These tillers were securely packed in custom mailing tubes (57cm in length and 10cm in diameter) and transported to the University of Melbourne. Five main tillers per genotype were reserved for Instron^® experiments, while another five tillers per replication were used to measure key peduncle traits.

A modified bending protocol was developed for the Instron 5569A universal testing frame equipped with a 2kN load cell. The protocol induced breaks at any point along the peduncle, differing from the traditional three-point bending experiment, which induces breaks at the centre (Robertson et al. 2015). Custom grips and holders were designed to secure the fragile barley samples, which included the spike, peduncle, and second internode in a single piece (Figure 1). The bending test involved applying a downward force at a constant rate. During the experiment, the Bluehill^® v3.0 software recorded the force required to induce breakage, compressive displacement, and the time to failure.

Results and discussion

Head loss percentage based on manual counting was retained as our important trait and an analysis of co-variance was carried out. Only the data from Esperance (WA) were used for analysis, as Inverleigh (Vic) was completely lost due to severe lodging during the season. The moisture content recorded for our samples, peduncle curvature angle and spike angle were kept as confounding factors to determine their effect on head loss. The head loss percentage was classified into five groups (Figure 2) namely, very low head loss (<5%), low head loss (5-15%), medium head loss (15-25%), high head loss (25-35%), very high head loss (>35%) to visualise the distribution. The majority of the genotypes were grouped under the 5–15% category. None of the commercially released varieties recorded less than 5% head loss.

A scatterplot with peduncle length, plant height and head loss revealed the distribution of genotypes (Figure 3). The plot was divided into four quadrants (I - IV) based on the average mean of plant height and peduncle length. The genotypes were coloured based on the head loss percentage value of the individuals and dot size represent head loss values. Quadrant I consists of genotypes with short plant height and short peduncle length. Quadrant II consists of short genotypes with long peduncles. Quadrant III consists of tall genotypes with long peduncles. Quadrant IV consists of tall genotypes with short peduncles. The general trend is that short (dwarf) genotypes (i.e. Quadrant I) show the least occurrence of head loss.  However, the data shows that there are also tall genotypes that exhibit low head loss (i.e. Quadrants III and IV).  The ability to genetically disconnect plant height/stature and head retention is important as it means new varieties could be developed with regional agronomic advantages of tall versus short, while in parallel improving head retention characteristics. For example, genotypes in Quadrant IV, with tall plant height and short peduncle length, could act as a potential parent to develop varieties with higher weed competitiveness and improved head retention.

Figure 1. Modified Instron bending experiment to estimate peduncle strength.

Figure 2. Distribution of genotypes for head loss percentage (Esperance, 2022).

Figure 3. Scatter plot for peduncle length (PL) x-axis (cm) and plant height (PH) y-axis (cm). The horizontal red line indicates the general mean for plant height. The vertical blue line indicates the general mean for peduncle length. Head loss percentage represented as a colour gradient and the dot sizes are directly proportional to the head loss values.

Conclusion

Head loss analysis revealed that the majority of genotypes exhibited a low head loss rate of 5–15%, with no commercial variety achieving less than 5%. This highlights significant opportunities for improvement in breeding programs. Genotypes with minimal head loss were predominantly characterized by shorter plant stature and peduncles, though promising lines with taller stature and shorter peduncles were also identified. These taller genotypes hold potential for developing resilient, weed-competitive barley varieties with reduced head loss. This finding underscores the importance of targeted optimisation of structural traits and environmental adaptability to enhance barley's resilience and productivity.

Acknowledgements

The scholarship for this work was provided through the GRDC and their support gratefully acknowledged. The primary author also thanks the University of Melbourne for support via a Melbourne Research Training Scholarship. The authors would also like to thank Intergrain Pty Ltd for providing funding for field trials and valuable breeding materials.

References

McCallum M, Tucker M, Porker K (2022) Development of a barley head loss susceptibility index for Southern Australia. Proceedings of the 20th Agronomy Australia Conference, Toowoomba Qld, 2022 (https://www.agronomyaustraliaproceedings.org/images/sampledata/2022/Climate/Assessing-and-managing-heat/ASAmccallum_m_401s.pdf)

PIRSA (2021) Crop and pasture report South Australia. Primary Industries and Regions SA (PIRSA), Adelaide. https://www.pir.sa.gov.au/__data/assets/pdf_file/0007/393604/crop-and-pasture-report-crop-establishment-2021-22-july.pdf

Robertson DJ, Smith SL, Cook DD (2015) On measuring the bending strength of septate grass stems. American Journal of Botany 102(1), 5-11. doi: 10.3732/ajb.1400183.

Minimising head loss in barley (https://www.grainsearch.net.au/2014/08/07/minimising-head-loss-in-barley/)

Murdoch University News: Research delivers barley head loss insights (https://www.murdoch.edu.au/news/articles/research-delivers-barley-head-loss-insights)

Contact details

Nirmal Raj Rajendran
The University of Melbourne
Royal Parade, VIC 3010
0404 721 674
nrajendran@student.unimelb.edu.au
@nimmu307