Defoliation Frequency Research Articles

Distribution of Non-Structural Carbohydrates and Root Structure of Plantago lanceolata L. under Different Defoliation Frequencies and Intensities.

Plantago lanceolata L. (plantain) increases herbage dry matter (DM) production and quality during warm and dry conditions due to its deep roots and drought tolerance and reduces nitrogen losses in grazing systems compared to traditional pastures. However, plantain density usually declines after the third growing season, mainly due to defoliation management. The effects of defoliation frequency and intensity on water-soluble carbohydrate (WSC) reserves and below-ground plant responses need further research to optimize grazing strategies for improved productivity and sustainability of grazing systems. Our study investigated the effects of defoliation frequencies (15, 25, and 35 cm of extended leaf length, ELL) and intensities (5 and 8 cm of residual heights) on morphological traits and WSC concentrations in plantain biomass under controlled environmental conditions. Defoliation frequency significantly influenced morphological and chemical characteristics and biomass distribution more than residual height. Less frequent defoliations promoted above-ground herbage DM production, reproductive stems, and root biomass. Root architecture showed adaptations in response to defoliation frequency, optimizing resource acquisition efficiency. Frequent defoliation reduced high molecular weight WSC concentrations in leaves, affecting regrowth capacity and DM mass. A defoliation frequency of 25 cm ELL (~15 days) balances herbage production and root development, promoting long-term pasture sustainability.

Marandu palisade grass‐forage peanut mixed pastures: Forage intake, animal behaviour, and canopy structure as affected by grazing intensities

AbstractIntegrating forage legumes into grasslands offers numerous ecosystem services. However, the management of grass‐legume pastures is challenging because the interaction between the defoliation frequency and severity may affect the legume persistence and forage intake by grazing animals. This 2‐year study evaluated the most effective grazing intensity to increase forage intake while maintaining a balanced legume proportion in tropical pastures consisting of Brachiaria brizantha and Arachis pintoi. Three treatments (grazing intensities) were assessed: severe, moderate, and low, corresponding to stubble heights of 10, 15, and 20 cm, respectively, and a pre‐grazing height of 25 cm. Two purebred Tabapua heifers were used for each treatment, and additional put‐and‐take animals were introduced, as required. Canopy structure, grazing behaviour, nutritive value, and forage intake were evaluated. The legume proportion in herbage mass remained consistent across all intensities, averaging 35% pre‐ and 32% post‐grazing. The grazing duration remained consistent across treatments on the first and third days, with an average grazing time of 390 and 440 min, respectively. Under post‐grazing conditions, low‐intensity grazing had a 52% greater intake rate than severe‐intensity grazing, which was attributed to a greater biting rate (37.9 bites/min) and bite weight (0.9 g DM/bite). Moderate grazing intensity (stubble height of ~15 cm associated with a pre‐grazing height of ~25 cm) is recommended to manage a mixture of B. brizantha and A. pintoi when the objective is to couple high forage intake with a balanced legume proportion in the pasture.

Selection environment impacts genetic gains for persistence in three temperate perennial forage grasses

Grasses and legumes are regularly grown in association, despite often being unbalanced or incompatible. This study was intended to conduct selection for persistence in three perennial grass species that have compatibility issues with alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.) and are poorly adapted to management-intensive rotational grazing. Five cycles of selection for persistence were conducted over a 24-year period for three grass species (reed canarygrass, smooth bromegrass, and timothy) in each of five selection environments: alfalfa mixture, red clover mixture, monocultures with a three-harvest hay management, tall fescue mixture with frequent defoliation, and monocultures with frequent defoliation. Selection in the two legume mixtures led to significant increases in persistence only when evaluated in legume mixtures. Selection responses were strongest when evaluated under the same legume as in the selection environment, indicating some species specificity. Selection for persistence under frequent defoliation was successful under both conditions (monoculture and tall fescue mixture), but led to no changes when evaluated with legumes. For these three grass species, cultivar development for use in mixture with legumes or for use in management-intensive rotational grazing should be based on a selection environment that mimics, as close as possible, the target production environment.

Tailoring defoliation and nitrogen management for large canopy radiation use and biomass production of perennial systems destined for biorefinery

Perennial leafy grasses and legumes are promising for extraction of proteins with a green biorefinery amid increasing demand for local production. However, which defoliation strategy increases radiation-use efficiency and biomass production still needs to be determined. A multi-year field experiment started in 2019 in Denmark with fertilized grasses (perennial ryegrass, tall fescue), unfertilized legumes (alfalfa, red clover) and their fertilized mixture (grass-legume), each defoliated at 2-, 4- or 8- weeks interval, at either 7-9 or 12-14 cm defoliation height. The main aim was to quantify effects of species, defoliation height and frequency, and nitrogen application rate on canopy radiation interception, aboveground biomass and crude protein production, as well as aboveground radiation use efficiency (RUEA).The results showed that accumulated intercepted photosynthetically active radiation (AIpar) increased with increasing defoliation height and decreasing frequency for most systems, with the largest AIpar obtained seasonally by red clover (730-815 MJ m−2) and grass-legume mixture (790-826 MJ m−2) defoliated at medium to low frequency. Tall fescue defoliated at medium to low frequency yielded the highest aboveground biomass (10-14 Mg ha−1) and red clover defoliated at medium to high frequency yielded the most crude protein (2.5-2.9 Mg ha−1) compared to all other systems annually. RUEA decreased with increasing defoliation height and maximum values were achieved at medium frequency, with tall fescue having significantly higher RUEA (1.6-2.1 g MJ−1) than the other systems. Seasonally, RUEA was variable at high defoliation frequency, being consistently higher at the start and the end of the season for ryegrass and the mixture systems.This study pinpoints tall fescue and red clover as perennial crops with high agronomic productivity among tested species for green biorefinery and also concluded that red clover in temperate humid climate maximize productivity and sustainability for biorefining with defoliation at low height and medium frequency.

Nitrogen fertilisation reduces the contribution of root-derived carbon to mineral-associated organic matter formation at low and high defoliation frequencies in a grassland soil

Abstract Background and aims Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass. Methods We applied N fertiliser (0 vs. 40 kg N ha−1 yr−1) and defoliation frequencies (3–4 vs. 6–8 clipping events year−1, simulating low and high grazing intensity) for three years, then used a 13CO2 pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions. Results With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition. Conclusion A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools.

Does long-term drought or repeated defoliation affect seasonal leaf N cycling in young beech trees?

Forest trees adopt effective strategies to optimize nitrogen (N) use through internal N recycling. In the context of more recurrent environmental stresses due to climate change, the question remains of whether increased frequency of drought or defoliation threatens this internal N recycling strategy. We submitted 8-year-old beech trees to 2 years of either severe drought (Dro) or manual defoliation (Def) to create a state of N starvation. At the end of the second year before leaf senescence, we labeled the foliage of the Dro and Def trees, as well as that of control (Co) trees, with 15N-urea. Leaf N resorption, winter tree N storage (total N, 15N, amino acids, soluble proteins) and N remobilization in spring were evaluated for the three treatments. Defoliation and drought did not significantly impact foliar N resorption or N concentrations in organs in winter. Total N amounts in Def tree remained close to those in Co tree, but winter N was stored more in the branches than in the trunk and roots. Total N amount in Dro trees was drastically reduced (-55%), especially at the trunk level, but soluble protein concentrations increased in the trunk and fine roots compared with Co trees. During spring, 15N was mobilized from the trunk, branches and twigs of both Co and Def trees to support leaf growth. It was only provided through twig 15N remobilization in the Dro trees, thus resulting in extremely reduced Dro leaf N amounts. Our results suggest that stress-induced changes occur in N metabolism but with varying severity depending on the constraints: within-tree 15N transport and storage strategy changed in response to defoliation, whereas a soil water deficit induced a drastic reduction of the N amounts in all the tree organs. Consequently, N dysfunction could be involved in drought-induced beech tree mortality under the future climate.

Agronomic characteristics of Tamani grass managed under different combinations of frequency and intensity of defoliation

Background: Management strategies may affect plant growth and herbage characteristics. Thus, understanding its impact may help to define appropriate management. Objective: To evaluate the effect of different defoliation intensities and frequencies on the structural characteristics, biomass components and the potential use of NDVI (normalized difference vegetation index) in pastures with Megathyrsus maximus cv. BRS Tamani. Methods: A randomized block design in a 2x3factorial arrangement was adopted, with two defoliation frequencies (85 and 95% of interception of photosynthetically active radiation (IPAR) and three defoliation intensities (residual leaf area index (LAIr) of 0.8, 1.3 and 1.8). Results: The frequency of defoliation affected the pre-defoliation leaf area index, height, total harvestable forage biomass (HTFB), and harvestable leaf blade (HGLB), with greater values for pastures managed at 95% of IPAR. The effect of intensity of defoliation was observed for the HTFB and HGLB variables, where pastures with lesser LAIr presented greater biomass values. Pastures managed at 95% of IPAR and higher LAIr reached the level of saturation of the normalized difference vegetation index more quickly. Pastures managed under the combination of 95% IPAR and LAIr of 0.8 showed greater production of harvestable green stem biomass and harvestable dead forage biomass. The combination of 95% of IPAR with LAIr of 0.8 or 1.8 enabled a greater number of new live leaves when compared to pastures with 85% of IPAR. Conclusions: Tamani grass must be managed with a frequency of defoliation of 95% of the interception of photosynthetically active radiation, maintaining a residual leaf area index between 0.8 and 1.3.

Endophytic Fungal Infection of Meadow Fescue in the Driftless Area of the Upper Mississippi River Valley: Impacts on Agronomic Fitness

Meadow fescue, Schedonorus pratensis (Huds.) P. Beauv., has recently been discovered as a common but previously unknown pasture grass in the Driftless Area of the upper Mississippi River Valley, USA. Preliminary data also indicated that many meadow fescue pastures were infected with an endophytic fungus, Epichloë uncinata (W. Gams, Petrini & D. Schmidt) Leuchtm. & Schardl. Therefore, the objective of this study was to determine if the endophyte impacts agronomic fitness of the host meadow fescue. Meadow fescue plants from eight farm sites were intensively sampled, and endophyte infection levels were determined to range from 82 to 95%. Paired endophyte-infected (E+) and endophyte-free (E−) meadow fescue subpopulations from each collection site were then created, and were subsequently compared for greenhouse and field drought tolerance, forage mass, and persistence under frequent defoliation. There was no impact of the endophyte under a wide range of drought conditions for either greenhouse or field studies. Furthermore, there was a small forage-mass-enhancement effect in the E+ subpopulation for only one of the eight collection sites. The only consistent effect was an average of 9% increased ground cover (persistence) in endophyte-infected meadow fescue under frequent defoliation. As per other studies, enhanced root growth, fungal-disease resistance, and/or reduced insect feeding could be mechanisms for this increased survivorship. We conclude that the meadow fescue endophytes present in the Driftless Area do not help protect their host from drought or provide any consistent forage-growth enhancement; however, we found evidence that the endophyte provides some protection against frequent defoliation at low residual sward heights.

Feed quality of modern varieties of Festuca arundinacea and Phleum pratense as an alternative to Lolium perenne in intensively managed grassland with different defoliation schemes

AbstractGrassland production based on cutting-only and an increasing frequency of prolonged periods of drought due to climate change could lead to decreased productivity in Lolium perenne. In field experiments, we tested whether Festuca arundinacea and Phleum pratense could be suitable alternatives to L. perenne on intensively managed grassland on clay, peat and sandy soil. The three grasses were sown in mixture with Poa pratensis and Trifolium repens and subjected to different frequencies of defoliation representing a cutting-only system, simulated grazing system and a mixed system. We found that in systems with at least six defoliations, F. arundinacea represented an acceptable compromise between feed quality (6.4 MJ net energy/kg dry matter (DM), 19.3% crude protein, CP), persistence (mass proportion >90%), and DM yield (12.7 Mg/ha). However, for dairy production based on intensive cutting-only systems, the quality of F. arundinacea was insufficient (5.9 MJ net energy/kg DM, 15.6% CP). Mixtures with P. pratense as the main sown species did not differ significantly in production of net energy and CP from L. perenne in cutting-only systems on sandy soil. On peat land, all sown mixtures were invaded by Holcus lanatus. We found that under frequent defoliation conditions, H. lanatus-rich swards had comparatively good DM yields (9.2 Mg/ha) and a feed quality that would be sufficient for dairy cow nutrition (net energy, 6.2–6.4 MJ/kg DM; 18.8–20.4% CP). We conclude that there is potential to adapt the choice of grasses and mixtures in different production systems to meet the challenges of climate change.

Continuous remobilization from below‐ground provides more than half of all carbon and nitrogen in regrowing shoots after grassland defoliation

Abstract Remobilization of carbon (C) and nitrogen (N) from roots and crowns to regrowing shoots is an important strategy for grassland plants to tolerate herbivory. While this remobilization is thought to terminate once plants have the capacity to photosynthesize again and to take up N from the soil, this has rarely been quantified. Here we investigated the remobilization of C and N to regrowing shoots of a perennial grassland under two defoliation frequencies (every 15 and 30 days) and two N fertilization levels (0 and 2.8 g m−2) by growing intact plant–soil cores for 60 days in a growth chamber. We measured the C remobilization from roots and crowns to regrowing shoots using a novel continuous 13C labelling method, and quantified the N remobilization to regrowing shoots and plant N uptake from soil with a 15N tracer method. Regrowing shoots relied strongly on below‐ground C reserves, where up to 52% of C in regrowing shoots came from below‐ground during the first 30 days after defoliation. Below‐ground C was still remobilized when plant shoots had fully regrown and retained full capacity to photosynthesize. Plants relied more on C remobilization with shorter times between defoliation events at the expense of root biomass, particularly without N fertilization. Large amounts of C remobilization were accompanied by large amounts of N remobilization (up to 89% of total shoot N) that were positively related, suggesting a coupled C and N transfer from roots and crowns to shoots. Synthesis. The unexpectedly high dependency on C and N remobilization occurred continuously after defoliation and did not terminate once plants were fully regrown. Our results indicate that remobilization of C and N from below‐ground has profound effects on long‐term productivity and C sequestration that needs to be considered for managing similar grasslands worldwide.

Prediction of the nutritional values by INRA (2018) feed evaluation system of Megathyrsus maximus subjected to different grazing strategies

Grazing management is a key element to optimize growth cycle of forages, which are enhanced in their chemical composition leading to the reception of greater nutritive values for feeding ruminants. Several studies have showed that the accuracy and precision of the feeding values are critical to achieve this goal, unfortunately, in Ecuador there are not referential data, despite of Megathyrsus maximus is the most forage used by livestock farmers. The predominant aim of the current study was to approach and estimate the feeding values by INRA feed evaluation system of Tanzania grass (Megathyrsus maximus) subjected at different grazing strategies. The statistical design was a randomized complete block, with a 3 × 2 factorial arrangement. Where the treatments were combinations of three grazing frequencies (GF) (30 d; termed GF30, 45 d, GF45 and 60 d, GF60) and two cutting heights (CHs; 30 and 45 cm). The agronomic data did not vary by season effect (P = 0.24 to 0.82), but GF60 had higher plant heigh and dry matter (DM) contents (P < 0.001) than other GF, although with less tiller populations (241 vs. 304 tiller/m2). By CHs effect, Tanzania grass at 30 cm had a greater DM content than 45 cm of CHs (5565 vs. 4221 ± 603 kg/ha-1; P = 0.073). All chemical determinations were affected by GF, CHs and their interaction (P < 0.001 to 0.004) with the exception on ADF between CHs (P = 0.50). Whereas only the CP and ADF values were conditioned by season effect (P = 0.001 to 0.011). Subsequently, Tanzania grass subjected at moderate rest periods and low defoliation intensities showed greater energy and proteins values than traditionally used GF60. Based in these findings, the defoliation frequency had influence on agronomic and chemical characteristics, whilst the feeding values was mainly affected by grazing intensities and weather conditions. So, the best combination regarding nutritive values should be at GF30 and GF45 combined with 45 cm of CHs.

Dry‐matter production and botanical composition of multispecies and perennial ryegrass swards under varying defoliation management

AbstractMultispecies swards, comprised of different plant functional groups have comparable production potential to high N input L. perenne swards at reduced N when legumes are included. However, information on the appropriate management of multispecies swards is limited. The effect of differing management practices on the herbage dry‐matter (DM) production and botanical composition of different sward types was investigated using a 3 × 2 × 2 factorial design plot experiment. The experiment consisted of three sward types: L. perenne‐only receiving (LP; 250 kg N ha−1 a−1); L. perenne‐Trifolium repens (LP‐TR); multispecies sward containing L. perenne, Phleum pratense, Trifolium repens, Trifolium pratense, Plantago lanceolata and Cichorium intybus (MSS). LP‐TR and MSS received 90 kg N ha−1 a−1. Harvesting of plot herbage took place every 21 or 28 days to a defoliation height of 4 or 6 cm, over two growing seasons (March to November 2020 and 2021 inclusive). Annual herbage produced by both LP and LP‐TR was not significantly affected by defoliation frequency. However, MSS produced 1923 kg DM ha−1 a−1 more herbage when harvested less frequently. Unsown species contributed significantly less to the herbage DM of MSS compared to LP and LP‐TR and remained similar in 2020 and 2021, whereas the contribution of unsown species to herbage DM increased in LP and LP‐TR from 2020 to 2021, with noxious species more associated with LP and LP‐TR than MSS. Results demonstrate the role of multispecies swards in improving the sustainability of grass‐based agricultural systems in an environment of reduced fertilizer inputs.

Frequent defoliation of perennial legume-grass bicultures alters soil carbon dynamics

Frequent defoliation of perennial legume-grass bicultures alters soil carbon dynamics

Forage intake and nitrogen metabolism of beef cattle grazing palisadegrass‐calopo mixed pasture managed using canopy light interception

AbstractTo define the best grazing management strategy, it is important to assess animals' responses to variations in the structure and composition of tropical forages. This 2‐year study evaluated animal response to Marandu palisadegrass (Urochloa brizantha)‐calopo (Calopogonium mucunoides) mixed pastures managed under rotational grazing. Treatments consisted of three defoliation frequencies defined by rest periods interrupted at 90% (90LI), 95% (95LI) and 100% (100LI) of photosynthetically active radiation interception (LI). The stubble post‐grazing height target was 15 cm. Statistical difference was declared at p < .10. The 100LI had lowest crude protein (CP) and in vitro dry matter digestibility (IVDDM) for the Marandu palisadegrass (p = <.001 both) and calopo (p = .003 and p = .067, respectively). Also, the OM digestibility decreased 7.0% in the 100LI condition than 90LI and 95LI treatment (p = .005). There was no difference in forage, grass, and legume intakes between the treatments (p > .10). The 100LI treatment decreased CP intake from grass in 33.3% (p = <.001) compared to other treatments. Greatest production of microbial N (p = .093) occurred with the 90LI treatment. The CP/digestible organic matter (DOM) ratio, urinary N excretion and retained N were lowest in the 100LI treatment (p = <.001, p = .007 and p = .014, respectively). The recommendation for grazing between 90 and 95% of LI is recommended because of greater CP intake and N utilization for the animals and improved the nutritive value of Marandu palisadegrass and calopo mixed pastures.

Grazing patterns, diet quality, and performance of cow-calf pairs using continuous or rotational grazing in the Texas panhandle USA

This study 1) measured diet quality and performance of cow-calf pairs; and 2) compared defoliation patterns and changes in standing crop of functional groups relative to the proximity of water over a grazing season under continuous (CG) or rotational (HSD) grazing at the same stocking rate and stocking intensity. Eighty lactating cow-calf pairs were weighed and assigned among shortgrass-dominated rangeland pastures (three replicates per treatment). Standing crop by species was measured at study initiation and completion. Frequency of grazed plants was measured along 0.39km point transects radiating from water troughs. Cow and calf weights and average daily gains were compared between treatments. Mean dietary CP was higher for CG cattle. Forage quality for major species (blue grama and gummy lovegrass) did not differ between treatments. Y-intercepts of regression lines predicting probability of defoliation differed between treatments following the 1st, 6th, and 7th 16-day intervals of the 112-day grazing study. Final standing biomass of gummy lovegrass was lower in HSD. Final cool-season annual grass biomass was lower in CG. HSD grazing can regulate the frequency and timing of defoliation, thereby changing relative proportions of plant species, inter-species competition and animal performance over time.

Growth Responses of the Perennial Grass, Phalaris Aquatica L., to Cutting Frequency and Influence on Secondary Metabolites and Antioxidant Activity

Highlights Defoliation initiates physiological recovery and chemical defense mechanisms in Phalaris aquatica. Under infrequent defoliation treatment, P. aquatica has high DM production. Defoliation severity on herbage regrowth was associated with variation of secondary metabolite content and antioxidant activity. Phalaris may be suited to conservation pasture systems; the interval between cuts is about six weeks to maximize rates of regrowth. Results indicate that Phalaris may be used as fodder crop to sustained production and food security. Abstract. Perennial grasses are the key to the economic and environmental sustainability of pastures for livestock, and in arid and semi-arid environments, they can provide multiple ecosystem services more effectively than production systems based on annual crops. The objective of this study was to evaluate the effect of different defoliation frequencies on forage production and nutritive value of the Phalaris aquatica L. variety Soukra under field conditions in Tunis, Tunisia, over a period of 12 weeks. We tested four defoliation frequencies: (1) severe, (2) moderate, (3) infrequent, and (4) control. The growth responses measured were plant tiller number (NT), dry matter production (DM), and relative leaf regrowth rate (RLR). DM under the severe and moderate defoliation frequencies was 7% and 41% less than under control defoliation, respectively. However, DM production under infrequent defoliation was 91% and 43% higher than under severe and moderate defoliation. The relative leaf regrowth rate was affected by defoliation frequency; the highest regrowth rate was under severe treatment. However, tillering of P. aquatica was reduced under the severe and moderate frequencies of defoliation. Under increased defoliation frequencies, concentrations of secondary metabolites significantly decreased; total polyphenol content, flavonoid content, and tannin contents were higher in control and infrequent than in moderate and severe treatments. Antioxidant activity also decreased significantly with defoliation compared to the control treatment. There were no significant differences (P > 0.05) in ABTS (3-ethylbenzothiazoline-6-sulfonic acid) among the defoliation frequencies. Pearson's r correlation and PCA (Principal component analysis) data revealed that growth parameters, secondary metabolites, and antioxidant activity have positive and negative correlations in distinguishing the control and defoliation treatments. Results indicate that P. aquatica management should target moderate harvest rates in the adoption of perennial grass forage production systems in Tunisia. Use of perennial grasses for forage production can contribute to sustained production, food security, and rural livelihoods, and move farming systems towards providing multiple economic, environmental, and social benefits. Keywords: ABTS, Defoliation frequency, DPPH, Flavonoids, Growth responses, Matter production, Perennial grasses, Phalaris, Polyphenols.

Development of a lucerne model in APSIM next generation: 3 Biomass accumulation and partitioning for different fall dormancy ratings

Modelling lucerne growth requires response functions that represent seasonal partitioning of biomass into above-ground and below-ground organs. An additional challenge is to parameterize perennial organ responses across contrasting fall dormancy (FD) genotypes. Current models use empirical approaches to simulate biomass accumulation and partitioning. This research integrated knowledge of lucerne biomass accumulation and partitioning into the Agricultural Production Systems sIMulator (APSIM) next generation (APSIM NextGen) model framework. Biomass supply was calculated from light interception and total radiation use efficiency (RUEtotal), and then allocated based on the relative demand of each organ. Leaf biomass demand was parameterized as a function of specific leaf area (SLA). Stem biomass demand was parameterized as a positive power function of shoot biomass. Root biomass (taproots and crowns) showed a strong seasonal pattern. The observed decrease of root biomass in periods of an increasing photoperiod (mid-winter to mid-summer) was assumed as remobilization to shoots and carbon loss from maintenance respiration. Periods of decreasing photoperiod showed increased biomass of root caused by greater carbon partitioning to this organ. To capture this, a model optimization approach was used to fit required parameters. Fitted parameters included a remobilization coefficient (percentage of storage biomass per day) of 0.01common to all FD cultivars tested (FD5, FD2 and FD10). The regrowth coefficient (remobilization duration) remained constant at 0.01 post-defoliation until 250 °Cd for FD5, 200 °Cd for FD2 and 300 °Cd FD 10, and then declined to 0 after another 50 °Cd. The model was parameterized to have maximal root demand in a decreasing photoperiod to capture carbon partitioning. The model had good prediction of shoot biomass (NSE=0.70) and fair prediction (NSE=0.60) of root biomass for 42 day defoliation treatments. It was less accurate for predictions of shoot biomass under a frequent (28 day) defoliation regime. This highlights the importance to include the response to limitations caused by depleted root N reserves in future model versions. The APSIM NextGen lucerne model provided a mechanistic framework to model perennial organ biomass dynamics with structural and storage components, root maintenance respiration, remobilization in spring, partitioning in autumn and the regrowth effect. This framework accounted for differences in fall dormancy of genotypes and provided a methodology that can be integrated into models of other perennial crops.

Canopy dynamics of lucerne (Medicago sativa L.) genotypes of three fall dormancies grown under contrasting defoliation frequencies

This study evaluated the canopy dynamics of three lucerne (Medicago sativa L.) genotypes of different fall dormancy (FD) ratings when grown under three contrasting defoliation regimes over five regrowth seasons at Lincoln University, Canterbury, New Zealand. Crops were sown in October 2014 in a split-plot design with main plots as defoliation frequencies (28 days [DF 28], 42 days [DF 42] and 84 days [DF 84]) and genotypes as subplots. Genotypes varied in fall dormancy (FD) rating and were classified as dormant (cv. AgR Palatable, FD2), semi-dormant (cv. Grassland Kaituna, FD5), or non-dormant (cv. SARDI 10, FD10). All crops were grown through an establishment phase until January 2015, when defoliation treatments were imposed. The experiment terminated in May 2019. Defoliation treatments were imposed from early spring (August-September) to autumn (May) with a single clean-up graze in late June/July. Defoliation frequency x genotype interactions showed FD10 was the most productive in regrowth season 1 but least persistent and productive in regrowth season 5, particularly under the DF 28 and DF 42 treatments. Results suggest different partitioning responses amongst genotypes. In a decreasing photoperiod, the growth rate of FD2 decreased by 0.77 ± 0.07 kg DM ha−1 °Cd−1 compared with 0.67 ± 0.09 kg DM ha−1 °Cd−1 for FD5 and FD10. However, FD10 showed no response to an increasing photoperiod under the DF28 defoliation regime. The lower rate of partitioning to perennial biomass of FD10 was suggested by the rapid decline in shoot density (−0.000112 shoot m−2 °Cd−1) over time compared with − 0.000049 shoot m−2 °Cd−1 for FD2 and FD5. FD10 had heavier individual shoot mass over the last two years, but its canopy plasticity was insufficient to control the shoot size/density balance, so there was greater weed ingress. The DF 84 treatment enabled FD10 to produce > 15 t DM ha−1 year−1 over the five regrowth seasons, which was not different to FD2 and FD5. Under more frequent defoliations, FD2 and FD5 were more persistent and had higher yields than FD10.

Forage productivity and morphogenesis of Mesosetum chaseae under defoliation frequency

The effect of defoliation frequency (14, 21, 28, 35, 42, 49 and 56 days) on forage production and morphogenic and structural characteristics of Mesosetum chaseae (Luces) was evaluated under natural field conditions during the dry season, in the savannas of Roraima. The reduction in pasture defoliation frequency resulted in higher green dry matter (GDM) yields, number of tillers plant-1, number of leaves tiller-1, tiller leaf area, leaf senescence rate and average leaf size. The relationship between defoliation frequency and GDM production was quadratic, with the maximum value estimated at 53.8 days of regrowth. Absolute rates of growth and leaf appearance and expansion were inversely proportional to defoliation frequencies. The highest rate of leaf expansion was obtained at 49.7 days of regrowth. Aiming to reconcile forage productivity with the optimization of the morphogenic and structural characteristics of the grass, the most appropriate period of use of its pastures is between 42 and 49 days of regrowth.

Nitrogen addition and defoliation alter belowground carbon allocation with consequences for plant nitrogen uptake and soil organic carbon decomposition

Grassland plants allocate photosynthetically fixed carbon (C) belowground to root biomass and rhizodeposition, but also to support arbuscular mycorrhizal fungi (AMF). These C allocation pathways could increase nutrient scavenging, but also mining of nutrients through enhanced organic matter decomposition. While important for grassland ecosystem functioning, methodological constraints have limited our ability to measure these processes under field conditions. We used 13CO2 and 15N pulse labelling methods to examine belowground C allocation to root biomass production, rhizodeposition and AMF colonisation during peak plant growth in a grassland field experiment after three years of N fertilisation (0 and 40 kg N ha−1 year−1) and defoliation frequency treatments (“low” and “high”, with 3–4 and 6–8 simulated grazing events per year, mimicking moderate and intense grazing, respectively). Moreover, we quantified the consequences for plant nitrogen (N) uptake and decomposition of soil organic C (SOC). Nitrogen fertilisation increased rhizodeposition and AMF colonisation (by 63 % and 54 %), but reduced root biomass (by 25 %). With high defoliation frequency, AMF colonisation increased (by 60 %), but both root biomass and rhizodeposition declined (by 35 % and 58 %). Plant N uptake was highest without N fertilisation and low defoliation frequency, and positively related to root biomass and the number of root tips. Therefore, when N supply is low and the capacity to produce C through photosynthesis is high, belowground C allocation to root production and associated root tips was important to scavenge for N in the soil. In contrast, the strong positive relationship between the rate of rhizodeposition and SOC decomposition, suggests that rhizodeposition may help plants to mine for nutrients locked in SOC. Taken together, the results of this study suggest that belowground C allocation pathways affected by N fertilisation and defoliation frequency affect plant N scavenging and mining with important consequences for long-term grassland C dynamics.