Tropical Grass Pastures Research Articles

Physical and biochemical characteristics of tropical grass and legume pastures grazed by lambs

AbstractLarge areas of tropical grass pastures are not grazed by lambs because of the difficulties in managing high growth rate swards with high selective animals. The objective of this study was to evaluate the physical and biochemical characteristics of tropical pastures offered to lambs in continuous grazing. An upright grass and a shrub legume were set out in three pasture types and grazed by lambs: (1) aruana grass monoculture (AG—Panicum maximum Jacq. cv. IZ‐5), (2) pigeon pea monoculture (PP—Cajanus cajan [L.] Millsp. cv. Anão) and (3) contiguous swards (CS), half of the paddock with AG (CSAG) and half with PP (CSPP). The pastures were evaluated for structural characteristics, production, nutritional composition and antioxidant concentrations in four periods over 92 days of continuous grazing by lambs. Hand plucking samples, similar to animal diet, were collected for chemical analysis. Regarding height, the PP legume monoculture after 42 days of grazing had uncontrollable growth by the lambs, reaching 1.2 m in height. This same legume but as a CS beside AG (CSPP) was maintained at a lower and similar height throughout the experimental periods. In general, the leaf:stem ratio of the different pastures decreased over the experimental periods from 0.7 to 0.2. In most periods, the CS showed intermediary nutritional quality compared to AG and PP. The alpha‐tocopherol content was similar among swards, with an average of 137.2 ± 13.67 mg/kg of green matter (p > .05). Pigeon pea showed the highest levels of total tannin and condensed tannin at 63 and 92 days (p < .05). The use of tropical grass together with a legume provides a better physical structure of the pigeon pea for grazing lambs than when monoculture of this species.

Nitrogen niche partitioning between tropical legumes and grasses conditionally weakens under elevated CO2

Abstract Plant community biodiversity can be maintained, at least partially, by shifts in species interactions between facilitation and competition for resources as environmental conditions change. These interactions also drive ecosystem functioning, including productivity, and can promote over‐yielding‐ an ecosystem service prioritized in agro‐ecosystems, such as pastures, that occurs when multiple species together are more productive than the component species alone. Importantly, species interactions that can result in over‐yielding may shift in response to rising CO2 concentrations and changes in resource availability, and the consequences these shifts have on production is uncertain especially in the context of tropical mixed‐species grasslands. We examined the relative performance of two species pairs of tropical pasture grasses and legumes growing in monoculture and mixtures in a glasshouse experiment manipulating CO2. We investigated how over‐yielding can arise from nitrogen (N) niche partitioning and biotic facilitation using stable isotopes to differentiate soil N from biological N fixation (BNF) within N acquisition into above‐ground biomass for these two‐species mixtures. We found that N niche partitioning in species‐level use of soil N versus BNF drove species interactions in mixtures. Importantly partitioning and overyielding were generally reduced under elevated CO2. However, this finding was mixture‐dependent based on biomass of dominant species in mixtures and the strength of selection effects for the dominant species. This study demonstrates that rising atmospheric CO2 may alter niche partitioning between co‐occurring species, with negative implications for the over‐yielding benefits predicted for legume‐grass mixtures in working landscapes with tropical species. Furthermore, these changes in inter‐species interactions may have consequences for grassland composition that are not yet considered in larger‐scale projections for impacts of climate change and species distributions. Read the free Plain Language Summary for this article on the Journal blog.

Hard-seeded temperate annual legumes establish better in a tropical grass pasture when autumn-sown than spring-sown in a summer dominant rainfall zone, Australia

ABSTRACT Temperate annual legumes can be effective companion species in tropical perennial grass pastures. These legumes are commonly sown in autumn following establishment of the grasses. However, it may be more effective to establish a seed bank before sowing the grass, or possibly sow the legumes as hard seed/pod in spring when the tropical grass pasture is sown. A study was conducted in northern New South Wales to compare establishment, regeneration and productivity of three hard-seeded legumes: bladder clover (Trifolium spumosum) cv. Bartolo, biserrula (Biserrula pelecinus) cv. Casbah and French serradella (Ornithopus sativus) cv. Margurita. The legumes were sown four times: either one or two autumns before digit grass (Digitaria eriantha) cv. Premier was established, the autumn following grass establishment or sown at the same time as the grass as either hard seed/pod segments or scarified seed (total of five treatments). While seasonal conditions influenced establishment success, autumn was the optimal time to sow temperate annual legumes to achieve a productive mixed pasture. The legumes can be sown 1–2 autumns before or in the autumn following grass establishment. Legume plant populations of spring sown hard seed/pod segments were lower than those autumn sown but they increased the following year.

Seedling emergence of tropical perennial pasture species in response to temperature used to determine sowing time recommendations

Tropical pasture grasses and legumes can be highly productive and persistent and fill the summer-autumn feed gap typical of temperate pasture systems in southern Australia. However, more information is needed on optimum temperature range for seedling emergence because this will influence sowing time recommendations. A replicated field experiment was conducted at 5 locations in New South Wales over a 12-month period to determine the optimum temperature for emergence of a range of tropical species: Rhodes grass (Chloris gayana Kunth), Makarikari grass (Panicum coloratum L. var. makarikariense Gooss.), kikuyu grass (Cenchrus clandestinus (Hochst. ex Chiov.) Morrone), digit grass (Digitaria eriantha Steud.), panic grass (Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. Jacobs), paspalum (Paspalum dilatatum Poir.), Urochloa hybrid (Urochloa decumbens × U. ruziziensis × U. brizantha), 2 cultivars of Desmanthus virgatus (L.) Willd. (cultivars ‘Marc’ and ‘JCU2’), D. bicornutus (S. Watson), D. leptophyllus (Kunth) and D. pernambucanus (L.) Thell.). Rhodes grass emerged satisfactorily over the longest time across all sites, exhibiting the greatest temperature range over which emergence occurred, while Makarikari grass and panic grass had the narrowest temperature range for emergence. The temperature for 50% emergence differed between the tropical species and whether the soils were warming or cooling. Rhodes grass had the lowest 50% emergence temperature (17 °C) while paspalum had the highest (22 °C). Results showed that temperature for 50% emergence is a useful indicator for determining sowing time in warming soils.

Behavioural compensation of extruded urea level and the effects of supplementation presencefor finished lambs in tropical grass pastures

The objective was to investigate how different levels of soybean meal replacement with extruded urea (EU) in sheep supplementation affected pasture characteristics (Brachiaria spp.), consumption, ingestive behavior and animal performance. Sixty Texel crossbred lambs were randomly divided between treatments: 0, 6, 12, 18 and 24 g of 100 kg−1 body weight (BW) EU supplement under continuous stocking in Brachiaria spp. The animals receiving a supplement in the proportion of 24 g 100 kg−1 BW from the EU had the highest pasture height (39.5 cm) as compared with other treatments. The levels of replacement of soybean meal by the EU had no effect on the supply of forage, forage accumulation rate or structural characteristics. The forage dry matter intake (DMI) as percentage of BW and metabolic BW were adjusted to quadratic equations, and the minimum levels of DMI in relation to BW (1.5 %) and metabolic (55.6 g DM/kg0.75) were estimated at 10 g and 9.4 g 100 kg−1 BW of the EU, respectively. The grazing time of the lambs decreased linearly (P = 0.006) by 4.88 min for each increase in the EU level. In contrast, as EU levels in lamb supplementation increased, the time taken to consume the supplement and the bite rate increased by 4.38 min and 0.28 bites/minutes, respectively. The average daily gain increased linearly up to 18 g 100 kg−1 BW (146 g/day) (P = 0.034). For lambs finishing in Brachiaria spp., the EU can be used in place of soybean meal up to 18 g 100 kg-1 BW without altering animal performance, consumption, or the structural characteristics of the pasture.

Herbage intake and performance of grazing lambs in tropical erect grass pastures maintained at different heights

One of the main constraints for sheep production in tropical and subtropical areas is the lack of knowledge of how to manage erect tropical grasses for lambs. The objective of this study was to assess the effect of pasture height and structure of an erect tropical grass, Aruana Guinea grass (Megathyrsus maximus cv. IZ-5) on the intake and performance of young weaned lambs. The experiment was repeated in two consecutive years. Thirty young weaned lambs were equally assigned, each year, to three pasture heights: (1) Tall 75 cm; (2) Medium 50 cm and (3) Short 25 cm. Herbage mass (HM), pasture height, plant morphological composition, pasture nutritional quality, lambs’ average daily gain, gain per hectare and herbage intake were measured. The Short treatment had higher (p < 0.05) leaf:stem ratio (L:S; 2.6 ± 0.27) and average daily gain (ADG; 92 ± 10 g day−1) than the Tall treatment (0.9 ± 0.11 and 40 ± 13 g, respectively). The Medium treatment showed intermediate ADG (69 ± 11 g) and L:S ratio (1.4 ± 0.09). The height of the pasture affects the rate at which the number of stems increases relative to the number of leaves. There was a significant interaction between treatment and period (summer and early autumn) for herbage intake. Intake decreased markedly on the Tall treatment from the first to the second period. This study showed that for a better performance of weaned young lambs during summer–autumn, it is important to maintain tropical swards at a short height (25 cm). However, if the grass height exceeds this condition, the use of cattle or a mower are good options to maintain the sward height.

Spatiotemporal variation in aboveground biomass in a tropical grass pasture over a 22‐year period

AbstractThis study monitored spatiotemporal variation in aboveground biomass of a paddock (1.1 ha) in a sown tropical grass pasture under cattle grazing over a 22‐year period (1995–2016) by nondestructively estimating biomasses of 182 fixed locations (0.5 × 0.5 m each) in the paddock at about monthly intervals during the annual stocking season (May to October or November). The mean aboveground biomass of the paddock varied seasonally and interannually between 33 and 605 g DM/m2 and was explained by a regression model including positive effects of mean air temperature, N fertilizer rate and total precipitation during the preceding 60 days (R2 = 0.311, P &lt; 0.001). The coefficient of variation of aboveground biomass, as an indicator of spatial (within‐paddock) variability, varied seasonally and interannually between 0.16 and 0.94, and was described by a model incorporating a positive effect of cumulative stocking density during the preceding 40 days and negative effects of mean air temperature, total precipitation during the preceding 20 days and N fertilizer rate during the preceding 60 days (R2 = 0.277, P &lt; 0.001). The spatial distribution pattern of aboveground biomass in the paddock was persistent (P &lt; 0.05) or not persistent over the study period, almost without being reversed. In detail, patterns in the mid stocking season tended to maintain mutual similarity in the entire range of the intervals between the measurements (364–7,694 days), whereas those at the beginning and near end of the stocking season lost mutual similarity as the interval increased. There was no indubitable evidence that the changes in botanical composition destabilized the spatial distribution pattern of aboveground biomass. The results indicate the need for assessing how the long‐term stability of the within‐paddock distribution pattern of aboveground biomass affects agronomic and ecological performance of the grazing system.

Effect of the Interaction between Excreta Type and Nitrogen Fertilizer on Greenhouse Gas and Ammonia Emissions in Pastures

This study aimed to evaluate the emission factor of N2O, CH4, and the volatilization of NH3 for the combination of feces or urine with increasing doses of ammonium nitrate in tropical palisade grass pastures. The emission of greenhouse gases was assessed in eight treatments combining feces and urine with doses (75 and 150 kg of N ha−1) of ammonium nitrate, (32% N). The emission factor of N2O was 0.11, 0.19, and 0.17% for feces, urine, and 75 kg N ha−1 year−1 (as ammonium nitrate) and showed an additive linear effect when feces or urine were combined with increasing doses of N fertilizer. The emission factor of CH4 of feces (0.18 kg CH4 animal−1 year−1) was similar irrespective of combination with ammonium nitrate. The N loss by volatilized NH3 has a decreasing linear effect (p &lt; 0.05) for the combination of feces or urine with ammonium nitrate. We concluded that N2O and CH4 emission factors of feces and urine in tropical climate conditions are lower than those reported by the IPCC. However, their N2O emission factors are sharply enhanced when combined with ammonium nitrate. These results may contribute to improvements in national and regional greenhouse gas inventories of livestock production.

Evaluation of the productivity and feed value of Wondergraze and Redlands leucaena cultivars under grazing

Context Leucaena leucocephala (leucaena) is a leguminous shrub used for beef grazing in low-rainfall regions (&lt;600–700 mm). Newer cultivars have the potential to extend adoption of the species to higher rainfall (&gt;600–700 mm), frost-free areas of Australia. Aim We compared productivity, nutritional value and animal performance of two leucaena cultivars, new psyllid-resistant Redlands and the 2010-released Wondergraze, under continuous grazing management in a higher rainfall environment. Methods Growing steers were allocated to replicated established stands of Wondergraze or Redlands with inter-row mixed grass–legume pasture from January to July 2021. Pasture and leucaena were characterised for biomass and nutritive characteristics. Botanical composition was measured. Liveweight gain, rumen fermentation, and leucaena mimosine breakdown products were measured in grazing steers. Key results At the beginning of the study, leucaena edible biomass was similar for both cultivars (P &gt; 0.05), but at subsequent samplings, biomass of Redlands was lower than of Wondergraze (P &lt; 0.01). Biomass of both cultivars declined rapidly over the grazing period. Pasture biomass increased between February and July and was significantly higher in Wondergraze paddocks (P &lt; 0.05). Animal performance was not significantly different between cultivar treatments, averaging 0.8 kg/day, but declined over time. Patterns of mimosine conversion to DHP isomers and their conjugation were similar for the two cultivars, suggesting that effectiveness of detoxification did not differ between them. Conclusion The results demonstrate that leucaena can sustain high levels of animal performance when included in tropical grass pastures in a higher rainfall environment if present in a sufficient quantity (&gt;2 t leucaena edible dry matter/ha established leucaena). Implications Grazing leucaena–grass pastures is an effective means of increasing animal productivity in parts of subtropical Australia. However, managing grass and/or leucaena growth to match animal requirements can be challenging.

Current Scenario and Perspectives for Nitrogen Fertilization Strategies on Tropical Perennial Grass Pastures: A Review

Traditional Nitrogen (N) fertilization practices for tropical perennial grass are still based on annual amounts, following flat N rates instalments. This strategy does not consider variations in climatic conditions along the growing season, their impacts on the plant’s demand and the soil N availability. At regrowth cycles where the amount of soil N released from mineralization or through animal excreta surpass the plant’s demand, most of the N may be lost (as ammonia, nitrate, and nitrous oxide), increasing production costs and environmental pollution. This paper examines current N fertilization and discusses possible gaps in knowledge for the definition of more precise fertilization guidelines in pasture-based livestock systems based on tropical perennial grasses. More precise fertilization practices, based upon site and seasonal-specific recommendations, will substantially contribute to the establishment of best fertilization guidelines. Sustainable approaches can be defined by combining the identification of regrowth cycles where high N rates are required, with enhanced efficiency fertilizers, and/or using grass species with the potential for biological nitrification inhibition. The lack of information on tropical grasses requirements, and soil and climatic factors driving the N fate into the mineralization and immobilization processes and how these factors affect plant’s N demand, still prevents opportunities for tactical applications and the establishment of best management guidelines.

Fermentation Quality of Silages Produced from Wilted Sown Tropical Perennial Grass Pastures with or without a Bacterial Inoculant

High growth rates and rapid reproductive development and associated decline in feed quality of sown tropical perennial grass pastures present management challenges for livestock producers. Conservation of surplus forage as silage could be an effective management tool. Experiments were conducted to evaluate the fermentation quality of silages produced from tropical grasses. Five species (Chloris gayana, Megathyrsus maximus, Panicum coloratum, Digitaria eriantha and Cenchrus clandestinus) were ensiled without additives after a short, effective wilt at dry matter (DM) contents ranging from 302.4 to 650.1 g kg−1. The fermentation profile of all silages in 2019 was typical for high DM silages, but in 2020 ammonia (% of total nitrogen: NH3-N), acetic acid and pH levels were higher. In 2020 M. maximus (302.4 g kg−1 DM) was poorly preserved with 20.2% NH3-N. The DM content of all other silages exceeded 350 g kg−1 and fermentation quality was generally good. In a second experiment, M. maximus was ensiled at 365 g kg−1 chopped and 447 g kg−1 DM chopped and unchopped, either without or with Pioneer 1171® (Lactobacillus plantarum and Enterococcus faecium) or Lallemand Magniva Classic® (L. plantarum and Pediococcus pentasaceus) bacterial inoculant. Inoculants increased lactic acid production, reduced pH and improved fermentation compared to Control, but D-lactate, L-lactate and acetic acid production differed between inoculants. Unchopped silages had higher pH and NH3-N and better preserved protein fraction than chopped silages at the same DM content. In both experiments, wilting increased water soluble carbohydrates by 0.5–31.5 g kg−1 DM and ensiling increased degradation of the protein fraction. We concluded that a rapid and effective wilt combined with a bacterial additive resulted in well preserved tropical grass silages.

Availability to ruminants of nitrogen in senesced C

Context Nutritional standards usually assume that ∼10% of the total nitrogen (TN) in forages is indigestible and hence not available to the ruminant. Senesced tropical C4 grasses often contain TN concentrations that are marginal or deficient to meet the nutrient requirements of ruminants, and low TN availability will exacerbate N deficiencies. Aim The aim of the study was to estimate the availability (i.e. digestibility) to ruminants of TN in mature and senesced C4 grasses, using data derived from previous experiments and published data. Methods In Dataset 1, forages grown in subtropical or tropical environments, including C4 (n = 143) and C3 (n = 15) grasses, were analysed for parameters including TN, acid detergent fibre (ADF), and N insoluble in ADF solution (ADIN). ADIN was used as a measure of unavailable TN. The Dataset 2 analysis included published measurements of TN and ADIN in C4 (n = 187) and C3 (n = 45) grasses. Key results In Dataset 1, TN averaged 9.7 and ADIN 1.45 g N/kg diet DM in C4 grasses. ADIN concentration could be predicted from TN and ADF concentrations by multiple regression (R2 = 0.50; P &lt; 0.001). The ratio ADIN/TN averaged 170 g/kg and increased exponentially with increasing ADF concentration (R2 = 0.43; P &lt; 0.001). Also, ADIN/TN was inversely related to TN concentration and DM digestibility. In C4 grasses containing &gt;400 and &gt;500 g ADF/kg DM, ADIN/TN averaged 190 and 230 g/kg, respectively, and in those containing &lt;10 g TN/kg DM, ADIN/TN averaged 194 g/kg. In these low-quality C4 grasses only ∼80%, and as little as ∼50%, of TN was available to ruminants. Dataset 2 produced similar results, although C4 grass forages were generally of higher quality, most having been harvested during vegetative growth. ADIN/TN was much lower in C3 grasses (89 g/kg). Conclusions In senesced, low-quality C4 grasses containing &gt;400 g ADF, &lt;10 g TN or &lt;530 g digestible DM/kg, the proportion of TN available to ruminants is substantially lower than that assumed for forages in general. Implications Low availability of TN in many senesced C4 grasses needs to be considered when evaluating the adequacy of dietary TN for ruminants grazing senesced tropical grass pastures.

Tannin-rich forage as a methane mitigation strategy for cattle and the implications for rumen microbiota

Context Methane from ruminant livestock systems contributes to the greenhouse effect on the environment, which justifies the adoption of novel feed strategies that mitigate enteric emissions. Aims We investigated the effects of the condensed tannin (CT)-rich legumes Flemingia macrophylla, Leucaena leucocephala, Stylosanthes guianensis, Gliricidia sepium, Cratylia argentea, Cajanus cajan, Desmodium ovalifolium, Macrotyloma axillare, Desmodium paniculatum and Lespedeza procumbens on in vitro methane emissions and rumen microbiota for beef cattle. Methods Four rumen-cannulated Nellore cattle grazing a tropical grass pasture were used as inoculum donors. Key results Real-time quantitative polymerase chain reaction analysis revealed that the abundance of Ruminococcus flavefaciens, methanogenic archaea and protozoa populations were reduced (P £ 0.05), whereas total ruminal bacteria were enhanced in the presence of CT. Our study also revealed a positive (P £ 0.05) relationship between CT and Fibrobacter succinogenes abundance. Reactive CT from L. leucocephala, D. paniculatum and L. procumbens resulted in decreased (P £ 0.05) isoacid content and methane production. Conclusions L. leucocephala, D. paniculatum and L. procumbens have the potential to suppress rumen methanogenesis. However, in vitro fermentation of L. leucocephala resulted in greater (P £ 0.05) degradability percentages than the other two species. Implications CT in legume species will have potential as part of an overall nutritional strategy to manipulate rumen microbiota and mitigate enteric methanogenesis in livestock production systems.

How lamb production systems can affect the characteristics and sward structure of Tifton 85 pasture?

Abstract Aimed at understanding how lamb production systems can affect sward structure in tropical grass pastures, the productive, structural and morphological characteristics of Tifton 85 (Cynodon spp.) pasture were evaluated in four systems: non-weaned and non-supplemented lambs (NW); non-weaned lambs receiving concentrate supplementation in creep feeding (CF); early-weaned lambs not supplemented (W); and early-weaned lambs supplemented with concentrate (WS). The trial lasted 105 days and was set out in a randomized block design with four treatments (systems), three replications (paddocks) and five lambs per replication. In the systems with weaning, lambs were weaned at 46 ± 6 days of age and immediately allocated to the paddocks; in the systems with supplementation, lambs received daily a protein–energy concentrate at 2% body weight (BW) on a dry matter (DM) basis. Continuous stocking management was used and the forage allowance was kept at 12% BW in DM/day in all systems. Changes in the structural and morphological characteristics of pasture were more intense up to 63 days of grazing. Sward height and forage mass were higher in WS compared to NW and CF systems, and forage accumulation rate was also higher in WS compared to the other systems. The mass of leaves and stems were higher in the WS system than in NW. Higher heterogeneity of pasture morphological characteristics was observed when weaning was performed. Forage mass was concentrated up to 20 cm sward height in the systems without weaning, with stem mass concentrated in the 0–10 cm stratum and leaf mass highly exposed to grazing in the 10–20 cm stratum. The systems with weaning presented the greatest dispersion of leaves and stems in all sward strata, which increased when the concentrate supplementation was provided to the lambs. Independent of concentrate supplementation to the lambs, the systems without weaning resulted in a better sward structure, with higher leaf/stem ratio for the animals on Tifton 85 pasture. In addition, supplementation of weaned lambs had a deleterious effect on sward structure.

Impact of a tannin extract on animal performance and nitrogen excretion of dairy cows grazing a tropical pasture

Context Tannin extracts have been investigated as natural feed additives with the potential to decrease the enteric emission of methane and urinary N and to improve the productive performance of ruminants. However, the impact of this additive in dairy cattle grazing tropical grass pastures has not been extensively evaluated. Aims To evaluate the impact of the Acacia mearnsii bark extract (TA) on productive and nutritional variables in dairy cows grazing a Cynodon dactylon pasture and receiving supplementary corn silage and concentrate. Methods Fourteen multiparous Holstein dairy cows were assigned in a randomised block design to either of the following two treatments: concentrate without TA (control) or with 10 g of TA/kg dry matter (DM). The concentrate and corn silage accounted for ~0.30 and 0.30 of total DM intake respectively. Key results The dietary concentration of TA was 2.9 g/kg DM and it did not affect nutrients intake, milk production and composition, rumen microbial protein synthesis, plasma urea concentration or faecal N excretion. Total N and urea N excreted in urine were higher, whereas total N excreted in manure tended to be higher in TA treatment. Conclusions No productive advantage was obtained by including TA in the diet of dairy cows grazing Cynodon dactylon pasture. Instead, it increased the excretion of labile urinary N. Implications The effect of the use of TA as a feed additive for cows fed tropical grass-based diets is negative from the environmental point of view.

Beneficial soil profile differences associated with tropical grass pastures on sodic texture contrast soils in Northern New South Wales

Volunteer native pastures on widespread sodic texture contrast soils in northern New South Wales slopes and plains are known for their limited agricultural production. Fertilised tropical grass pastures on these soils are reported to have much increased pasture production, deeper, more abundant root mass and greater soil profile moisture storage. The subsoil physical differences between native and tropical grass pastures are not well understood. This observational study compared root abundance, soil structure and soil physical parameters (dispersion, bulk density, porosity and pore distribution) in sodic texture contrast soils under native and adjacent, well established and fertilised tropical pastures in a 14-year chronosequence. The physical differences observed may have contributed to improved soil water storage reported by other authors. Fourteen years after establishment, mean root abundance was significantly lower in soils under native pasture and greater in the tropical grass pasture system with 4600 and 8400 m of roots m–3 respectively. Dispersion values were high in native pastures but soils under tropical pastures had to be physically worked to cause dispersion. Bulk density under native pasture was significantly higher than in tropical grass pastures by 0.08 g cm–3 at 0–10 cm and by 0.2 g cm–3 in the upper B horizons. Total soil porosity of topsoils and upper B horizons was consequently lower in native than in tropical grass pasture. Tropical grass pasture upper B horizons had a three-fold greater macroporosity (pores &amp;gt; 30 µm), than under native pastures. This is equivalent to significantly greater potential water flow through stable macropores in dense sodic B horizons in tropical pastures. These findings indicate that pasture system selection and management positively affects deep soil structural properties which promote pasture productivity. The study contributes to a better understanding of mechanisms of published deeper water storage in tropical grass pasture systems on these normally low production soils.

Methane production and nitrogen balance of dairy heifers grazing palisade grass cv. Marandu alone or with forage peanut.

Livestock production systems are an essential agribusiness activity in Brazil, but a critical challenge of Brazilian farmers is to maintain the equilibrium of the ecosystem, using herbage resources efficiently with a minimum impact on the environment. Nitrogen (N) fertilization and the inclusion of forage legumes into tropical grass pastures are management strategies which increase the productivity and nutritive value of pastures and may also affect methane (CH4) production by ruminants. The objective of this study was to examine the effects of either fertilizing palisade grass pastures with N or including the forage peanut (Arachis pintoi) into grass pastures on enteric CH4 emission, microbial protein production in the rumen via purine derivatives in the urine, and N balance. Twenty-one nonlactating crossbred dairy heifers were used in a completely randomized design with 3 treatments. The treatments consisted of pastures of palisade grass without N fertilization (control), fertilized with urea (fertilized), and palisade grass mixed with forage peanut (mixed). Seven animals (replications) were used to evaluate dry matter intake, digestibility, CH4 emission, urea, purine derivatives, and volume of urine, and N ingestion and excretion. Four paddocks (replications) were used to measure herbage mass; morphological, botanical, and chemical composition of herbage; and herbage allowance. The CH4 emissions were determined using the sulfur hexafluoride (SF6) tracer gas technique. The efficiency of N utilization (ENU) was calculated using the N balance data. Crude protein (CP) concentration of herbage increased with fertilization or legumes inclusion (P < 0.0001) while neutral detergent fiber (NDF) concentration decreased (P = 0.0355). The leaf allowance was higher in the fertilized treatment (P = 0.0294). Only uric acid excretion increased with N fertilization (P = 0.0204). The ENU was not affected by fertilized or mixed compared to control and averaged 55% (P = 0.8945). The enteric CH4 production was similar between treatments and averaged 129 g/d (P = 0.3989). We concluded that the changes in chemical composition of herbage provided by N fertilization or the inclusion of the legume showed no reduction in enteric CH4 emissions, but the ENU was more significant than previous studies with palisade grass, suggesting that different management strategies might alter the ENU under grazing conditions.

&lt;i&gt;Leucaena leucocephala&lt;/i&gt; introduction into a tropical pasture in the Chaco region of Argentina. Effects on soil carbon and total nitrogen

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.The introduction of leucaena (Leucaena leucocephala), apart from increasing animal production, improves soil fertility through biological nitrogen (N) fixation and its deep-rooted system. There is limited information on carbon and N dynamics in hedgerow silvopastoral systems, particularly in the subsoil profile. The concentrations and vertical distribution of organic carbon (OC) and total N , and their fractions (particulate and associate forms) in the profile (0‒100 cm) of a 4-year-old leucaena stand in a Urochloa brizantha-Chloris gayana pasture were compared with those in the adjacent pure tropical grass (U. brizantha) pasture. Leucaena introduction increased the OC concentration in the subsoil (20‒100 cm) by 45%, particularly the stable form (associate OC) in the deepest horizon (50‒100 cm). This was attributed to a greater abundance of leucaena roots deeper in the profile than for grass. Leucaena also enhanced by 7.6% the N concentration (from 0.131 to 0.141%) in the topsoil (0‒20 cm) associated with an increment in the labile form (particulate organic N), due to leaf deposition, recycling of animal feces and nodule-N turnover from N fixation. Leucaena establishment has the potential to improve soil fertility and hence availability of N to companion grass growth, and can be utilized as a greenhouse gas mitigation strategy.

Optimum plant density of Digitaria eriantha for herbage accumulation and hydrological performance in a summer dominant rainfall zone

AbstractIn the frost‐prone, summer dominant rainfall zone of northern NSW and southern Queensland, sown tropical grass pastures commonly establish with plant densities &gt;26 plants per m2, yet the optimum density for maximum herbage accumulation and hydrological performance is not known. A replicated, field study was established in northern NSW in November 2011, using five densities of Digitaria eriantha (digit grass): 0, 1, 4, 9 and 16 plants per m2 and a range of agronomic and hydrologic measures was assessed. The results showed that the largest differences between plant densities in herbage accumulation, root depth, soil water extraction and rainfall refill efficiency occurred during the first 2 years; after this time, digit grass pastures with densities of ≥4 plants per m2 responded similarly. Plant frequency increased with increasing plant density but the treatments did not converge. During the first two growing seasons, there was no effect of plant density on water use efficiency; however, by the third growing season, pastures with 4 plants per m2 were the most water efficient. Overall, the results indicated that 1 plant per m2 was too low to efficiently use resources, 16 plants per m2 utilized the resources quickly then tended to stall, while 4–9 plants per m2 was a good compromise between agronomic and hydrological response, achieving both herbage accumulation and sustainability goals. A practical management advantage of pastures with 4–9 plants per m2 is they allow soil water resources to be available for a longer period of time and potentially provide the opportunity to establish legumes in the ensuing 24‐month period.

Who will win where and why? An ecophysiological dissection of the competition between a tropical pasture grass and the invasive weed Bracken over an elevation range of 1000 m in the tropical Andes.

In tropical agriculture, the vigorously growing Bracken fern causes severe problems by invading pastures and out-competing the common pasture grasses. Due to infestation by that weed, pastures are abandoned after a few years, and as a fatal consequence, the biodiversity-rich tropical forest is progressively cleared for new grazing areas. Here we present a broad physiological comparison of the two plant species that are the main competitors on the pastures in the tropical Ecuadorian Andes, the planted forage grass Setaria sphacelata and the weed Bracken (Pteridium arachnoideum). With increasing elevation, the competitive power of Bracken increases as shown by satellite data of the study region. Using data obtained from field measurements, the annual biomass production of both plant species, as a measure of their competitive strength, was modeled over an elevational gradient from 1800 to 2800 m. The model shows that with increasing elevation, biomass production of the two species shifts in favor of Bracken which, above 1800 m, is capable of outgrowing the grass. In greenhouse experiments, the effects on plant growth of the presumed key variables of the elevational gradient, temperature and UV radiation, were separately analyzed. Low temperature, as well as UV irradiation, inhibited carbon uptake of the C4-grass more than that of the C3-plant Bracken. The less temperature-sensitive photosynthesis of Bracken and its effective protection from UV radiation contribute to the success of the weed on the highland pastures. In field samples of Bracken but not of Setaria, the content of flavonoids as UV-scavengers increased with the elevation. Combining modeling with measurements in greenhouse and field allowed to explain the invasive growth of a common weed in upland pastures. The performance of Setaria decreases with elevation due to suboptimal photosynthesis at lower temperatures and the inability to adapt its cellular UV screen.