Introduction

Pasture-based livestock production provides a sustainable way to supply the global market with high-value nutritional foods such as meat and milk, and tropical forage grasses are an essential component of these systems1. However, beyond the seasonality of forage production caused by the concentration of rainfall during specific periods of the year, which defines the dry season with severe water deficits1,2, increasing variability in precipitation during the rainy season has led to periods of water restriction that limit forage accumulation in pastures, including in Central Brazil3,4. These periods can be defined as intraseasonal drought.

Adequate water status in plants is essential for maintaining their biochemical and physiological processes. Water deficits can impair nutrient absorption and carbon (C) assimilation5, altering the elemental homeostasis among C, nitrogen (N), and phosphorus (P). Elemental homeostasis represents the plant’s ability to maintain nutritional balance despite environmental fluctuations6. Losing this balance can impair nutrient use efficiency in grasses7,8, subsequently reducing growth, tillering, and dry mass production9. However, the few studies addressing the relationship between elemental homeostasis and drought tolerance in tropical forage grasses have been conducted under severe water deficits simulating drought conditions8, highlighting the need for field trials that account for periods of water deficit during the rainy season.

Plants under water restriction activate strategies such as the accumulation of organic solutes, including amino acids, which help reduce leaf water potential to maintain cellular functions10,11 as well as carbohydrates12. Another strategy is the accumulation of silicon (Si), a beneficial element with anti-stress action in plants, which optimizes stress response in grasses under water deficits8,13. Although some Urochloa cultivars have shown tolerance to short-term water restriction14,15, further studies are required to deepen the understanding of tolerance mechanisms, extend it to widely used species such as Megathyrsus sp., and incorporate recently developed cultivars.

The biological strategies of grasses under water stress are essential for their longevity. However, these strategies demand energy and result in losses to photosynthesis16. One practice that mitigates these losses is pasture irrigation, which can be carried out using efficient systems such as subsurface drip irrigation17,18,19. It is known that irrigation can mitigate forage production seasonality during the dry season20, but further studies are needed to elucidate the benefits of irrigation in pastures subjected to water restrictions due to drought. In recent years, new tropical forage grass cultivars have been developed to increase production in grazing systems21. Among these are Megathyrsus maximus cv. Zuri and Urochloa brizantha x U. ruziziensis cv. Mavuno, which have high production potential and respond well to more intensive management practices such as fertigation22,23. However, the mechanisms employed by these cultivars during short water deficit periods, or those that most contribute to their production under non-restricted conditions remain unknown.

This study was motivated by the need to understand how short-term water restrictions (intra-seasonal droughts) during the spring–summer season affect tropical forage grasses and how irrigation can mitigate these effects. We hypothesized that intra-seasonal droughts impair nutritional balance and productivity in Mavuno and Zuri grasses, that species differ in their tolerance due to contrasting mechanisms of elemental homeostasis, nutrient use efficiency, and solute accumulation, and that irrigation may counteract drought-induced damage by sustaining these physiological and nutritional processes. Therefore, the objectives of this research were to (i) evaluate the impact of intra-seasonal drought on the nutritional status and productivity of Mavuno and Zuri grasses; (ii) investigate tolerance mechanisms to water deficit in forage species; and (iii) assess the potential of subterranean drip irrigation to mitigate water deficit damage by optimizing the physiological responses of forage grasses during the spring–summer season.

Material and methods

Growing conditions

The research was conducted in field conditions with forage grasses cultivated at the Teaching, Research, and Extension Farm of São Paulo State University in Jaboticabal, São Paulo, Brazil. The experimental area is located at coordinates 21º 14′ 54′′ S and 48º 17′ 06′′ W, with an altitude of 560 m.

The region’s climate is classified as tropical wet and dry (Aw) with dry winters featuring a rainy season during summer, from November to April, and a distinct dry season in winter, from May to October. Meteorological data were recorded using the university meteorological station throughout the experimental period, from October 2022 to April 2023. During this time, cumulative rainfall reached 1415.4 mm, while cumulative potential evapotranspiration totaled 820.6 mm. In the irrigated treatment, 175 mm of water was applied through irrigation. The average global solar radiation was 20.4 MJ m−2, and the mean maximum and minimum air temperatures were 30.4 °C and 18.8 °C, respectively.

The experimental soil was classified Oxisol24. Soil samples were analyzed for chemical properties before the experiment25, yielding the following results for 0–20 cm and 20–40 cm depths, respectively: pH: 5.8 and 6.0, Organic matter (OM): 22 and 22 g dm−3, P: 78 and 51 mg dm−3, K (potassium): 5.5 and 3.6 mmolc dm−3, Ca (calcium): 46 and 44 mmolc dm−3, Mg (magnesium): 15 and 15 mmolc dm− 3, S: 3 and 4 mg dm−3, B (boron): 0.32 and 0.25 mg dm−3, Cu (copper): 3.4 and 3.5 mg dm−3, Fe (iron): 6 and 5 mg dm−3, Mn (manganese): 25.5 and 18.6 mg dm−3, Zn (zinc): 1.6 and 1.0 mg dm−3, Potential acidity (H + Al): 21 and 18 mmolc dm−3, Cation exchange capacity (CEC): 88 and 81 mmolc dm−3, Base saturation (V): 76% and 77%, Clay: 582 and 591 g kg−1, Silt: 235 and 231 g kg−1, Sand: 183 and 178 g kg−1.

The soil was prepared by plowing in February 2022, and a subsurface drip irrigation system was installed, with driplines buried 20 cm deep. The drip lines, spaced 80 cm apart, used Petroisa Durazio emitters with a flow rate of 1.45 L h−1 and spaced 30 cm along the line. The system showed excellent uniformity, with a Christiansen Uniformity Coefficient (CUC) of 94.7% and a Distribution Uniformity Coefficient (DUC) of 93.4%26.

Treatments, experimental design, and management

The treatments were organized in a randomized complete block design in a 2 × 2 factorial arrangement, consisting of two forage species (Urochloa brizantha x U. ruziziensis cv. Mavuno and Megathyrsus maximus cv. Zuri) and two water conditions (irrigated and rainfed). The treatments were distributed across four randomized blocks, serving as replicates. Plots measuring 3 × 4.8 m (14.4 m2) were sown in April 2022 by manually spreading 10 kg ha−1 of pure viable seeds for each forage species. After initial plant growth, the plots were cut to stimulate basal tillering using a manual mower.

In October 2022, a uniformization cut was performed, respecting a residue height of 35 cm for Zuri grass and 15 cm for Mavuno grass27,28. Treatments were then applied, with plots receiving subsurface drip irrigation or remaining rainfed. Soil moisture storage capacity was defined based on field capacity and permanent wilting point, determined using water retention curves from undisturbed soil samples in a Richards pressure chamber29. In irrigated plots, water was applied to replenish 100% of evapotranspiration, maintaining a soil water field capacity of 50 mm (up to 0.4 m depth). The frequency and amount of irrigation in the irrigated plots were determined to fully replace evapotranspiration losses, using a management allowable depletion of 0.6, resulting in a total irrigation water amount of 175 mm during the experimental period (Fig. 1). Under rainfed conditions, soil moisture remained below the critical threshold of 20 mm during October and November 2022, as well as in March 2023.

Fig. 1
figure 1

Soil available water content during the experimental period (October 2022 to April 2023) at the experimental site located in Jaboticabal, Brazil. The rainfed condition is represented by the yellow line, and the irrigated condition by the black line. The blue dashed lines indicate total water availability (TWA) and the red dashed lines indicate critical soil water availability (CWA).

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Maintenance fertilization was performed three days after each pasture cut, replenishing the N, K, P, and S removed in the dry matter using urea, purified monoammonium phosphate, potassium chloride, and ammonium sulfate30,31. In irrigated treatments, fertigation was applied, while rainfed plots received manual broadcast fertilization, replacing monoammonium phosphate with single superphosphate.

Data collection

Biometric, productive, and morphological biomass composition parameters

The number of tillers per m2 was estimated in each treatment using 625 cm2 plastic frames to delineate grass clumps. After each cut, live and dead tillers were counted to calculate the average number of live tillers per m232. Plant height (cm) was measured at five points per plot before pasture cutting using a graduated ruler. Forage production was assessed by collecting the grazeable layer using a sickle between 7:00 and 9:00 a.m when the pasture reached entry heights of 85 cm for Zuri and 35 cm for Mavuno, with residue heights of 35 cm and 15 cm, respectively27,28. A total of ten cuts were made for irrigated treatments and seven for rainfed treatments. Fresh forage was immediately weighed from sampling areas of 2.25 m2 (Zuri) and 1.0 m2 (Mavuno). After sampling, residue height was standardized using a manual mower.

A 100 g sample of fresh forage was dried in a forced-air oven at 65 ± 5 °C until a constant weight to determine dry matter (DM). Based on DM content, pasture height, and cycle duration, the following were calculated: forage mass (FM, Mg ha−1 DM), forage mass density (FMD, kg ha−1 cm−1 DM), forage accumulation rate (FAR, kg ha−1 day−1 DM), and support capacity (SC, LU ha−1, where AU = animal unit or 450 kg live weight). Another fresh forage sample was used to determine morphological composition by separating leaf blades, stems, and dead material. These fractions were dried in an oven at 65 ± 5 °C until constant weight, and their proportions in the dry matter were calculated33.

Nutrient content determination

Dry forage samples were ground in a Willey mill. Nitrogen was analyzed using sulfuric acid digestion and Kjeldahl distillation with NaOH 15 N, followed by titration with H2SO4 0.05 N. Phosphorus, Ca, Mg, S, K, Cu, Zn, Mn, and Fe were extracted using nitric-perchloric acid digestion and analyzed via spectrophotometry, and B was determined after dry-ashing the plant material in a muffler furnace at 400 °C and spectrophotometric determination after colorimetric reaction with azomethine H34. Carbon was determined by wet digestion with K2Cr2O7 solution and FeSO4 titration using the modified Walkley-Black method35. Silicon content was determined by alkaline digestion in hydrogen peroxide at 120 °C, followed by spectrophotometry at 410 nm using ammonium molybdate36,37.

Stoichiometric ratios and nutrient use efficiencies

The C: N, C: P, N: P, and C: Si ratios were calculated using C, N, P, and Si contents in dry matter. Nutrient accumulation (kg ha−1) was obtained by multiplying dry matter by element concentrations. Nutrient use efficiencies were calculated as forage mass/nutrient accumulation for C (CUE), N (NUE), P (PUE), K (KUE), Ca (CaUE), Mg (MgUE), and S (SUE) in kg kg−138.

Total carbohydrates and amino acids

Carbohydrates and total amino acids were extracted from 1.0 g of dry plant material using a methanol-chloroform-water solution (60:25:15%). Carbohydrates were quantified via reaction with phenol and sulfuric acid, followed by spectrophotometry at 490 nm39. Amino acids were quantified using citrate buffer, ninhydrin solution, and KCN, with spectrophotometric reading at 570 nm40.

Data analysis

Data were tested for normality, homogeneity, and residual independence41,42 before analysis of variance using F-tests (p < 0.05). Tukey’s test (p < 0.05) was applied for mean comparisons, using SPEED Stat version 3.243. Pearson correlation analysis was performed for variable relationships in each water condition (p < 0.05). Principal component analysis (PCA) used covariance matrices, Z-score standardization, and NC < 120 ratios. Hierarchical cluster analysis was performed using Euclidean distance and single-linkage methods, analyzed in R Studio version 4.3.3.

Results

Macronutrient and Si contents and accumulations

A significant interaction (p < 0.05) was observed between species and water condition for Si, S, and Mg contents in dry matter (DM), as well as for the accumulation of P, Si, K, and Mg (Figs. 2a–h and 3a–h).

In irrigated conditions compared to rainfed, Mavuno grass showed a decrease in N (-7%), K (-17%), and S (-11%) contents, but an increase in P (+ 18%) and Si (+ 27%) contents, with no changes in C, Ca, and Mg contents in DM. Additionally, Mavuno exhibited an increase in the accumulation of N, P, C, Si, K, S, Ca, and Mg (Figs. 2a–h and 3a–h).

In Zuri grass under irrigation, there was a decrease in K content (-22%), but an increase in P, Si, and Mg contents compared to the rainfed condition. Accumulations of P (+ 15%), Si (+ 53%), and Mg (+ 64%) increased, while K accumulation decreased, without changes in N, C, S, and Ca contents or accumulations (Figs. 2a–h and 3a–h).

When comparing species under rainfed conditions, Zuri grass had lower contents of P, C, and S, but higher contents of Si and Ca, as well as higher accumulations of N, C, Si, K, and Ca compared to Mavuno in the same condition. Under irrigation, Zuri exhibited higher contents of N (+ 6%), Si (+ 70%), Ca (+ 32%), and Mg (+ 54%), and lower contents of P (-15%) and C (-3%), with higher accumulations of P and K and lower accumulations of Si, Ca, and Mg compared to Mavuno. Accumulations of N, C, and S were not affected by irrigation (Figs. 2 and 3a–h).

Fig. 2
figure 2

Contents of nitrogen (a), phosphorus (b), potassium (c), sulfur (d), calcium (e), magnesium (f), carbon (g) and silicon (h) in the shoot of Mavuno grass and Zuri guineagrass under condition irrigated or water restriction in field conditions in Jaboticabal, Brazil. ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. S, C, and S×C denote species, water condition, and their interaction, respectively. Means were tested by the Tukey test at 5% error probability. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. Bars represent the standard error of the mean, n = 4.

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Fig. 3
figure 3

Accumulation of carbon (a), silicon (b), nitrogen (c), potassium (d), phosphorus (e), sulfur (f), calcium (g) and magnesium (h) in the shoot of Mavuno grass and Zuri guineagrass under condition irrigated or water restriction in field conditions in Jaboticabal, Brazil. ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. S, C, and S×C denote species, water condition, and their interaction, respectively. Means were tested by the Tukey test at 5% error probability. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. bars represent the standard error of the mean, n = 4.

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Contents and accumulations of micronutrients, contents of amino acids and carbohydrates

A significant interaction (p < 0.05) was observed between species and water condition for Cu, Mn, and Fe contents in dry matter (DM) (Fig. 4a, d, e).

Irrigation, compared to rainfed cultivation, led to a decrease in Cu and Fe contents and an increase in Cu (+ 30%), Zn (+ 39%), and B (+ 40%) accumulation in Mavuno grass (Figs. 4a–g and 5a-e). In Zuri grass, irrigation resulted in higher Cu and Mn contents, with a decrease in total amino acid content (Fig. 4a, d, f), as well as higher accumulations of Cu (+ 22%) and Mn (+ 51%) compared to the rainfed condition (Fig. 5a, d).

Under rainfed conditions, Mavuno grass exhibited higher contents of Zn, Fe, and total carbohydrates (+ 13%), and a greater accumulation of Fe (+ 105%) compared to Zuri grass in the same condition. In contrast, Zuri showed higher Mn content and higher accumulations of Cu (+ 22%), B (+ 34%), and Mn (+ 90%) compared to Mavuno under rainfed conditions (Figs. 4a–g and 5a–e).

In irrigated conditions, Zuri grass had higher Cu and Mn contents and greater Mn accumulation compared to Mavuno. On the other hand, Mavuno showed higher Zn, Fe, amino acids (+ 61%), and total carbohydrates (+ 16%) contents (Fig. 4a–g), as well as greater Zn and Fe accumulations compared to Zuri under irrigated conditions (Fig. 5b, e).

Fig. 4
figure 4

Contents of copper (a), zinc (b), boron (c), manganese (d) and iron (e), and contents of amino acids (f) and carbohydrates (g) in the shoot of Mavuno grass and Zuri guineagrass under condition irrigated or water restriction in field conditions in Jaboticabal, Brazil. ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. S, C, and S×C denote species, water condition, and their interaction, respectively. Means were tested by the Tukey test at 5% error probability. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. Bars represent the standard error of the mean, n = 4.

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Fig. 5
figure 5

Accumulation of copper (a), zinc (b), manganese (c), boron (d) and iron (e), in the shoot of Mavuno grass and Zuri guineagrass under irrigated condition or water restriction in field conditions in Jaboticabal, Brazil. ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. Means were tested by the Tukey test at 5% error probability. S, C, and S×C denote species, water condition, and their interaction, respectively. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. bars represent the standard error of the mean, n = 4.

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Stoichiometric ratios and macronutrient use efficiencies

There was no significant interaction between species and water condition for stoichiometric ratios (Fig. 6a–d), but there was a significant interaction (p < 0.05) for nutrient use efficiencies, except for the EUP and EUK (Fig. 7a–g).

When comparing stoichiometric ratios under irrigated versus rainfed conditions, both species showed a decrease in C: P, C: Si, and N: P ratios, with a 6% increase in the C: N ratio in Mavuno grass (Fig. 6a–d). Irrigation also promoted an increase in the efficiencies of C, N, P, K, Ca, Mg, and S use in Mavuno grass, and K use efficiency in Zuri grass (Fig. 7a–g).

When comparing rainfed forage species, Zuri grass had lower C: N (-6%) and C: Si (-29%) ratios, and a higher N: P ratio (+ 16%) compared to Mavuno, with no difference in the C: P ratio between the species. Under irrigation, Zuri exhibited higher C: P (+ 13) and N: P (+ 24%) ratios, as well as higher C: N (+ 9%) and C: Si (76%) ratios in Mavuno (Fig. 6a–d).

For nutrient use efficiencies, under rainfed conditions, Zuri showed higher efficiencies in C (+ 5%), P (+ 13%), K (+ 10%), and S (+ 21%), and water use efficiency (WUE) (+ 35%) compared to Mavuno, with no differences between species for N and Mg use efficiencies (Fig. 7b, f). On the other hand, under irrigation, Mavuno exhibited higher efficiencies of N (+ 5%), Ca (+ 32%), and Mg (+ 65%) compared to Zuri. Zuri had higher WUE (+ 16%) under rainfed conditions, while Mavuno increased WUE by 19% under irrigated conditions (Fig. 7h).

Fig. 6
figure 6

Stoichiometric ratios carbon: phosphorus (a), carbon: nitrogen (b), carbon: silicon (c) and nitrogen: phosphorus (d), in the shoot dry mass of Mavuno grass and Zuri guineagrass under condition irrigated or water restriction in field conditions in Jaboticabal - SP, Brazil. ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. S, C, and S×C denote species, water condition, and their interaction, respectively. Means were tested by the Tukey test at 5% error probability. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. bars represent the standard error of the mean, n = 4.

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Fig. 7
figure 7

Use efficiency of carbon (a), nitrogen (b), phosphorus (c), potassium (d), calcium (e), magnesium (f), sulfur (g), and water (h), in Mavuno grass and Zuri guineagrass under condition irrigated or water restriction in field conditions in Jaboticabal – SP, Brazil. ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. S, C, and S×C denote species, water condition, and their interaction, respectively. Means were tested by the Tukey test at 5% error probability. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. bars represent the standard error of the mean, n = 4.

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Production variables and morphological composition of dry mass

There was a significant interaction (p < 0.05) between water condition and forage species for the variables of forage dry matter, forage accumulation rate, support capacity, and percentage of dead material in the biomass (Fig. 8a–i).

When comparing irrigated versus rainfed cultivation, Mavuno grass showed an increase in the number of tillers (+ 30%), dry matter production (+ 41%), forage accumulation rate (+ 30%), and support capacity (+ 30%), as well as a decrease in the percentage of dead material (-34%) in the forage (Fig. 8a, c, d, f, i). Zuri grass under irrigation showed an increase in leaf blade percentage and a decrease in stem percentage (-14%) in the forage, with no effect of irrigation on the productive parameters of this grass (Fig. 8g, h).

Under rainfed conditions, Megathyrsus maximus cv. Zuri had a lower number of tillers (− 44%), but higher canopy height, dry matter production (+ 29%), forage accumulation rate (+ 26%), and support capacity (+ 26%) compared to Mavuno (Fig. 8a–d, f). Rainfed Mavuno grass had higher forage mass density compared to Zuri (Fig. 8c). No difference in dry matter production was found between the species when grown under irrigation (Fig. 8c), but Mavuno had a higher number of tillers and density (Fig. 8a, e).

Both under rainfed and irrigated conditions, Mavuno exhibited a greater number of tillers and higher forage mass density compared to Zuri (Fig. 8a, e). Conversely, Zuri had a higher percentage of leaf blade (+ 40%), and lower percentages of stem (− 73%) and dead material (− 97%) in the forage compared to Mavuno (Fig. 8g, i).

Fig. 8
figure 8

Number of tillers (a), height canopy (b), forage mass (c), forage mass density (d), forage accumulation rate (e), support capacity in animal units per hectare (f), percentages of leaf blead (g), stem (h) and dead material (i) of grazing stratum in Mavuno grass and Zuri guineagrass under condition irrigated or water restriction in field (Jaboticabal, Brazil). ns, * and ** indicate non-significant F-test, and significant F-tests at 5 and 1% probability levels, respectively. S, C, and S×C denote species, water condition, and their interaction, respectively. Means were tested by the Tukey test at 5% error probability. Uppercase letters compare means between conditions and lowercase letters compare the effect of species. Bars represent the standard error of the mean, n = 4.

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Principal component analysis

Variables were selected after a prior assessment of redundancy, based on the ratio between the largest and smallest eigenvalues, or the condition number (CN). Only variables contributing to CN < 120, indicating low or negligible multicollinearity, were retained, facilitating the analysis of the most important variables in the study. The eigenvalues of the two principal components extracted were greater than one, indicating that they can be grouped into models that explain the data variation.

When analyzing the cultivation conditions (Fig. 9), the Principal component analysis (PCA) identified patterns of variation among grass traits under different water conditions. The PC1 explained 51.3% of the variance and was positively associated with variables linked to productivity, such as carbohydrate content, number of tillers, and C:N and C:Si ratios, while negatively correlated with the percentage of leaves in the grazable layer and the N: P ratio. This suggests that PC1 primarily reflects the capacity for biomass accumulation and metabolic adjustments to maintain nutrient use efficiency.

The PC2 accounted for 27.6% of the variance and was positively associated with amino acid content as well as N:P and C:Si ratios, while negatively correlated with forage mass and the C:N ratio, indicating that this component represents physiological strategies for drought tolerance, such as osmotic solute accumulation and adjustments in nutrient stoichiometry. The distribution of species along these two components showed that M. maximus cv. Zuri maintains better homeostasis and more efficient use of C, N, and other nutrients under drought conditions, whereas U. brizantha × U. ruziziensis cv. Mavuno responds more strongly to irrigation, with increased productivity and tillering.

Fig. 9
figure 9

Vector loadings and cluster Euclidean distance in principal component analysis in Mavuno grass and Zuri guineagrass under condition irrigated or water restricted in field conditions in Jaboticabal-SP, Brazil. Aas: amino acids content; CHO: carbohydrates content; FM: forage mass, Lr: leaf rate, Tll: tillers; C/N: carbon/nitrogen, C/Si: carbon/silicon and N/P: nitrogen/phosphorus. M w/o I: Mavuvo rainfed, Z w/o I: Zuri rainfed, M Irrig: Mavuno irrigated and Z Irrig: zuri irrigated, clusters of Euclidean distance of treatments.

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Discussion

Differential nutrient uptake, homeostasis and solute accumulation in forage grasses under water restriction

In each region of forage cultivation, there are patterns for leaf nutrient contents that guide the evaluation of the forage’s nutritional status. In São Paulo State (Brazil), recommended nutrient levels for forage grasses (genera Megathyrsus and Urochloa) are provided for macronutrients (g kg−1): N: 15–25; P: 1–3; K: 15–30; Ca: 3–8; Mg: 1.5-5.0; S: 1–3, and micronutrients (mg kg−1): B: 10–30; Cu: 4–14; Fe: 50–200; Mn: 40–200; Zn: 20–50)30. These standards are intended for both rainfed and irrigated cultivations, but there are doubts regarding their applicability for species more responsive to irrigation, which are typically species whose production is limited by water deficits and are less tolerant to this restriction.

It was observed that irrigation, although it altered the leaf contents of some nutrients like P, K, and S in both forages (Fig. 2b, c, d), did not change the nutritional status of the crops. For instance, irrigation reduced the leaf N content in Mavuno grass (from 28 to 27 g kg−1), but this did not alter the nutritional status since it remained above the recommended range (15–25 g kg−1), and this trend was observed for other nutrients as well. Therefore, both irrigated and rainfed conditions in spring-summer did not affect the nutritional status of the grasses, as no nutritional deficiencies were observed. However, Mavuno grass grown under rainfed conditions showed limited production due to lower nutrient use efficiency (Fig. 7a-g). Thus, the current nutritional standards remain adequate for evaluating the nutritional status of Zuri grass, with or rainfed, and also for Mavuno when irrigated. “However, the results suggest that this may not hold true for irrigated Mavuno grass, as production was limited by water restriction even when nutrient levels were adequate and comparable to those under irrigation. Therefore, this should be taken into account in future long-term studies, as it may be necessary to update nutritional standards according to species/cultivars and cropping systems under irrigated or rainfed conditions.

Irrigation can improve nutrient absorption by forage grasses, but few field studies elucidate its benefits for plant nutrition, particularly during drought periods, or which nutrients are most impacted by irrigation depending on the species or cultivar. This is important because the species cultivated may or may not influence absorption, as it is a process regulated by membrane transporters, which are affected by genetic factors44. In this study, irrigation promoted higher absorption of P and Si in both species (Fig. 2b, h), which supports the better diffusion of phosphate under optimal soil water availability, enhanced by subsurface drip fertigation that places nutrients in the root zone, promoting their absorption and accumulation in biomass18,45.

Irrigated cultivation also favored Si absorption (silicic acid, H4SiO4) by both Mavuno and Zuri grasses due to the maintenance of transpiration flow46. Irrigation may have also facilitated the dissolution of phytoliths, which are an important Si reservoir in the soil47, benefiting grasses by improving nutrient use efficiency48,49. A positive correlation was observed between Si content and Ca (r = 0.92**), Mg (r = 0.95**), and the accumulation of Si with C use efficiency under irrigation (r = 0.64**).

The species M. maximus cv. Zuri absorbed more Ca and Si than Mavuno, regardless of water condition, which is an important ability because Ca influences cell division and elongation, stress hormone signaling, and the absorption of other nutrients44, while Si is an important anti-stress element in grasses under water deficit8 including by promoting the production of compatible solutes and, consequently, water uptake13. Although Poaceae plants are known Si accumulators46, the superior capacity of M. maximus cv. Zuri to absorb and accumulate Si compared to U. brizantha x U. ruziziensis cv. Mavuno indicates species-level differences among grasses, which could be the focus of future studies.

The higher capacity of Zuri grass to maintain the accumulation of nutrients like N, C, and K compared to Mavuno under rainfed conditions (Fig. 2a, g, c) was a significant advantage, as these accumulations strongly correlated with forage production under rainfed conditions (N: r = 0.99**, C: r = 0.99**, K: r = 0.95**). In contrast, under irrigation, while Zuri benefited from higher accumulations of P and Si, Mavuno showed increased accumulations of all macronutrients, indicating greater responsiveness of U. brizantha x U. ruziziensis cv. Mavuno to irrigation. These results support the first and second hypotheses, showing that short-term intra-seasonal droughts cause significant nutritional losses in forage grasses, and that tolerance to water deficit varies between species based on nutrient absorption and accumulation patterns.

Water deficit impairs nutrient absorption, especially those absorbed via mass flow or diffusion50. However, as observed in this study, the damage to forage grasses varies by species. For example, M. maximus cv. Zuri had lower absorption of Zn and Fe compared to Mavuno under rainfed conditions (Fig. 5b, e), while Cu and B accumulation was higher in Zuri (Fig. 5a, c). These results confirm the first and second hypotheses of this study, as water deficit during drought periods can limit nutrient absorption, with variations depending on the forage species.

Irrigation benefits were observed in optimizing Cu accumulation (in both species), Zn and B in Mavuno, and Mn in Zuri. Specifically, Zuri exhibited a superior capacity to absorb Mn from the soil compared to Mavuno, regardless of water condition, indicating a strong species effect, which has also been studied in other cultivars of Megathyrsus51. The decrease in Cu and Fe levels in Mavuno under irrigation is likely due to a dilution effect, which occurs due to increased biomass in irrigated cultivation.

It was also shown that forage grasses utilize the solute accumulation mechanism when subjected to water restriction, as Zuri prioritized amino acid concentration under rainfed conditions, while Mavuno exhibited higher total carbohydrate content compared to Zuri in the same condition (Fig. 4g, h), confirming the second hypothesis tested. This concentration of solutes in the aerial parts helps the plants maintain nutrient absorption from the soil even under water deficit, due to osmotic potential regulation, stabilization, and less oxidative damage to cell membranes12,52. Similarly, this was observed in Cynodon dactylon in a controlled environment10. The observed species-specific changes in micronutrient and amino acid concentrations under water deficit conditions support the first and second hypotheses, highlighting that drought impairs nutrient uptake and that forage species respond differently through physiological mechanisms.

Effects of water condition and species on elemental homeostasis and nutrient use efficiency in forage grasses in spring–summer

Homeostasis between C, N, and P has been a useful approach to understand how plants tolerate or not the restrictions imposed by cultivation systems, due to the importance of this balance for plant production53. This study demonstrated that water deficit during droughts in the rainy season compromises the elemental homeostasis of grasses, as nutrient absorption is limited (Fig. 3a–h), which had previously been observed only in dry-season typical deficits54,55. In this context, irrigation during the spring-summer period benefited C:N:P homeostasis by decreasing the C:P, N:P, and C: Si ratios in both Mavuno and Zuri grasses, due to optimized absorption of P and Si in both species.

Stoichiometric flexibility is important for drought tolerance in some forage grasses56. Indeed, both Mavuno and Zuri showed changes in the N:P stoichiometry under rainfed conditions compared to irrigated ones. This confirms their strategy in response to water restriction for these species, with high positive correlations observed between the N: P ratio and nutrient use efficiency for Ca (r = 0.91**), P (r = 0.91**), and S (r = 0.87**) in rainfed cultivation. However, this mechanism did not prevent a decrease in the productive potential of Mavuno, indicating its higher sensitivity to the loss of C: N:P balance compared to Zuri.

Mavuno exhibited significant improvements in elemental homeostasis under irrigation, with an increase in the C:N ratio and a decrease in the C:Si ratio due to a decrease in N content and an increase in Si content compared to rainfed cultivation. This was associated with an increase in C accumulation and improvements in the efficiency of C, N, P, and other macronutrients (Fig. 7a-g), consolidating the responsiveness of U. brizantha x U. ruziziensis cv. Mavuno to irrigation. Additionally, C accumulation and the C:N ratio showed high positive correlations with N (r = 0.94**) and Mg (r = 0.95**) use efficiency.

Zuri grass showed higher WUE, lower C:N and C:Si ratios, regardless of the water condition, which is related to its lower C content compared to Mavuno. This suggests a possible reduction in relative C due to a greater incorporation of Si in the plant’s C structures, a process of lower energy cost for the plant than lignin synthesis, resulting in an energy surplus57,58. This mechanism, which was previously described in grasses under water deficit55,59, has now been shown to contribute to drought tolerance imposed by drought periods in Zuri grass, allowing greater C accumulation than observed in Mavuno in this condition (Fig. 7h).

Modifications in elemental stoichiometry through Si incorporation are often associated with better plant efficiency in converting nutrients into dry mass49,54. This was confirmed under rainfed conditions for Zuri grass, which exhibited greater changes in C:N:P: Si stoichiometry and higher use efficiencies of C, N, P, K, and S compared to Mavuno, supporting the second hypothesis. The observed differences in nutrient use efficiency were further supported by positive correlations between the C:P and N:P ratios and the use efficiencies of Mg (r = 0.97**; r = 0.94**), K (r = 0.69**; r = 0.72**), and C (r = 0.81**; r = 0.83**), which corroborate the patterns observed across species and water conditions. These results indicate that alterations in elemental homeostasis and nutrient use efficiency under drought conditions. In contrast, the positive effects of irrigation confirm, demonstrating that irrigation enhances these mechanisms to mitigate intra-seasonal drought damage.

Irrigated and rainfed cultivation effects on forage production and morphological composition in spring-summer

Forage production in various regions worldwide is limited by periods of water deficit during the dry season. However, as shown in this study, water restriction due to short periods in the rainy season can reduce the productive potential of sensitive forage species. It was observed that under rainfed conditions during drought, Mavuno’s production was 29% lower than Zuri, despite having a higher number of tillers (Fig. 8a, c). This higher tolerance of Zuri is explained by its superior capacity for nutrient accumulation and efficient use of C and nutrients like N, which is fundamental for tillering in grasses60, and K, which is required for enzymatic activation in protein synthesis and photosynthesis61.

Nutrient use efficiency reflects the plant’s ability to convert them into dry matter, and this process is influenced by water condition and the species cultivated. In this study, the efficiencies of C, N, P, K, and S use showed high correlation with leaf blade percentage (r = 0.96**; r = 0.96**; r = 0.90**; r = 0.94**, and r = 0.90**, respectively), which in turn relates to forage production (r = 0.97**). Moreover, the higher canopy height of Zuri grass also contributed to the higher accumulation rate and, consequently, higher forage production (r = 0.96**), overcoming the effect of lower density compared to Mavuno, as well-established in the ecophysiology of forage plants62.

Irrigation led to a 29.4% increase in the forage accumulation rate of Mavuno compared to rainfed cultivation, resulting in a 41% increase in total forage production, equivalent to 6,703 kg ha−1 of dry matter. This gain was primarily attributed to a higher tiller density under irrigation (Fig. 8a), which showed a strong positive correlation with the forage accumulation rate (r = 0.94**). The number of tillers in the irrigated treatment was positively correlated with leaf concentrations of Fe (r = 0.96**), Zn (r = 0.96**), and Si (r = 0.96**), as well as with nutrient use efficiency for Mg (r = 0.90**) and Ca (r = 0.86**). Additionally, Mavuno exhibited improved P accumulation under irrigation, a key nutrient for tillering and photosynthetic function61. As a result of the increased productivity, the pasture’s carrying capacity increased by 2 AU ha−1 (Fig. 8f), emphasizing the potential of irrigation to enhance the performance of Mavuno grass during the spring–summer season under variable rainfall conditions. These findings support the third hypothesis, highlighting the suitability of Mavuno for intensified forage production in irrigated systems.

Conclusions

Water restriction imposed by drought during the spring-summer period impairs nutrient absorption and accumulation, disrupting the C:N:P balance in both Zuri and Mavuno grasses. M. maximus cv. Zuri exhibited greater tolerance to water deficit by employing mechanisms such as amino acid accumulation, adjustments in C:N:P: Si homeostasis, and maintenance of carbon and mineral nutrient use efficiency under rainfed conditions. Irrigation alleviated the nutritional damage caused by water deficit in U. brizantha x U. ruziziensis cv. Mavuno, increasing forage production by 41% during spring-summer through enhanced nutrient use efficiency and tillering. Additionally, irrigation improved the morphological composition of forage in both species. Future studies should examine the effects of intra-seasonal drought across multiple years of cultivation to strengthen the robustness and applicability of these findings.