Herbage accumulation , morphological composition , and nutritive value of Paspalum notatum CV

Citation Giorello D, Sbrissia A, Da Silva S. Herbage accumulation, morphological composition and nutritional value of Paspalum notatum cv. INIA Sepé. Agrociencia Uruguay [Internet]. 2021 [cited dd mmm yyyy];25(1):e348. Available from: http://agrocienciauruguay. uy/ojs/index.php/agrociencia/art icle/view/348. Herbage accumulation, morphological composition, and nutritive value of Paspalum notatum CV. INIA Sepé


Introduction
Paspalum notatum is a summer perennial grass, type C4, and one of the most common species in Uruguay's natural grassland. Taking into account P. notatum's potential, and to capitalize Uruguayan genetic resources, a national collection of germplasm was held in 2006 to characterize, value and preserve genetic resources. The characterization and evaluation processes were developed between 2007 and 2012. The regional evaluation stage began in 2012 along with an increase of the seed clone TB42, which led to INIA Sepé cultivar; it belongs to the botanical variety latiflorum, it is tetraploid and its reproduction is apomictic (1) . Once nutritional and water limiting factors are solved, and the productivity, quality and persistence of perennial pastures are determined, grazing management follows. Forage accumulation by forage crops subjected to intermittent cuttings or grazing was described as the result of the growth and senescence processes (2) . Defoliation intensity in rotational grazing can be divided into frequency and severity, which are related to the interval between cuttings and tissue proportion (forage mass, height, LAI [Leaf Area Index]) removed in each cutting or grazing period. The timing in which the regrowth is interrupted is crucial as it determines the amount of accumulated forage, its nutritional value and the animals' ability to harvest it (3) . Korte and others (4) postulated the concept of critical LAI as a determining criterion, which would correspond to the interception of 95% of the incident light by the forage canopy. Interrupting the regrowth after that point could result in forage accumulation reduction and a deterioration in pasture structure caused by an increase in stem proportion and senescence rates (5) . Parsons and Penning (6) demonstrated that interruption of regrowth at the critical LAI results in maximum forage accumulation rate for perennial ryegrass, a condition in which the balance between growth and senescence processes would be maximum. Managing pastures with more than 95% of light interception during regrowth (LI) could result in a higher total accumulation. However, a significant proportion of stems and dead material would be present, resulting in a lower accumulation of dry leaf matter and forage of lower nutritive value (7) . Although the concept of critical LAI was developed on temperate species, similar results were found on tropical and subtropical pastures through research that started in the early 2000s and continues to these days (8)(9)(10) (11) . Relations were established between critical LAI and forage canopy heights associated with these studies, transforming the latter into a reliable and easy-to-apply tool in the field when setting management goals (5) . To harvest better nutritive value forage at the expense of a possible loss of forage accumulation, studies were carried out evaluating targets of LI lower than critical LAI (90% vs 95% LI) (12) , which indicated a decrease in forage and leaf accumulation in the growth process. Regarding defoliation severity, Parsons and others (13) demonstrated that the lower the residual LAI was, the longer the regrowth period had to be before reaching the maximum forage accumulation rate, which was unlikely to vary in an extensive range of defoliation regimes. Despite using the pre-set canopy height as a management criterion for temperate and tropical pastures, the use of flexible post-grazing (cutting) heights is more recent, mainly in tropical pastures, where most studies were carried out with fixed heights, generating variable defoliation severities that hinder the definition of a single goal for different forage crops. A recent improvement Initial fertilization consisted of locating all experimental units at soil phosphorus sufficiency levels (minimum of 30 ppm) by applying differential doses of triple Superphosphate according to soil phosphorus level, measured with the Citric Acid method. Nitrogen fertilization was performed by applying granular urea, manually distributed in each experimental unit after each cutting and before irrigation at an equivalent dose of 1 kg of nitrogen per hectare per day of regrowth.
The experimental design was completely randomized, with 4 replications. As experimental units, 24 plots of 10×16 m were used. The treatments corresponded to all combinations between three cutting frequencies determined by the incident light interception percentage during regrowth (90%, 95% and maximum LI, the latter being defined when similar values were obtained in two consecutive measures -LI90%, LI95% and LImax, respectively), and cutting severity corresponding to 40 and 60% of the canopy height at cutting time (60 and 40% removal of precutting height corresponding to LI targets). Treatment application was performed using a Honda HRX217 propeller cutter, removing the cut forage. Before the start of the study, the experimental area was prepared and subjected to an adaptation period to experimental treatments and conditions. During the growth season between October 2016 and May 2017, the preparation of the experimental area began through a series of cuttings made on 10/24/2016, 1/3/2017 and 3/1/2017, at an average height of 6.5 cm with an average interval between cuttings of 49.6 + 14.9 days. On September 29, 2017, a homogenization cutting was carried out at 8.5 cm from the ground, and the control of the experimental conditions and treatment imposition were initiated throughout the growing season between October 2017 and May 2018. The effective experimental period corresponded to the following growth season, from November 2018 to March 2019.

Light interception (%) and forage canopy height (cm)
Light interception monitoring (LI) by the forage canopy was performed using the LAI-2000 Li-Cor equipment (LI-COR, USA), every 10 days on average, monitoring weekly on plots exceeding 85% of LI. Three measurements were taken on each plot at dawn (due to the need to measure under diffuse light condition). Each consisted of an above-canopy measurement recording incident photosynthetically active radiation (iPAR), and 5 consecutive measurements under the canopy, recording transmitted photosynthetically active radiation (tPAR) at ground level.
Canopy height (in centimeters) was determined at pre and post-cutting using a ruler, considering the highest sheet density point, registering 10 randomly chosen points per plot; the average of these observations being the average height of the plots.

Forage mass (DM kg.ha -1 ), morphological and botanical composition (%), and nutritive value
The evaluation of forage mass, botanical and morphological composition and nutritive value in precutting and post-cutting was carried out by extracting 3 samples per plot, cut at ground level with electric scissors powered by an internal battery and a rectangular steel frame of 35x70 cm. Once extracted, samples were conditioned in plastic bags, adding distilled water with a sprinkler and taken to a cold chamber until processing. This procedure was performed so as to minimize respiration and perspiration processes. All material was taken to the laboratory and stored in a cold chamber for further processing. The fresh weight of the entire sample was registered. Subsequently, subsamples were separated for botanical and morphological analysis. The remaining material was weighed for fresh weight and taken to the oven to dry for 48 hours at 65 °C. Finally, it was weighted once more in order to obtain the total dry matter percentage and total dry matter amount by relating this percentage to the total fresh weight of the sample. The subsamples selected for botanical and morphological composition analysis were weighted in fresh and then manually fractioned into weeds, dead material and Paspalum notatum (leaf, stem and inflorescence). Each fraction was sent to the oven to dry for 72 hours at 60 °C. Dried material was weighted, and the results used to calculate total dry matter percentage and total dry matter amount from each of the fractions. The results were expressed as percentage of the total dry matter. Once dry, the material obtained from the leaf fraction was ground and subsequently sent to the laboratory to determine: dry matter content (DM), crude protein (CP), acid and neutral detergent fiber (ADF and NDF), by using the equipment NIR PERTEN DA7250 (Perten, USA). The digestibility of the forage dry matter was estimated through the formula: DMD(%) = 88.9 -(0.779×ADF%) (19)

Forage accumulation (kg DM.ha -1 ) and daily rates of forage accumulation (kg DM.ha -1 .day -1 )
During the pre-cutting evaluation period, sampling was carried out to evaluate the total forage mass accumulation by using a HRX217 Honda propeller cutter with forage collector, where the 0.52 m wide and 8 m long central plot surface was harvested, thus avoiding the effect of the edges. For each sampling, the residual height was determined according to the height set in each treatment for the corresponding severity.
The daily forage accumulation rate was calculated by dividing the total harvested forage mass between the days of duration of the regrowth period, then a daily weighed average was calculated for each plot corresponding to each period.

Statistical analysis
The basic prerogatives of the analysis of variance (normality of the data and homogeneity of variance) were tested and the model additivity and independence of errors were assumed. When necessary, data were transformed before the analysis of variance. ANOVA was performed using the Mixed Models package of the Infostat software (20) . The results were analyzed considering the periods of greatest growth activity (November to March), which determined two periods (Period 1: from November 15, 2018, to January 31, 2019; Period 2: from February 1 to March 31, 2019). Frequency and severity of defoliation were considered as fixed effects, while period as a random effect. The comparison of means, when necessary, was carried out through the Tukey test (P<0.05).

Transparency of data
Data not available: The data set that supports the results of this study is not publicly available.

EXPERIMENTAL control
In general, LI pre-cutting values were close to those proposed and on average corresponded to 92.9, 96.1 and 97.8 for treatments with LI90%, LI95%, and LIMax, respectively. Pre-cutting height varied (p<0.05) with defoliation frequency, severity and the interaction frequency × severity × growing period. In general, higher values of pre-cutting height were found in lower defoliation frequencies. Regarding defoliation severity, there were no differences between severity, except in the treatment LIMax during period 2, where the highest value was recorded for the defoliation severity of 60% (Table 2). Post-cutting height varied (p<0.05) with frequency, defoliation severity, growth period and the interaction frequency × severity × growing period. In general, higher values were registered in lower frequency treatments and lower defoliation severity (Table 3). Post-cutting height values expressed as a percentage of the pre-cutting height were close to those proposed and corresponded, on average, to 40.7 and 58.4 for severity treatments of 40 and 60%, respectively.
The interval, in days, of each cycle, was greater for the less frequent (LIMax) and more severe (post-cutting height equivalent to 40% of the pre-cutting height) treatments.

Forage mass (kg DM.ha -1 ), botanical/morphological composition (%) and nutritive value of harvested forage at pre-cutting
Forage mass at pre-cutting varied (p<0.05) with growing period and the frequency × severity × growing period interaction. In general, forage mass was higher in Period 2 than in Period 1. While there were no differences between treatments in period 1, in period 2 a difference was found between levels of defoliation severity for the LI90% treatment (higher values for 60% relative to 40%), and between targets of LI for the severity level of 40% (higher values for LI95% relative to LI90%, with intermediate values for LIMax) ( Table 4).   (Table 6). The percentage of NDF in the forage varied (p<0.05) with the frequency × defoliation severity interaction and with the growth period × frequency × defoliation severity interaction. No difference between treatments was observed in period 1. In period 2, no difference was found between LI targets for the 60% severity, while for the 40% severity, higher values were recorded for LIMax relative to LI90% with intermediate values for LI95%. Differences in levels of defoliation severity were recorded only during period 2 for the LI90% treatments, with higher values recorded for 60% severity (Table 7). Dry matter digestibility varied (p<0.05) with the frequency × defoliation severity interaction. No difference was found between LI targets for the 60% severity, while for the 40% severity, higher values were recorded for the LI90% relative to LIMax with intermediate values for LI95% treatments. There was difference between levels of defoliation severity for the LI90% treatments, with higher values recorded for 40% (Table 8).

Forage mass (kg DM.ha -1 ) and botanical/morphological composition (%) at post-cutting
Forage mass at post-cutting varied (p<0.05) with frequency, defoliation severity and growing period. For the LI

Discussion
In general, experimental conditions were adequately controlled, with pre-cutting LI values close to the pre-established targets, ensuring the planned contrasts between defoliation regimes. Forage mass recorded values were generally high and within the range found for Paspalum notatum in other countries, such as Japan (21) and USA (22) , reaffirming this species' potential as a forage source during summer. Higher values were recorded in period 2 compared to period 1. This result could be related to the effect of leaf area recovery in period 1, since during the previous winter and due to frosts, plots did not have live (green) material above ground. Leaf percentage at pre-cutting was higher for the higher defoliation frequency (LI90%). This is a relevant aspect considering the importance the leaf component has for the plant regarding both its photosynthetic apparatus and effect on forage production; and for the grazing animal, given its preference for this type of plant structure and the associated benefits to animal performance (11) . Stem percentage did not vary between treatments and its average value was close to 23%, a result that is inconsistent with what happened in evaluations of other tropical pastures such as Panicum maximum, Pennisetum purpureum and Brachiaria brizantha, where stem percentage rapidly increased after the LI95% condition (5) . This aspect generates greater management flexibility given that there is no increase in stem percentage in forage mass with higher levels of LI, generating benefits in terms of both herbage production and intake.
On the other hand, dead material percentage (Table  5) was higher for the lower frequency (treatments of 40% defoliation severity), an aspect that could be due to the longer interval between cuttings, which explains the greater accumulation of senescent and dead material under those circumstances (5) (11) . Although stem percentage is relatively stable in the range of the defoliation frequencies studied, the increase in dead material with greater intervals between cuttings indicates greater losses due to senescence and nutritive value deterioration (reduction in CP and digestibility and increase in NDF and ADF). The recorded CP values are within the existing range in bibliography (21) (22) (between 10-15%), variable according to leaf age, time of year, cutting frequency and level of nitrogenous fertilization. Regarding ADF (Table 6) and NDF (Table 7), the highest values were recorded for the combination LIMax/40%, indicating a greater presence of structural tissue in the leaves which is consistent with larger leaves due to the greater intervals between cuttings. These values of fiber and dead material determined a lower dry matter digestibility (Table 8) for this treatment, indicating that the LI95% condition during regrowth should not be exceeded, even if there is no increase in stem percentage in the forage mass.
In general, the values obtained for leaf dry matter digestibility were similar to those obtained for the species in previous studies, varying between 55 and 65% for green leaves during summer (23) and within the range found in experimental studies (22) . In turn, for LI90%, ADF and NDF values (in period 2) were higher for the 60% severity level, resulting in lower digestibility values. This result is expected since lower tissue renewal caused by lower defoliation severity generates leaves with greater structural tissues accumulation (24) .
Values of forage mass at post-cutting, as described for pre-cutting, were higher in period 2, while highest values were registered for defoliation severity of 60%, a predictable outcome given the lower severity applied to treatments that had similar pre-cutting mass values. Furthermore, higher forage mass values for lower defoliation frequencies are a consequence of greater intervals between cuttings and higher pre-cutting forage mass, a common behavior in this type of study (7) . Leaf percentage at post-cutting was higher for the 60% severity, an expected result since less severe harvests do not reach the bottom half strata where a higher proportion of stems and dead material is normally accumulated. As expected, the percentage of inflorescences in the post-cutting forage mass was extremely low, not exceeding 0.5%.
The values recorded for both total forage accumulation and daily rates of forage accumulation are within the range of those observed by Hirata and others (20) in a review of experiments carried out with Paspalum notatum in Japan. Forage accumulation rate, as well as total accumulation during the experiment, was higher for higher the defoliation frequency treatments (LI90% and LI95%), probably as a result of greater shading and lower photosynthetic efficiency associated with greater cutting intervals for the LIMax treatments, which determine greater senescence and therefore higher dead material percentage in the pre-cutting forage mass. Overall, the results indicate greater production of high nutritive value forage in Paspalum notatum cv. INIA Sepé if the defoliation frequency used does not exceed the critical LAI (95% LI). This is an analogous result to that reported for other tropical forage grasses such as Panicum maximum, Pennisetum purpureum, and Brachiaria brizantha. The difference relies on the fact that the restriction not to exceed 95% of LI would not be an increase in stem accumulation, but in dead material instead, with consequent reduction in harvest efficiency and nutritive value of produced forage.

Conclusions
Paspalum notatum cv. INIA Sepé has large flexibility of defoliation management, and optimal pre-cutting condition should not exceed 95% of LI (about 30 cm of canopy height) as a way to ensure high nutritive value and forage production.