1. 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 apomictic¹. 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 processes². 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 it³. Korte and others⁴ 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 rates⁵. Parsons and Penning⁶ 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 value⁷. 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 goals⁵. 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 others¹³ 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 was the establishment of the defoliation ratio criterion based on the fact that severity greater than 50% of pre-grazing (cutting) height prevented maximizing intake in the short term^11)(14)(15, once leaves predominate in the upper half of the forage canopy⁵. The need to evaluate different pre-grazing goals with defoliation severity that did not exceed 50% of the pre-grazing height was perceived in this way. Applying different managing intensities (combination of frequency and defoliation severity) determines the structure of the forage canopy, which modulates growth processes variation, forage accumulation and nutritive value. inia Sepé cultivar, Paspalum notatum, has a high compensation capacity between number and size of tillers associated with tiller longevity, which ensures great flexibility to defoliation intensity¹⁶. In this context, it is hypothesized that defoliation frequency has greater relative importance than defoliation severity, condition in which forage nutritive value would be determinant of the optimum harvest point. The goal of this study is to determine forage accumulation, its botanical and morphological composition, and the nutritional value of Paspalum notatum cv. inia Sepé subjected to six defoliation regimes, product of the combination of three frequencies (90, 95, and maximum light interception during regrowth), and two severities of defoliation (40 and 60% of pre-cutting height).

2. Materials and methods

The experiment was carried out from November 15, 2018, to March 31, 2019, at Campo Experimental Tambores, dependent on inia Tacuarembó, in the department of Tacuarembó, Uruguay: latitude 31º54'41.15''S, longitude 56º13'39.35''W and altitude 253 meters.

The climate of this region of Uruguay is of Cfa type according to Köppen classification, with an average temperature of 18.5 °C and average annual rainfall of 1294 mm¹⁷. In order to maintain an adequate soil water level above 50% of available water, spray irrigation was used throughout the experiment period. The application was made according to water balance carried out daily and considering the soil physics data of the site such as water retention capacity (80 mm), infiltration speed, effective depth of root exploration (40 cm), and daily climatic variables such as potential evapotranspiration and daily rainfall, as well as applied irrigation. Climate data were collected with a Davies Pro-2 Plus meteorological station (Davis, usa) located 30 meters from the experiment within the experimental site (Figure 1).

Water balance was completed daily from Monday to Friday, and irrigation was carried out in 10 to 30 mm doses, so as to prevent depletion of available soil water below 50% and losses by surface runoff.

Figure 1: Average decadal temperature evolution of the experimental period, and average, minimum, and maximum of the historical series (1986-2019) of inia Tacuarembó climate records 

The experimental area was sown in November 2015, with a Semeato drill seeder, consisting of 13 lines separated by 17 cm at a density of 15 kg ha^-1, to place 150 viable seeds per square meter. The pasture was considered adequately established after a series of 3 cuttings made at 4.9 cm from the ground on 1/15/2016, 9/19/2016 and 10/3/2016. The soil of the experimental site is a Typic Eutric Vertisol (Hapluderts Typic), belonging to the Soil Unit Coneat Itapebí Tres Árboles, with 39% clay, 31% sand and 30% silt content. The chemical characteristics were those observed in Table 1.

Table 1: Result of soil chemical analysis used in experiment II 

Source: La Estanzuela INIA's Analysis Laboratory.

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 -li_90%, li_95% and li_max, respectively), and cutting severity corresponding to 40 and 60% of the canopy height at cutting time (60 and 40% removal of pre-cutting 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.

3. Results

3.1 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 li_90%, li_95%, and li_Max, 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 li_Max during period 2, where the highest value was recorded for the defoliation severity of 60% (Table 2).

Table 2:  Pre-cutting height (cm) for Paspalum notatum cv. inia Sepé subjected to intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column or lowercase in the row are not significantly different (p>0.05). SE indicates the standard error of the variable.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).

Table 3: Post-cutting height (cm) for Paspalum notatum cv. inia Sepé subjected to intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column or lowercase in the row are not significantly different (p> 0.05). SE indicates the standard error of the variable.

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 (LI_Max) and more severe (post-cutting height equivalent to 40% of the pre-cutting height) treatments.

3.2 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 li_90% treatment (higher values for 60% relative to 40%), and between targets of li for the severity level of 40% (higher values for li_95% relative to li_90%, with intermediate values for li_Max) (Table 4).

Table 4: Pre-cutting forage mass (kg dm.ha^-1) in Paspalum notatum cv. inia Sepé subjected to different intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column or lowercase in the row are not significantly different (p>0.05). SE indicates the standard error of the variable.

Leaf percentage in the pre-cutting forage mass varied (p<0.05) with defoliation frequency. Greater values were registered for li_90% relative to li_Max, with similar intermediate values for li_95% (48.3, 45.5 and 42.3 + 1.41% for treatments li_90%, li_95% and li_Max, respectively). Stem percentage in the pre-cutting forage mass did not vary between treatments, its mean value being 22.8 + 0.96%. Dead material percentage in the pre-cutting forage mass varied (p<0.05) with defoliation frequency and with the frequency × defoliation severity interaction. No difference was found between li targets for the severity of 60%, while for 40% severity, higher values were recorded for li_Max relative to li_95% with intermediate values for li_90%. There was a difference between levels of defoliation severity only for li_Max, with higher values recorded for the 40% relative to 60% treatments (Table 5).

Table 5: Dead material percentage in pre-cutting forage mass (kg dm.ha^-1 ) in Paspalum notatum cv. inia Sepé subjected to different intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column or lowercase in the row are not significantly different (p> 0.05). SE indicates the standard error of the variable.

Weed percentage in the pre-cutting forage mass varied (p<0.05) only with growth period, with greater values recorded during Period 1 relative to Period 2 (18.6, 14.0 + 1.43% for Period 1 and 2, respectively). Inflorescence percentage in the pre-cutting forage mass did not vary between treatments, its mean value being 3.0 + 0.33%.

Crude protein percentage in the forage varied (p<0.05) with defoliation frequency. Higher values were recorded for the li_90% relative to li_Max treatments (13.0, 12.7 and 12.1 + 0.25% for treatments li_90%, li_95% and li_Max, respectively). adf percentage varied (p<0.05) with defoliation severity and with the frequency × defoliation severity interaction. No difference was found between li targets for the 60% severity, while for 40% severity, higher values were recorded for li_Max relative to li_90%, with intermediate values for li_95%. There was difference between levels of defoliation severity only for the li_90% treatments, with higher values recorded for the 60% relative to 40% treatments (Table 6).

Table 6: adf percentage in forage in Paspalum notatum cv. inia Sepé subjected to different intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column or lowercase in the row are not significantly different (p> 0.05). SE indicates the standard error of the variable.

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 li_Max relative to li_90% with intermediate values for li_95%. Differences in levels of defoliation severity were recorded only during period 2 for the li_90% treatments, with higher values recorded for 60% severity (Table 7).

Table 7: ndf percentage in forage in Paspalum notatum cv. inia Sepé subjected to different intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column or lowercase in the row are not significantly different (p> 0.05). SE indicates the standard error of the variable.

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 li_90% relative to li_Max with intermediate values for li_95% treatments. There was difference between levels of defoliation severity for the li_90% treatments, with higher values recorded for 40% (Table 8).

Table 8: Dry matter digestibility (%) in Paspalum notatum cv. inia Sepé subjected to different intermittent defoliation strategies from November 15, 2018, to March 31, 2019 

Means with the same uppercase in the column and lowercase in the row are not significantly different (p> 0.05). SE indicates the standard error of the variable.

4. 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 Japan²¹ and usa²², 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 (li_90%). 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 performance¹¹. 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 li_95% condition⁵. 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 circumstances⁵⁾⁽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 bibliography²¹⁾⁽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 li_Max/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 li_95% 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 summer²³ and within the range found in experimental studies²². In turn, for li_90%, 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 accumulation²⁴.

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 study⁷. 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 others²⁰ 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 (li_90% and li_95%), probably as a result of greater shading and lower photosynthetic efficiency associated with greater cutting intervals for the li_Max 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.

5. 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.