SUMMARY
The nutritional quality of two tall fescue cultivars ( Festuca arundinacea) was evaluated.Schreb) during a summer regrowth (mean temperature: 21±2.8 °C), in relation to the morphogenesis of two cultivars (Traditional: El Palenque PlusINTA and Flexible leaves: Grasslands AdvanceGentos). The design was completely randomized (n=3) in 250 pots/cultivar in the open air, without water or N and P limitations. Eight destructive harvests of 300 vegetative tillers/harvest were carried out over a 14-week period. The frequency was adjusted to the ontogenic state of the leaves to obtain blades in 6 states of similar thermal age (from the beginning of growth to complete senescence) from 3 consecutive generations of leaves (G1, G2 and G3). Morphogenesis variables were measured in 15 marked tillers per cultivar. Age, Leaf Average Life (VMF) and Leaf Appearance Interval (AHI) were expressed in Growth Degree Days (GDC: ΣTem½ – 4 ºC). In the leaves, the length of the blades and sheaths was measured; and in the sheets the content of Neutral Detergent Fiber (NDF) and digestibilityin vitroNDF (DFDN) and MS (DAMS) by incubation for 24 hours in a DaisyII digester. Morphogenesis variables were evaluated as repeated measures over time; and for the quality ones, linear functions were adjusted with age and leaf length, and they were compared by means of dummy variable analysis. The cultivars did not differ in morphogenesis, with the exception of the VMF and the Number of Live Leaves per tiller (NHV), which were lower for Advance. The DFDN of both cultivars decreased linearly with age and with the increase in leaf length between successive regrowth leaves, with a higher rate in Advance. But the NDF content did not differ between cultivars, but remained constant during leaf life (54.5±3.2%) and increased during senescence (64.9±0.8%).
Keywords: Sheet quality; NDF; NDF digestibility; Leaf morphogenesis; Defoliation.
ABSTRACT
Leaf nutritive quality was evaluated in two tall fescue (Festuca arundinacea Schreb) cultivars, during a summer regrowth (average daily temperature: 21±2.8°C), in relation to morphogenesis parameters. Cultivars were El Palenque Plus INTA (traditional) and Grasslands Advance Gentos (soft leaf type). The experiment was conducted in pots (250/cultivar) arranged in a completely randomized design (n=3) under natural climate conditions, with out limitation of water, N and P. Swards were sampled 8 times (~300 tillers/harvest) over a period of 14 weeks, to obtain leaves in 6 ontogenic stages of similar age (from growing to complete senescence) corresponding to the 3 first sequentially appearing leaf generations (G1, G2, G3). Morphogenesis traits were measured in 15 marked tillers/cultivar. Leaf age, Leaf lifespan (LLS) and leaf appearance interval (LAI) were expressed in growing degree days (GDD: Σaverage temperature – 4.0 ºC). Leaf blades and sheaths were measured and leaf blade NDF and the in vitro digestibility (NDFD and DMD, incubation at 24 h DaisyII digester) were determined. The morphogenesis traits were evaluated using repeated measures data analysis. Quality traits were analyzed by linear function and dummy variables. The mean values of cultivars in each ontogenetic state were compared by ANOVA (Tukey, p <0.05). Cultivar did not differ in morphogenesis traits, with the exception of LLS and number of leaving leaves (NLL), which were lower in Advance. The NDFD declined with leaf age and length in both cultivars, with higher rate in Advance due to its shorter LLS. However, cultivars did not differ in NDF content, which remained stable throughout the leaf lifespan (54.5±3%) and increased during senescence (64.9±0.8%). Cultivars did not differ in NDFD at similar ontogenic stages, which indicates that Advance require more frequent defoliations than El Palenque to control changes of quality over time.
Keywords: Tall fescue; cultivars; Leaf quality; NDF; DNDF; Morphogenesis; Defoliation.
INTRODUCTION
Flexible-leaved cultivars of tall fescue ( Festuca arundinacea Schreb) were developed to improve palatability, consumption and animal production (Hopkins et al. , 2009). There are experiments that show that milk yields comparable to perennial ryegrass can be achieved with them (Milne et al. , 1997; Chapman et al ., 2007). However, the productive result is associated with the management of defoliation (Milne, 2001) which, as in perennial ryegrass (Fulkerson and Slack, 1994), depends on the synchronization between the frequency of defoliation and the dynamics of leaf turnover. (Donaghy et al. , 2008).
Biomass accumulates in successive cycles of leaf turnover. In these, the leaves gradually change in age and increase in length, which causes loss of forage quality (Wilson, 1976; Duru and Ducroq, 2002; Groot and Neuteboom, 1997). The increase in leaf length and the consequent loss of quality during regrowth is attributed to the progressive increase in pod length (Duru and Ducroq, 2002). Therefore, the reduction of the height of the pods with the management of defoliation decreases the length of the blades and increases the nutritional quality of the forage (Duru and Ducroq, 2002; Di Marco and Agnusdei, 2010).
The Leaf Half-Life (VMF), which is the period between leaf emergence and senescence, is a very useful parameter to synchronize defoliation frequency with leaf turnover dynamics (Lemaire and Chapman, 1996). . During the VMF, a Number of Live Leaves per tiller (NHV) accumulates, which depends on the quotient between the VMF and the Leaf Appearance Interval (IAH) of the species. For example, ryegrass can accumulate up to 3 live leaves per clump without the accumulation of senescent material (Fulkerson and Slack, 1994) because it has a VMF of 330 Growing Degree Days (GOD in ºCd) and an AHI of 110 GOD (330/110 =3). Therefore, the mentioned morphogenesis variables, or the number of leaves per tiller,
The objective of this work is to evaluate the nutritional quality of individual blades of two cultivars of Festuca arundinacea (traditional and flexible leaves) in relation to the morphogenesis of the cultivars. The study was carried out on individual leaves, in six ontogenetic stages of similar thermal age, which spanned from the beginning of growth to the complete senescence of the first three consecutive leaf generations of a regrowth cycle. This experimental protocol allows to analyze the quality of the blades of both cultivars in comparable ontogenetic states and to evaluate, in addition, if the leaf length has an additional effect on the leaf age.
MATERIALS AND METHODS
The nutritional quality of two cultivars of tall fescue (Festuca arundinacea Schreb.), a traditional cultivar (El Palenque PlusINTA) and another with flexible leaves (Grasslands AdvanceGentos), was evaluated. The trial was carried out at the INTA Experimental Agricultural Station (EEA) in Balcarce (Southeast Buenos Aires, 37º 45′ S; 58º 18′ W) during the summer of 2009/10.
The cultivars were sown (04/08/09) in 250 pots/cultivar (20 x 40 cm), with substrate extracted from the A horizon of a typical Argiudol soil, in a Completely Randomized Design (DCA, n=3). The trial was outdoors without water or nutritional limitations to simulate a non-restrictive environment for growth. Sprinkler irrigation was applied daily up to field capacity, and initial fertilization with Calcareous Ammonium Nitrate (150 kg N ha -1 ) and Diammonium Phosphate (50 kg P ha -1 ), plus weekly fertilizations with Calcareous Ammonium Nitrate. (35 kg N ha -1) throughout the trial. Prior to the beginning of the experimental period, 3 cuts were made to favor the increase in plant density and homogenize the age of tillers to form dense and leafy micropastures.
Prior to sampling, a homogenization and growth synchronization cut was made at 5 cm in height (12/8/09). The daily temperature was recorded, which was 21 ± 2.8 °C for the entire experimental period. Time was expressed as GDC, which was calculated as the accumulation of mean daily temperature above a base temperature of 4 °C. The AHI, the leaf elongation rate (TEF), the VMF and the NHV per tiller were recorded, with a frequency of three times per week in 15 tillers per cultivar. The VMF was measured as the accumulated GDC elapsed between the appearance of the visible lamina and the beginning of senescence.
At the same time, 8 destructive harvests were carried out during 14 weeks, in which ~300 vegetative tillers/experimental unit were obtained. The harvest frequency was adjusted to the state of development of the leaves to obtain the evolution of the first 3 generations of leaves that appeared sequentially during regrowth (G1, G2 and G3) in the following 6 ontogenetic states: growth, expanded, adult, pre -senescence, 50% senescence and 100% senescence.
Tillers were cut at the base level and immediately frozen in liquid N. Then, they were stored in a freezer (-20 °C) until their subsequent separation and processing. They were then thawed and kept in trays on ice during lamina and pod separation, by generation (G1, G2 and G3) in 6 age categories. Generations after G3 that appeared during this period were not studied and were discarded.
The sheets and pods were measured and weighed separately. The slices were lyophilized and ground with a Ciclotec type mill with a 1 mm mesh. Then the NDF content (in %) was determined (Van Soest et al ., 1991) and the NDF (DFDN) and true dry matter (DVMS) digestibility by incubation.in vitro of 250 mg of sample after 24 h of incubation in the DaisyII equipment. The DVMS became apparent digestibility (DAMS) by subtracting the metabolic factor 11.9 proposed by Van Soest (1994).
Data were analyzed with a linear statistical model for a DCA design with 3 replicates. The morphogenesis variables were analyzed as repeated measurements over time, taking into account the marked tillers as study subjects and the cultivar as a treatment factor. For the quality variables, the cultivars and the generations of the blades were evaluated by adjusting the linear functions with the GDC and making comparisons through the analysis of dummy variables. Data were analyzed with PROG REG and PROC GLM from version 8 of the SAS statistical package (SAS, 2001). A variance analysis was performed for a DCA of the variables of interest and the means were compared using Tukey’s test (p<0.05).
RESULTS
Variables of morphogenesis
Advance presented lower VMF and NHV than El Palenque Plus, without differentiating in TEF, AHI or leaf length. Blade and sheath length was similar between cultivars and increased between generations of successive regrowth leaves ( table 1 ).
Table 1 . Morphogenesis variables of “Advance” (A) and “El Palenque Plus” (EP). Different letters mean significant differences (p<0.05). SE: Standard error of the mean (n=3).
Blade quality at the same ontogenetic state
The cultivars did not differ in blade quality (DAMS, FDN and DFDN) at the same ontogenic state ( table 2 ). DAMS and DFDN decreased with the progress of leaf ontogeny (p<0.0001) and were highly related to each other (y = 0.68x (±0.02) + 26.4 (±0.68), R2 =0.95, p<0.0001). The NDF content did not change during VMF, but increased during the senescence process. Said parameter showed a moderate relationship with the DAMS (y = -1.91x (±0.17) + 162 (±9.8), R2=0.54, p<0.0001) and poor association with the DFDN (y = 2.2x (±0.3) – 163 (±17), R2=0.33, p<0.0001).
Table 2. Content and digestibility of NDF (DFDN) and DM (DAMS) of slices in different ontogenetic states of “Advance” (A) and “El Palenque Plus” (EP). SE: Standard error of the mean (n=3).
Effect of leaf age on the quality of the blades
The NDF content remained constant during the VMF at an average value of 54.5±3.2% for both cultivars, with no differences between them (p>0.05). However, during senescence, NDF increased to an average of 64.9±0.8% ( Figure 1a ). In contrast, DFDN decreased during VMF and senescence in both cultivars ( figure 1b). The loss rate during VMF was higher in Advance (parallelism p<0.001), but did not differ between cultivars during senescence (parallelism p>0.05). The decrease in DFDN with leaf age between generations of successive regrowth leaves was similar within each cultivar (Advance parallelism: p>0.05; El Palenque Plus: p>0.05), but the ordinates differed (Advance: G1 : 74.6, G2: 67.2, G3: 63.9, p<0.05, El Palenque Plus: G1: 69.1, G2: 62.2, G3: 57.1, p<0.05 ).
Figure 1 . Content (A) and NDF digestibility (DFDN, b) of “Advance” (black) and “El Palenque Plus” (white) slices in relation to leaf age. Arrows show the beginning of senescence. In B: G1, ●○; G2, ■□; G3, ▲Δ.
Effect of leaf length on blade quality
In adult leaves (from the expanded leaf to the end of VMF) the NDF content was not affected by the increase in blade length (54.5±3.2%) between generations. , but DFDN decreased at a rate of approximately 0.6 for every 10 mm increase in blade length ( figure 2 ), with no differences between cultivars (p>0.05 coincidence).
Figure 2. Effect of leaf length on NDF digestibility (DFDN) for “Advance” (black) and “El Palenque Plus” (white) in adult leaves. G1, ●○; G2, ■□; G3, ▲Δ. Y=-0.57x(±0.1)+61.2(±2.7), R2=0.67; p<0.0001.
DISCUSSION
Morphogenesis of the cultivars
The lower VMF of Advance compared to El Palenque Plus (490 vs 630 GDC) explained its lower NHV (2.5 vs 3.3 NHV, respectively), since the NHV arises from the ratio between the VMF and the IAH (Lemaire and Agnusdei , 2000). In both cultivars, the VMF was in the range of 500-600 GDC cited for tall fescue (Gao and Wilman, 1994; Lemaire and Chapman, 1996), which corresponds to a species with a moderate rate of leaf turnover compared to perennial ryegrass that it has a VMF of 330 and is classified as a species with a high rate of leaf turnover (Davies, 1988).
Quality of the sheets during ontogeny
During VMF (from emergence to pre-senescence) the DAMS decreased from approximately 70 to 50%, reaching 30% in the 100% senescent sheet ( table 2 ). Variations in this parameter were highly associated with variations in NDFD (R2=95%), and to a lesser extent with NDF content (R2=54%). Despite these variations, the cultivars did not differ to the same ontogenetic state ( table 2). NDFD decreased over time during VMF and NDF content remained unchanged, indicating that both variables are independent. For this reason, it is important to take NDFD into account for a better interpretation of the changes in forage quality that are commonly associated with an increase in NDF content.
Leaf age and quality of the blades
The NDF content of the green blades (during VMF) remained constant at an average of 54.5±3.2 for both cultivars ( figure 1a ), a value similar to that found by Buxton and Redfearn (1997) in tall fescue. The constancy of the NDF during leaf life agrees with that observed in other species (Di Marco and Agnusdei, 2010; Avila et al., 2010; Agnusdei et al., 2009). This is because NDF is formed in the growth zone located at the base of the lamina within the pod tube (Schnyder et al., 1987; Maurice et al.,1997). On the other hand, the increase in NDF during senescence is explained by the export of cell contents towards the growing leaves (Robson and Deacon, 1978).
The DFDN decreased progressively from 65% to 36% during the VMF, that is, from emergence to pre-senescence without differences between cultivars at the same ontogenetic state ( table 2 ). However, the ontogenic states of Advance passed faster because they had a lower VMF (-20%). This means that the aforementioned cultivar aged faster and, therefore, had a higher NDFD loss rate than in El Palenque Plus ( figure 1b). This suggests that the physical (Wilson and Mertens, 1995) and chemical (Akin, 1989; Jung and Allen, 1995; Buxton and Redfearn, 1997) processes that make the cell wall less digestible, occurred more rapidly in the cultivar with higher leaf turnover. fast. On the other hand, the marked drop in DFDN during senescence was due to the increase in the dry fraction of the lamina.
Leaf length and quality of the blades
The leaves of the successive generations increased in length and had less DFDN. Groot and Neuteboom (1997) and Duru and Ducroq (2002) maintain that the smaller NDFD of the longest leaves is associated with the increase in the length of the tube of sheaths, which determines the older leaf at the time it emerges from it. The results of the present work indicate that 0.6 NDFD points are lost for every 10 mm increase in length ( Figure 2 ), without changes in the NDF content. Similar results were obtained in other C3 and C4 species (Di Marco and Agnusdei, 2010; Avila et al., 2010; Agnusdei et al., 2009).
CONCLUSIONS
The results confirm that the quality of the two tall fescue cultivars under study does not remain static during the vegetative stage. The digestibility of the sheets decreases with the age of the leaf and with the increase in leaf length between generations of successive leaves that make up a regrowth cycle. The loss of quality of the sheets is due to the decrease in the NDFD. The NDF content only increases during the senescence process of the sheets, but remains stable in the sheets until the end of their life cycle. At the same ontogenic state, the cultivars present similar quality. However, the Advance cultivar has a lower VMF (490 vs 632 GDC), which means that it ages faster and therefore loses digestibility at a higher rate.
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