The effect of age and leaf length on the quality of wheatgrass blades ( Thinopyrum ponticum cv. Hulk) in an autumnal regrowth of two heights was studied. The trial was carried out in a greenhouse (17±4 ºC) in a BCA design (n=3) of 160 pots per block. Blades from 3 successive generations of leaves in 6 ontogenetic stages (from growth to advanced senescence) were studied. The blades were obtained from ~200 vegetative tillers/harvest, sampled over 52 days of regrowth, with a frequency of 80 GDC (growth degree days: Σ t1/2- 4.5 ºC). The foliar half-life (VMF), the interval of appearance (AHI), lamina and pod length, NDF content and in vitro digestibility were measured.of the FDN (DFDN). Linear adjustments were made between quality variables with age and leaf length, and slopes and ordinates to the origin were compared (PROC REG of SAS). The rest of the variables were analyzed according to BCA with factorial arrangement (2 heights x 3 leaf generations) using the SAS GLM procedure. The means were compared by Tukey’s test at 5%. The shorter pasture (241.8 vs 458.4 mm) had higher VMF (453.6 vs 337.8 GDC), similar NDF content (52.3%) and higher DFDN (~10%). In both pastures, DFDN decreased linearly with age and leaf length, but NDF was not affected during VMF. It is concluded that during regrowth the DFDN decreases with age and the increase in leaf length between successive leaves and that the reduction in pasture height delays senescence (>VMF).
Keywords : Growth; NDF; NDF digestibility; Age; Leaf length.
ABSTRACT
The effect of leaf age and length on leaf blade quality was studied in wheatgrass leaf blades leaves (Thinopyrum ponticum cv. Hulk) from swards of two heights. Swards were grown in plots in a greenhouse (17±4 ºC) in a randomized block design (n=3) of 160 plots/block. Leaves in 6 ontogenic stages from 3 consecutive leaf generations (from early growth to advanced senescence) were obtained from vegetative tillers (~200/harvest) harvested along 52 days of regrowth, with a frequency 80 GDD (growing degree days: Σ t1/2 – 4.5ºC). The leaf lifespan (LLS), leaf appearance interval (LAI), leaf and sheath lengths, NDF content and NDF digestibility (NDFD) were measured. The relationships among quality and morphogenic parameters were studied by linear regression and data analyzed as a complete randomized block design with factorial arrangement (2 heights x 3 leaf generations) and means compared by Tukey test (5%). The shorter leaf sward (241.8 vs 458.4 mm) had longer LLS (453.6 vs 337.8 GGD), similar NDF content and higher NDFD (~10%). In both swards the NDFD decreased linearly with leaf age and length, but NDF remained unchanged during the LLS. It is concluded that the NDFD declines with leaf age and length during regrowth, however shorter sward height delays senescence (>LLS), increases tiller leaving leaf number and improves leaf blade quality. 4 mm) had longer LLS (453.6 vs 337.8 GGD), similar NDF content and higher NDFD (~10%). In both swards the NDFD decreased linearly with leaf age and length, but NDF remained unchanged during the LLS. It is concluded that the NDFD declines with leaf age and length during regrowth, however shorter sward height delays senescence (>LLS), increases tiller leaving leaf number and improves leaf blade quality. 4 mm) had longer LLS (453.6 vs 337.8 GGD), similar NDF content and higher NDFD (~10%). In both swards the NDFD decreased linearly with leaf age and length, but NDF remained unchanged during the LLS. It is concluded that the NDFD declines with leaf age and length during regrowth, however shorter sward height delays senescence (>LLS), increases tiller leaving leaf number and improves leaf blade quality.
Keywords: Growth; NDF content; NDF digestibility; Leaf age; Leaf length.
INTRODUCTION
The elongated wheatgrass ( Thinopyrum ponticum ) is a perennial temperate forage grass of great importance in Argentine livestock due to its adaptability to soils with severe edaphic limitations (Mazzanti et al. , 1992). It is a species of rustic leaves that, if not correctly managed, progresses to structures of tall bushes of low forage value. However, it has high quality in the vegetative state (Aello et al. , 1981; Di Marco et al. , 1982; Garciarena et al. , 1984; Gándara and Gómez, 1987).
Previous works showed that the quality of forage grasses not only decreases with the advance of the phenological state, but also with the accumulation of leaf biomass in the vegetative state (Agnusdei et al., 2009 and Ávila et al.,2009). This is due to the fact that pastures, even in a vegetative state, progressively accumulate biomass in successive growth cycles, in which the leaves go through successive phases of growth, maturity and senescence (Chapman and Lemaire, 1993; Lemaire and Chapman, 1996; Lemaire and Agnusdei, 2000). In this way, as the regrowth time progresses, leaves accumulate in advanced ontogenetic stages that progressively increase in length. The present work was carried out to study the quality dynamics of wheatgrass blades during regrowth, in relation to age and the increase in length of successive leaves in two stabilized turf structures at different heights.
MATERIALS AND METHODS
The trial was carried out in a greenhouse with a plastic cover at the INTA Balcarce Agricultural Experiment Station (EEA) (southeast Buenos Aires, 37º 45′ S; 58º 18′ W). The long wheatgrass ( Thinopyrum ponticum cv. Hulk) pasture was sown on July 3, 2008 with a density of 430 seeds/m 2 . Plastic pots (20 x 40 cm) filled with black earth (horizon A of a typical Argiudol soil, Mar del Plata Series) were used, grouped into three complete random blocks of 160 pots per block. The test was conducted without limiting water or N and P (application of urea and diammonium phosphate at a rate of 400 kg ha -1 and 60 kg ha -1, respectively) to simulate a non-restrictive environment for growth. The average temperature during the test was 17 ± 4 ºC. The time was expressed in GDC (growth degree days: Σ t1/2- 4.5 ºC). The plants grew until November 12, 2008 when they were subjected to two defoliation regimes that lasted until March 2, 2009. Severe cutting was applied to half of the pots and light cutting to the other half, to make up two treatments. , which were two pasture structures of low (B) and high (A) heights. The severe cut was 5 cm from the soil with a frequency of 132 GDC, which represents a period equivalent to half of the Foliar Half-Life (VMF).
The slight cut was at the height of the sheath of the last expanded leaf (~15 cm from the ground) with a frequency of (396°GDC), which represents the complete VMF period. Subsequently, 8 harvests (~200 vegetative tillers) were carried out during 52 days (03/02/09 – 05/29/09) with a frequency of 80 GDC. Tillers from three successive regrowth generations (G1, G2 and G3) were harvested; each harvest was about 10 pots/block/treatment, which were not used again. The leaves were separated from the tillers and, from these, the blades and pods. In turn, the blades were separated into 6 age categories: growing, recently expanded or with a visible ligule, mature, at the end of the VMF and senescent blade, with less (S<30) or more than 30% (S> 30) of dry fraction.
VMF and leaf appearance interval (AHI) were recorded with a frequency of three times per week (5 marked tillers/replication). The VMF was measured as the accumulated GDC elapsed between the appearance of the visible lamina and the beginning of senescence.
The length of the blades and pods were measured separately. The slices were lyophilized and ground with a Ciclotec-type mill with a 1-mm mesh to determine the Neutral Detergent Fiber content (NDF, Van Soest et al., 1991) and the NDF digestibility (DFDN) by in vitro incubation of 250 mg of sample after 24 h of incubation in the DaisyI I equipment .
Linear functions were fitted between quality variables with age (GDC) and leaf length, and slopes and ordinates to the origin (PROC REG of SAS) were compared. The rest of the data were analyzed according to BCA with factorial arrangement (2 heights x 3 leaf generations) using the SAS GLM procedure. The means were compared by Tukey’s test (p<0.05).
RESULTS
Pasture B had a height 50% lower than A, higher VMF and number of live leaves, but did not differ in the rest of the morphogenic parameters under study ( table 1 ).
Table 1 . Morphogenetic and structural characteristics of an autumn regrowth of wheatgrass ( Thinopyrum ponticum ) pastures of different heights.
In pasture A there were no differences in leaf length between leaf generations, on the other hand, in pasture B the adult leaf length increased significantly from the first to the third generation of successive leaves (135.3 mm, 196.8 and 256, 7mm, respectively). In both structures, the blade length was highly associated (y = 3.20x + 20.60; R2=.0.80) to the sheath length.
The NDF content remained unchanged with age, or ontogenic state, at an average of 52% (SE: 1.6) in both structures, with no interaction between the pasture height and regrowth generation factors. However, NDF increased by 12% (y = 0.01x + 47.8; R² = 0.74) with leaf length (lamina + sheath).
The average DFDN of the set of leaves of the three successive generations of the regrowth was higher in pasture B ( fig. 1 ), in which it remained constant during the VMF of the blades in an average of ~57% and decreased to ~48 % in the green fraction of the leaves in senescence.
Figure 1. Digestibility of NDF (DFDN) of blades of wheatgrass ( Thinopyrum ponticum ) pastures of different heights. ◊: low ●: high. Each point represents the average (± ds) of three consecutive generations of leaves in the same ontogenic state (C: growing, E: recently expanded, M: mature, end of VMF and S<30% and S>30%: fraction senescent sheet green with less or more than 30% senescence, respectively).
In contrast, in pasture A the DFDN was ~52% during leaf elongation (until the appearance of the ligule) and subsequently decreased to an average of ~42% in the mature leaf, remaining at this level in the green fraction of the leaf in senescence. In individual slices, the effect of age on DFDN ( Fig. 2) showed interaction between the height and leaf generation factors. In G1 and G2, DFDN decreased linearly with the advancement of ontogenetic states. The slope was similar between pasture heights, but the intercept was ~12% greater in B than in A (65.8% vs 58.7%), indicating that the shorter blades emerged with greater NDFD than the longer ones. . On the other hand, in G3 the DFDN, which was lower than in the first two generations (50-55%), there was interaction with age. In B it remained constant (54.4%) and in A it decreased.
Figure 2. NDF digestibility (DFDN) of elongated wheatgrass ( Thinopyrum ponticum ) sheets in relation to leaf age. ◊: low structure; ●: tall structure. G1, G2 and G3: Consecutive leaf generations of an autumn regrowth from the beginning of growth to advanced senescence.
The DFDN decreased 0.024 percentage units/mm in G1 and G2 with the increase in leaf length, but did not show changes in G3, which presented the lowest value (52.1%). According to the equations shown for G1 and G2 ( fig. 2 ), the average NDFD would decrease from ~60% to ~50% with an increase in length between 100 and 500 mm.
DISCUSSION
The elongated wheatgrass is a very widespread species in soils with severe edaphic limitations in our country that drastically decreases its quality with the accumulation of biomass (Aello et al., 1981; Di Marco et al., 1982; Garciarena et al., 1984; Gandara and Gomez, 1987). However, under the conditions of this experiment, it showed phenotypic plasticity as pointed out by Nelson and Moser (1994) in other grasses. That is, it modified morphogenic and forage quality parameters in response to the applied mechanical defoliation regime.
In the first place, the height of the pasture decreased 50% with the cut at 5 cm from the ground and a frequency of ½ VMF. This is because the low cut reduces the length of the pod and consequently the size of the growth zone within it (Arredondo and Schnyder, 2003), which determines that the successive leaves are shorter (Wilson and Laidlaw , 1985 and Duru and Ducrocq, 2002). This photomorphogenic response explains the association between pod length and blade previously described in results, which has been previously highlighted by Wilson (1976); Groot; Neuteboom (1997) and Duru; Ducroq (2000).
Second, it increased the leaf life cycle (VMF) by 30%, which shows that the time of onset of senescence is not static. This effect has not been reported in the literature, in which it is accepted that the VMF expressed in thermal time is relatively constant under a wide range of environmental and management conditions (Lemaire and Chapman, 1996). The results shown here indicate that senescence is earlier in pastures with high structure and delayed in the lower one, which may be associated with the degree of shading of the leaves of the lower stratum and/or the quality of lighting, which changes the relationship red/far red of light (Deregibus et al., 1983).
Third, the decrease in pasture height caused an increase in the number of live leaves. This is to be expected since the VMF increased and the number of live leaves per tiller is determined by the quotient between the VMF and the leaf appearance interval (Lemaire and Chapman, 1996). An increase in the number of tillers in the low structure could also be expected, although it was not measured in this experiment, since the decrease in their weight (smaller leaves) is compensated by an increase in their number (Davies, 1988; Lemaire and Chapmann, 1996).
Finally, the height of the pasture affected the quality of the blades through its incidence in the DFDN, since the NDF content remained relatively stable in both pasture structures at an average of 52% (SE: 1.6), regardless of the ontogetic state. This is to be expected because cell wall (NDF) accumulation occurs in the maturation zone within the pod when the lamina is not yet visible (MacAdam and Nelson, 1987; Nelson, 1992). However, the leaf length had an effect of moderate practical importance, since according to the equation previously shown in results, the NDF would increase from 48.8% to 53.8% for a leaf length range between 100 and 600 mm. The results agree with previous work on other temperate and megathermal species (Agnusdeiet al., 2009; Avila et al., 2009 and Insua et al., 2012)
The DFDN increased approximately 10% with decreasing pasture height and also decreased with the advancement of the ontogenic state and with the increase in leaf length within both heights. of pasture ( fig. 3and 4). Once the lamina emerged, its NDFD decreased during the VMF with a similar rate in generations 1 and 2. In generation 3, on the other hand, the NDFD remained constant at the minimum level. This generation emerged with the lowest NDFD and was longer than the previous ones. Which can be explained because it grew inside a longer pod. Therefore, it leads to the assumption that in the maturation zone, located at the base of the pod, changes occur in the cell wall of the sheets in formation that make the leaves emerge with less DFDN. This can be interpreted as a photomorphogenic response to give greater support capacity to the longer leaf organs. Among the processes mentioned in the bibliography that make the cell wall less digestible (<
Figure 3. NDF digestibility (DFDN) of elongated wheatgrass ( Thinopyrum ponticum ) sheets in relation to leaf length. ◊: low structure; ●: tall structure. G1, G2 and G3: Consecutive generations of autumn regrowth leaves from early growth to advanced senescence.
The decrease in DFDN with the increase in leaf length was 0.02 units per mm of increase, with the exception of generation 3. The results of both treatments were located on the same descending line with the increase in leaf length. The short blades were located in the upper portion of the line and the long blades in its lower part. This indicates that the increase in leaf length caused by light competition negatively affects DFDN, as has been observed in other species (Groot; Neuteboom, 1997; Avila, 2009; Agnusdei, 2009).
The commented variations in the DFDN determined the quality of the material offered, with consequences on the consumption of dry matter and therefore on animal productivity (Oba and Allen; 1999). These authors estimate that for each unit increase in NDFD there is an increase of 0.177 kg/day in DM consumption and of 0.230 kg/day in milk productivity.
CONCLUSIONS
The results show that as the regrowth of a vegetative wheatgrass pasture advances, the NDFD decreases with age and leaf length, without affecting the NDF content. However, wheatgrass is a plastic species that modified its structure and nutritional quality in response to management. Defoliation with a frequency within the VMF period and an intensity that controlled pod elongation reduced pasture height and increased blade quality in two ways. Directly, by improving the digestibility of the NDF without affecting its content; and indirectly, by delaying senescence and increasing the number of live leaves per tiller. The opposite was also observed, that is,
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