Yields and chemical composition of different parts of the common vetch at flowering and at two seed filling stages

In annual forage legumes such as the common vetch (Vicia sativa L.), in which the pods represent a substantial proportion of the total biomass, the optimum harvesting stage may not adjust to the classical model of decreasing nutritive value after flowering. The effect of harvest time on the yield of the main chemical components of common vetch was evaluated under field conditions typical of the Castilian Plain (Mediterranean continental-type climate under rain-fed conditions). Over two growing seasons (1996-1997 and 1997-1998), plants were harvested at flowering and at two stages during seed filling (characterized by a progressively higher dry matter content of the seed; 280 and 380 g kg respectively). The results show that the greater quality and quantity of the pods offsets the decrease in quality occasioned by the aging of the vegetative plant parts (stems plus leaves). Crude protein yields were not affected by delaying the harvest-time. Starch yields were higher when the plant was harvested during seed-filling. Sugar yields increased between flowering and seed filling in the first growing season but decreased in the second. Yields of structural components such as neutral-detergent f ibre, acid-detergent f ibre and acid-detergent cellulose, followed a pattern similar to that of crude protein. There therefore appears to be a flexible harvest period ranging from flowering until the beginning of seed filling (seeds with 380 g dry matter kg), which may be advantageous when trying to select optimum haymaking conditions.


Introduction
Common vetch (Vicia sativa L.) is an annual legume cultivated under rain-fed conditions in the semi-arid regions of Spain and other Mediterranean countries.In these areas, vetch is integrated into the conventional cereal-fallow crop rotation system.This species helps alleviate seasonal forage def icits and therefore contributes to the economic sustainability of cereal-sheep farming systems (Caballero, 1999).Moreover, this leguminous crop provides soil cover during the rainy season, f ixes atmospheric nitrogen, increases soil organic matter and breaks pest, disease and weed cycles (Lacasta, 1995;ICARDA, 1998).
In the mixed cereal and sheep production systems of the Castilian Plain, complementary vetch forage provides the bulk of the protein in sheep rations.Common vetch is harvested as hay in May-June and used to meet the winter-time forage def icit.Previous studies have illustrated the lack of quality standards for on-farm hays (Caballero et al., 1996a).Two main factors causing hay quality variation have been identif ied.The f irst, seeding rate, was addressed in a previous paper (Caballero et al., 1995); the second, harvesting time, is dealt with in the present paper.
Unlike in other leguminous forage crops such as alfalfa (Medicago sativa L.) or different types of clover (Trifolium spp.), common vetch pods account for a high proportion of the total plant biomass (Caballero et al., 1996b).The conventional model of decreasing quality of biomass after flowering may not be applicable to common vetch forage.We hypothesized an offsetting effect of the pods on the decreasing quality of the vegetative plant parts (stems plus leaves) after flowering.
The aim of the present study was to investigate how delaying harvesting beyond flowering and into the seed-f illing phase affects the chemical composition and yield of common vetch forage.Quantitative data were collected to assess the forage quality of common vetch for use in animal feeding systems.
The rainfall recorded during the three months before harvest was 1.5, 35.4 and 33.5 mm for March, April and May in 1997, and 19.2, 33.9 and 76.0 mm for the same months in 1998.The corresponding average mean temperatures were 13.8, 15.5 and 16.7ºC for 1997 and 12.7, 11.9 and 14.3ºC for 1998.Common vetch cv.Vereda was sown on November 27 th 1996 and November 25 th 1997 at a seeding rate of 80 kg ha -1 in rows 0.17 m apart.Before sowing, the experimental area was fertilized with 24.4 kg P ha -1 and 23.2 kg K ha -1 using a compound fertilizer (N-0, P-6.1, K-5.8).Linuron [N'-(3,4-dichlorophenyl)-Nmethoxy-N-methylurea] (0.875 kg ha -1 ) was applied for pre-emergence weed control.Vetch forage was harvested at three stages: at flowering (F) (more than 50% of plants with flowers), and at two seed filling stages def ined by a progressively higher seed dry matter (DM) content (means of 280 and 380 g DM kg -1 for seed filling stages 1 and 2 [SF1 and SF2] respectively).More advanced growth stages were not studied since vetch reaches full pod growth before 500 g DM kg -1 seed is attained (Caballero, 1989;Caballero et al., 1996b).
Harvesting stage treatments were allocated following a completely randomised design with three replications in single plots measuring 4 × 20 m.Forage yields were determined by harvesting a 1.5 × 10 m area in the centre of the plots.All plants were cut approximately 5 cm above the level of the soil with a scythe.
Plants assigned to the flowering stage were harvested on May 28 th 1997 and May 8 th 1998.Twenty individual plants were randomly selected from each single plot and separated into leaves and stems.The dry matter contents of fresh whole forage and plant parts were determined by oven-drying at 60ºC for 22 h and subsequently at 80ºC for 2 h.Mean leaf to stem ratios were calculated.
Plots assigned to SF1 and SF2 were harvested on the 12 th and 21 st June 1997 and on the 20 th May and 4 th June 1998 respectively.In all cases, 30 randomly selected plants from single plots were separated into leaves, stems and pods.Whole fresh forage and plant part samples were oven-dried as described above.
Approximately 3 kg of fresh forage were randomly collected from each plot at the two seed-filling stages and the vegetative parts (stems plus leaves) and pods separated by hand.Samples for chemical analyses were placed in freezer bags, frozen at -20ºC, freezedried and milled to pass a 1 mm screen.Samples of the whole plant at the flowering stage were similarly treated.

Yields and statistical analyses
Chemical component yields for the whole plant were calculated as the product of the DM yield of the plant and the concentration of the corresponding chemical components (also in terms of DM).
The variables analysed statistically were the content in CP, starch, sugar and cell-wall components (NDF, ADF, ADC, and ADL), and the yields of these chemical components.A two-factorial f ixed effects model including interactions (Y ij = µ + A i + B j + AB ij + ε ij , where i = 1,2,3 and j = 1,2) was used to examine the chemical component content.The main effects were harvesting stage (A) and plant parts (B).The mean squares of the main effects, and those of the interaction, were compared to that of the residual.Analyses of variance were performed by year since differences between years were detected, as shown by year × treatment interactions and the heterogeneity of the error.The 2V program of the BMDP statistical package was used (BMDP, 1992).
Values for indigestible neutral detergent f ibre (INDF) were estimated using linear (Chandler, 1980) and non-linear log-log (Traxler et al., 1998) equations, and expressed as percentages of neutral detergent fibre.

Yields of whole plant and morphological parts
Table 1 shows the dry matter yields for the whole plant and the proportion of its morphological parts (leaves, stems and pods).Overall mean whole plant yields of 5,730 and 8,501 kg DM ha -1 were recorded in the first and second season respectively.Mean leaf to stem ratios at the flowering stage (in terms of DM) were 1.64 and 1.23 in 1997 and 1998 respectively.Table 1 shows that the relative proportion of vegetative parts (leaves plus stems) decreased from SF1 to SF2, this effect being less intense in the first (160 g kg -1 ) than in the second growing season (254 g kg -1 ).The relative contribution of pods was much higher in the second than in the f irst season at both seed f illing stages studied.The annual mean DM content of fresh whole forage was 173 g kg -1 .

Chemical components
Table 2 shows the chemical composition of the whole plant and its vegetative parts at the three harvesting stages studied.The CP content decreased significantly (P < 0.001) over the two growing seasons as maturity increased, with mean differences of 30.4 and 69.4 g kg -1 DM in WP and WP-P respectively from F to SF2.As expected, the CP content of the pods (Table 3) was higher than that of the whole plant and its vegetative parts, but remained almost unchanged over the two pod maturation stages studied.Significant differences (P < 0.001) were also found in the CP contents of the different plant parts (Table 2).The vegetative parts showed a lower CP content than the whole plant at both seed f illing stages and in both growing seasons (the mean differences were 26.7 g kg -1 DM at SF1 and 39.0 g kg -1 DM at SF2).The rate of decrease of the CP content of the vegetative plant parts was more pronounced in the first (F-SF1) than in the second interval (SF1-SF2), with values of 3.78 and 1.70 g CP kg -1 DM day -1 respectively.The CP content  reduction rate of the whole plant was much lower than that of its vegetative parts (1.75 and 0.63 g CP kg -1 DM day -1 in the first and in the second interval respectively).
Table 2 shows the non-structural carbohydrate contents (starch and sugars) for the whole plant and its vegetative parts.The starch content of the whole plant showed an analogous pattern in both seasons, increasing significantly (P < 0.001) at a similar rate in both intervals as maturity advanced: 4.4 and 4.1 g kg -1 DM day -1 at F-SF1 and SF1-SF2 respectively.Across maturity stages, the starch content was significantly different in the two growing seasons, the maximum content for the whole plant being reached at SF2 (99.1 and 167.0 g kg -1 DM in the first and second season respectively).During the f irst growing season, the sugar content increased (P < 0.001) between F and SF1 but remained unchanged from SF1 to SF2; it decreased (P < 0.001), however, between flowering and SF1 in the second season.With respect to the pods, Table 3 shows there were significant differences (P < 0.01) in starch content between the two seed f illing stages (greater at SF2 than at SF1 in both seasons).On the contrary, the sugar content was not affected by seed maturity.With respect to cell-wall components (Table 2), the data for each plant part were analysed separately since the interaction harvesting stage x plant part was significant (due at least in part to the different trends observed in the WP and WP-P results, especially in the second season).Across seasons and harvesting stages, the vegetative parts showed a significantly (P < 0.05) greater proportion of structural components than the whole plant (mean differences: 46.6, 34.0, 26.1 and 8.8 g kg -1 DM for NDF, ADF, ADC and ADL respectively).For the vegetative parts, NDF, ADF and ADL contents increased from flowering to seed filling in both growing seasons.In the whole plant, however, the NDF content remained unchanged with advancing maturity in both seasons.The ADF and ADC content did not change in the first season and decreased in the second.The lignin (ADL) content of the whole plant increased from F to SF in the first season but remained unchanged in the second.In the pods (Table 3), the cell-wall components remained approximately unchanged in the f irst season but decreased significantly (P < 0.05) in the second.The higher seed proportion with increasing maturity (149 and 340 g kg -1 at SF1, and 309 and 594 g kg -1 at SF2, in the first and second seasons respectively) (Table 1) might balance out the progressively higher content in structural components of the vegetative parts, counteracting the effect of maturity on the whole plant fibre content (with the exception of ADL in the first season) (Table 2).

Yields of chemical components
Table 4 shows the whole plant chemical component yields.Besides the diminishing trend in the CP content mentioned above, CP yields were not signif icantly affected by maturity.For the three harvesting stages of the two study years, the overall mean value was 1,347 kg ha -1 .Yields of non-structural carbohydrates, however, were significantly affected by plant maturity.Across seasons, the highest whole plant starch yield was obtained at SF2 (mean 975 kg ha -1 ).This can be attributed to the accumulation of starch in the seeds at this stage.Sugar yields increased from F to SF in the first season, and decreased at SF2 in the second season.In both seasons, the yields of most f ibre components showed a similar pattern.The NDF, ADF and ADC yields were not signif icantly affected by maturity, whereas ADL yield increased from F to SF in the first season.
Large differences between the two growing seasons were found in the yields of chemical components.Thus, across harvesting stages, the yields reached in the second season were higher than those attained in the first (Table 4), the relative differences being 31% for CP, 327% for starch, 54% for sugars and 35-44% for all cell-wall components.

Discussion
The mean whole plant DM yield in the second growing season was 2,772 kg ha -1 greater than in the first (Table 1).Dry matter yields of common vetch and other annual legumes in this semi-arid environment are highly correlated to the quantity and distribution of rainfall during the months (March-April-May) prior to harvesting (Caballero, 1984).In the present study, the rainfall recorded for this period was 70.4 and 129 mm in 1997 and 1998 respectively.In addition, in the second growing season, the rainfall was more evenly distributed; in the first growing season most fell near flowering and had less influence on yield.
The whole plant CP content at flowering (mean = 209.6 g kg -1 DM; Table 2) was in the range of values reported in crop maturity studies performed by Thomson et al. (1990).The reduction in whole plant CP content from F to SF2 was 30.4 g kg -1 DM; a similar reduction was found by the above authors between early flowering stage and the late flowering-early pod stage.The general trend towards a lower CP content with increasing maturity was more evident in the vegetative parts than in the whole plant, the rate of CP reduction being higher in the first interval (F-SF1) than in the second (SF1-SF2).
Pod CP (Table 3) showed an overall mean value of 248.7 g kg -1 DM -98.5 g kg -1 higher than that of the vegetative parts.In contrast with the variation in CP observed in the vegetative parts (Table 2), pod CP changed only slightly between the two reproductive stages studied.This might be explained by the fact that during the period of rapid seed growth (SF1 to SF2), most of the CP accumulated in the grain had been redistributed from the vegetative to the reproductive structures (Caballero et al., 1998).The results of the present study also suggested a trade-off between the decreasing CP content of the vegetative parts and the increasing pod content (mean pod contribution to total harvested forage was 245 and 452 g kg -1 DM at SF1 and SF2 respectively) (Table 1).Haj Ayed et al. (2001) reported a similar effect in samples of common vetch and hairy vetch field-cured hays.
The non-structural carbohydrate results (Table 2) show the combined effect of morphological changes and of photosynthetic activity between flowering and SF2.As expected, a trend towards a higher whole plant starch content was seen with advancing maturity in both growing seasons.This is attributable to the combination of an increasing contribution of the seed to whole plant values (Table 1) and the accumulation of this carbohydrate in the seed during pod development (Table 3).Nevertheless, a greater starch content did not correlate with a proportional decrease in sugars (Table 2); this agrees with the results of earlier work (Caballero et al., 1998).In the latter paper, we indicated that most of the starch was derived from photosynthesis and not from sugar redistribution.Although starch and sugars represent the main digestible carbohydrate fractions, their ruminal degradation rates are very different.While sugars are rapidly fermented, starch must be hydrolysed to simple sugars before fermentation.The proportion of rapidly fermented carbohydrates relative to degradable CP in the rumen can affect the efficiency of microbial protein synthesis and CP utilisation (Gill, 1990).
An offsetting effect for the structural carbohydrate content of the whole plant was also observed.The trend towards higher contents in the vegetative parts of the plant with maturity (Table 2) was compensated by the greater contribution of the seed to the whole plant DM yield (Table 1).These results are in agreement with those of Thomson et al. (1990), who reported that the modified acid detergent content of vetch forage did not change between the early flowering stage and the late flowering-early pod stage.
In forages such as vetch, the energy supplied to ruminants by structural carbohydrates is related to the degree of NDF digestion.It is generally accepted that lignin is the primary entity in reducing the proportion of potentially digestible fibre.The estimated INDF values (Fig. 1), based on the lignin and NDF contents, increased from flowering to SF1 and remained almost unchanged during the two seed filling stages studied, regardless of the equation (linear or non-linear) used for predicting the amount of unavailable fibre.The present results also illustrate that the log-log equation used to estimate the energy content of the vetch gave a lower value than the linear equation, although it is unclear which has the best predictive power.Traxler et al. (1998) reported that both equations under-predicted actual cattle weight gains (the log-log equation to a greater extent), since the higher the INDF content, the lower the forage energy value.
During the two growing seasons studied, whole plant CP yields (Table 4) were not affected by maturity and showed an overall mean value of 1,166 and 1,528 kg CP ha -1 in the first and second seasons respectively.These values are higher than those reported by Moreira (1989) for hairy vetch (Vicia villosa Roth.) (799-1,204 kg CP ha -1 ) cultivated in northeastern Portugal, and by Piñeiro et al. (2002) for common vetch cv.Jaga (832-952 kg CP ha -1 ) in northwestern Spain.No harvesting stage effect was seen on structural carbohydrate yields; a significant effect was seen, however, on ADL and non-structural carbohydrate yields (Table 4).This contrasting behaviour might be explained by the combined effects of plant composition (in terms of chemical components) and the partitioning changes that occur.Hintz and Albrecht (1994) reported that maturity at harvest had the greatest effect on DM partitioning and chemical composition in soybean (Glycine max (L.) Merr) plant parts.In common vetch and soybean, the seed fraction accounts for a considerable proportion of the harvested biomass, and contributes substantially to the total yields of the most valuable chemical components.The relative contribution of the pods to whole plant yields in this study at SF1 and SF2 (mean of two seasons) were 32% and 60% for CP, 53% and 79% for starch, and 24% and 43% for sugars.
In conclusion, the DM and CP yields of common vetch depend more on the year and less on the growth stage at harvest (between flowering and SF2).In this period, a compensatory effect between the decay of the vegetative parts and seed development is seen.Growth stage, however, has a signif icant effect on nonstructural carbohydrate yields.This pattern does not agree with the conventional model for other forage legumes, such as alfalfa, in which harvesting after flowering involves a loss of energy and protein biomass.This harvest flexibility of common vetch may be an advantage when trying to determine the optimum haymaking conditions between flowering and seed-filling (the latter corresponding to a seed DM content of 380 g kg -1 , i.e., close to full pod growth but before the seed starts to change colour).

Table 1 .
Whole plant and morphological part (leaves, stems and pods) yields of common vetch at the three harvesting stages

Table 4 .
Chemical component yields of whole common vetch at the three harvesting stages (kg ha -1 on DM)