- Email: [email protected]
Changes induced by defoliation in the yield and digestibility of leaves and stems of perennial ryegrass (Lolium perenne L.) during reproductive development
EUropean
Journal
of
&P--Y
ELSEVIER
European Journal of Agronomy 6 (1997) 257-264
Changes induced by defoliation in the yield and digestibility of leaves and stems of perennial ryegrass (Lolium perenne L.) during reproductive development T.J. Gilliland
*
Department of Agriculture for Northern Ireland, Plant Testing Station, 50 Houston Road, Crossnacreevy, Bevast, BT6 9SH, UK Accepted
25 November
1996
Abstract Ten identical perennial ryegrass plots (cv. Frances) were sequentially harvested for first cut silage at 7 day intervals, with second cuts after 6 weeks regrowth and further cuts until the growing season ended. Total herbage dry matter and digestible organic matter yields increased and digestibility decreased with delayed chtting, as was expected. The opposite and counterbalancing response occurred at the second cut. This pattern of yield change was mirrored by changes in the amount of stem tissue, whereas leaf yield did not change significantly at the first harvest but declined at the second, in response to delayed cutting. Furthermore, stem digestibility declined at the first cycle of harvests
from ca. 75 to 62-64% in the most delayed cutting treatments. In contrast, leaf digestibility remained high (ca. 70%) until after seed-head emergence but then decreased rapidly to ca. 56%. This decline may have been associated with accelerated leaf senescence and redistribution of assimilates, though this needs to be examined. It was concluded that although manipulating first harvest date determined the proportioning of yield and digestibility in the first and second cuts, the observation that the combined yield and digestibility in these two harvests did not vary substantially is an important result for farming practice. 0 1997 Elsevier Science B.V. Keywords:
Conservation;
Development;
Quality;
Ryegrass;
1. Introduction
The technical requirements for converting fresh grass into properly fermented silage are well understood (Whittenbury et al., 1967) and the late timing of the first silage cut to increase yield and the consequent depressed digestibility of the harvested material has also been reported (Harkess,
* E-mail: [email protected]. 1161-0301/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ S1161-0301(96)02053-9
Timing; Yield
1981). Chestnutt et al. (1977) have also reported that an early first cut and a 4-6 weekly regrowth cycle maintained high digestibility with no significant penalty in yield of digestible material in comparison to an 8 weekly regrowth regime. Furthermore, much information is available on leaf turnover and tiller regeneration, though most of the detailed study into these aspects has been conducted on grazed swards (Jones et al., 1982; Johnson and Thornley, 1983). Despite this body of knowledge, farming prac-
258
T. J. Gillilond 1 Europeun Journal of Agronomy 6 i 1997) 257-264
tice still varies greatly. To optimise yield, first cuts are often taken when swards are at the fully headed stage, while early first cutting is often practised in order to achieve high digestibility but at a perceived large yield sacrifice. However, Gilliland (1995) reported that timing of the first silage cut pre-determined the rate of regrowth and herbage digestibility at the second cut. Furthermore, compensatory regrowth at the second and subsequent cuts was found to largely reverse any advantageous effects in terms of yield or quality resulting from manipulating timing of the first cut. It seems likely that these responses were determined by the proportion of reproductive tillers remaining after the first cut, which could potentially limit the digestibility at the following cut. However, as leaf and stem fractions were not separated in that study, the objective of the work reported here was to determine the relative importance of these fractions in controlling compensatory regrowth at the second and subsequent cuts following differently timed initial cuts. In agricultural practice, it is assumed that stem tissue is always of lower digestibility than leaf tissue. However, this is an oversimplification, as the quality of the leaf fraction is unlikely to be a constant as it ages and as stems are initiated as soft, green and highly digestible tissue, which progressively and substantially declines in quality (Harkess, 1981). The purpose of the present study was to provide better knowledge of the dynamic interactions between changing digestibility and yield of each of these fractions, when modified by differently timed harvests. The results were expected to provide a basis for formulating optimal cutting strategies for silage production, particularly when seasonal factors such as very late or very early spring growth induce large changes in the normal leaf-to-stem ratios present in developing silage swards.
2. Materials and methods An early maturing perennial ryegrass (Lolium was autumn sown at
perenne L. cv. Frances)
22 kg ha
’ on a medium loam soil, of boulder clay origin, pH 6.0 at Crossnacreevy, County Down, N. Ireland (54”32’N, Y52W). A fully randomised block design was used comprising four replicates of 10 identical plots ( 5 x 1.5 m) to which 10 different cutting time treatments were applied. In two consecutive years ( 1989, 1990), the plots were sequentially taken for the first silage cut of the growing season at 7 day intervals, producing a range of 63 days between the first and last cutting treatment (Table 1). Second cuts were taken 6 weeks after the first cut of each plot, so maintaining the 63 day range in cutting time, and imposing the same fixed period of regrowth for each plot. Further cuts were taken at varied intervals thereafter, as detailed in Table 1, to synchronise all plots to the same end of season date for the final cut. All cuts were taken at a target height of 60 mm above soil level by a Haldrup plot harvester with a reciprocating cutter-bar. In each harvest year a total of 400 kg ha-’ N, and 516 kg ha-’ K,O was 188 kg ha-’ P,O, applied, split into four dressings to provide N, 47 kg ha-’ P,O, and 129 kg 100 kg ha-’ ha- ’ K,O in spring and after each of the first three harvests. At each cut the total herbage of fresh material was sub-sampled to provide a 300 g sample for DM determination and a 500 g sample for separation into leaf (lamina from ligule to tip) and stem tissues. For the present study, ‘stem’ is defined as comprising true stem and leaf sheaths, but not leaf laminae, as these were removed and included in the ‘leaf’ fraction. These fractions were subsequently dried, weighed and the dry matter (DM) of total shoot tissue and of the leaf and stem fractions was calculated. The dry matter subsamples from the first three harvests of each plot were also subjected to modified acid detergent fibre (MADF) analysis, following exactly the methodology described by Alexander ( 1969). These data were used to assess herbage digestibility (percentage digestible organic matter in the dry matter, DOMD), expressed as g (kg OM)- ‘, and digestible organic matter (DOM) yield in the stem and leaf fractions. Analysis of variance was carried out to compare the yield responses in the various
259
T. J. Gilliland / European Journal of Agronomy 6 (1997) 257-264
Table 1 Schedule for sequential timing of cutting treatments
cut 1
Treatment
Date
1 2 3 4 5 6 7 8 9 10
2 May 9 May 16 May 23 May 30 May 6 June 14 June 19 June 21 June 4 July
cut 2
Treatment
Date
1 2 3 4 5 6 I 8 9 10
6 June 14 June 19 June 21 June 4 July 10 July 17 July 25 July 2 August 8 August
cut 3
cut 4 Cut 5 Cut 6
tissues and parameters attributable to the different cutting time treatments.
3. Results 3.1. Seasonal variation The mean date of ear emergence for cv. Frances, recorded from 60 singly sown spaced plants grown in the same location, was 17 May in 1989 and 5 May in 1990. In the first harvest year, the experimental plots were higher yielding than in the second harvest year, as would be expected. However, this feature resulted in treatment responses being most clearly expressed in the first harvest year, though the same pattern response was observed in both years. 3.2. DM yield Examination of the total DM yield production (Figs. 1(a) and 2(a)) showed the same pattern of accumulation as recorded in a previous study (Gilliland, 1995). Delaying the first cut significantly increased DM yields at that first cut (p
Treatment
Date
1+2 3 4+5 6+1 8 9+lO l-5 6-10 l-5 6-10 l-5
10 July 17 July 25 July 16 August 21 August 29 August 16 August 19 September 19 September 31 October 31 October
the initial cut did not change with delayed cutting date despite a difference of 65 days growth between treatments 1 to 10 (Figs. l(b) and 2(b)). Leaf yields at the second cut declined in line with the delayed initial cutting treatment, though this was more evident in the first harvest year (p ~0.05) than in the second harvest year, when the differences were not significant. Thereafter, leaf production appeared to be determined by the differing lengths of regrowth of each cut rather than any response to the timing of the treatments. As expected, stem production responded profoundly to delayed initial cut treatment (Fig. 1(c)Fig. 2(c)). The later the initial cut, the more stem was produced at that cut (p
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T. J. Giililand / European Journal qf Agronomy 6 ( 1997) 257-264
Fig. 1. Changes in total, leaf and stem tissue yield in response to date of initial cut, in the first harvest year. Standard errors: (a) cut 1, 0.214; cut 2, 0.157; cut 3,0.062; cut 4,0.041; cut 5 + 6, 0.051; (b) cut 1, 0.055; cut 2, 0.063; cut 3, 0.048; cut 4, 0.033; cut 5+6, 0.040; (c)cut 1. 0.189; cut 2, 0.024: cut 3. 0.009.
treatment, although this change was less than that found in the initial cycle of cuts. The digestibility of both the leaf (Figs. 3(b) and 4(b)) and stem (Figs. 3(c) and 4(c)) fractions declined in the initial cycle of cuts (p
Fig. 2. Changes in total, leaf and stem tissue yield in response to date of initial cut, in the second harvest year. Standard errors: (a) cut I, 0.243; cut 2, 0.121; cut 3.0.051; cut 4,0.036; cut 5+6, 0.049; (b) cut 1, 0.063; cut 2. 0.054; cut 3. 0.061; cut 4, 0.028; cut 5+6. 0.061; (c) cut 1, 0.095; cut 2, 0.031; cut 3, 0.018.
Conversely, the leaves retained their highest digestibility until the end of May, after treatments 4 or 5 had been cut, before declining to ca. 56%. At the second cut, digestibility in both fractions tended to increase with the delayed initial cut treatment, though this response was greater for the leaf fraction (~~0.05) than the stem fraction; differences were not statistically significant. There were no identifiable treatment effects at cut 3.
T. J. Gilliland 1 European Journal of Agronomy 6 (1997) 257-264
Fig. 3. Changes in total, leaf and stem tissue digestibility in response to date of initial cut in the first harvest year. Standard errors: (a) cut 1, 0.469; cut 2, 0.681; cut 3, 0.454; (b) cut 1, 0.541; cut 2, 0.595; cut 3, 0.270; (c) cut 1, 0.541; cut 2, 0.735; cut 3, 0.422.
261
I
I
Fig. 4. Changes in total, leaf and stem tissue digestibility in response to date of initial cut in the second harvest year. Standard errors: (a) cut 1, 0.555; cut 2,0.7735; cut 3,0.267; (b) cut 1, 0.509; cut 2, 0.530; cut 3, 0.349; (c) cut 1, 0.649, cut 2, 0.703; cut 3, 0.324.
3.4. DOM yield
The total digestible yields (DOM) followed exactly the same pattern of responses to timing of the initial cut as the DM yields (Figs. 5(a) and 6(a)) and this pattern was repeated in the stem fraction (Figs. 5(c) and 6(c)). However, in the leaf fraction DOM yield showed a slight though not significant downward trend at the first cut, and at the second cut, a small significant decline (p < 0.05)
in leaf DOM yield was recorded in the first harvest year (Figs. 5(b) and 6(b)).
4. Discussion The observed change in total herbage yield and digestibility with delayed first cut reported here is a well recognized response in forage species
262
T J. Gilliland 1 European Journal of Agronomy 6 ( 1997) 257-264
Fig. 5. Changes in total, leaf and stem tissue digestible yield in response to date of initial cut in the first harvest year. Standard errors: (a) cut 1, 0.187; cut 2, 0.132; cut 3, 0.008; (b) cut 1, 0.017; cut 2, 0.044; cut 3, 0.104; (c) cut 1. 0.125; cut 2, 0.024: cut 3, 0.102.
(Hornstein et al., 1985) and specifically in ryegrasses (Thomas et al., 1980). Furthermore, the dynamics of the compensatory growth induced in the second cut by the timing treatments has also recently been documented (Gilliland, 1993) and was thought to depend on the physiological stage of tillers at defoliation. The sooner a sward was cut, the greater was the number of short or immature reproductive tillers avoiding decapitation and
Fig. 6. Changes in total, leaf and response to date of initial cut Standard errors: (a) cut 1,0.192; cut 1. 0.102; cut 2, 0.057; cut 3, 0.018: cut 3. 0.133.
stem tissue digestible yield in in the second harvest year. cut 2, 0.150; cut 3, 0.016; (b) 0.017; (c) cut 1, 0.111; cut 2,
growing on for the second cut and, more importantly, inhibiting the renewal of vegetative tillers. As reproductive tillers had been shown to grow faster than vegetative ones (Stapleton and Jones, 1987), and as stem tissue is widely regarded as having a lower digestibility than leaf tissue, this explanation seemed a likely cause of these observations. Davies (i972) and Camlin (1978) had also come to similar conclusions. However, with reference to the results of the
T.J Gilliland / European Journal of Agronomy 6 (1997) 257-264
current study, the observation of Murdock (1989) that an early first cut and a 6 week regrowth will produce silage of high digestibility, would seem to oversimplify the interactions involved. Similarly, while Chestnutt et al. (1977) recorded no significant yield penalty between a high digestibility 4-6 week cutting cycle and an 8 week lower quality cycle, this does not conflict with the present study as the timing of the first cut was constant, and this is the important determinant of total yield and quality. While not contradicting previous findings, the present study showed that stem quality may not be the only factor important in determining sward quality in primary growth prior to the first silage cut. The changes in stem weight and digestibility occurred largely as had been predicted, but the absence of any change in leaf DM yield and the degree and speed of decline in leaf quality with delayed cutting was unexpected. Up to the end of May, the results complied with the commonly accepted concept that low quality stem was buffered or counterbalanced by high quality leaf material (Gilliland and Camlin, 1984). Thereafter, however, the large decline in leaf quality to lower digestibility values than the stems, was the primary cause of total herbage quality loss. The observation that the leaf DOMD remained high for a period of time before declining rapidly suggests that some threshold had been reached. This change occurred approximately 7 days later in the first harvest year than in the second harvest year. It was notable that the ear emergence dates for cv. Frances, measured on 60 individual spaced plants during each of these two experimental years, was separated by a similar number of days. Therefore, the most probable explanation of the decline in quality would seem to be leaf senescence and the associated loss of leaf protein to support the developing reproductive structures. However, if this was the complete explanation, it is surprising that the DM yield of leaf did not significantly decrease during the period of delayed cutting as leaf number is finite, being predetermined prior to entry to the reproductive mode, and leaf senescence and, presumably, loss of weight would be progressive. The leaf samples were not separated into green and senescent tissue to allow proper investigation
263
of this observation. If the proportion of senescent leaf was rising, this would obviously cause the digestibility of the total leaf fraction to decline rapidly. However, without direct measurements, it is not possible to discount the possibility that the quality of green leaf was also declining with age and contributing, in some degree, to this response. Further experimentation is clearly required to test if these observations are indeed simply a consequence of assimilate repartitioning, or whether, or in addition, leaf quality declines measurably with age over this time. At the second cut, the decline in leaf yield and increase in leaf and stem digestibility in association with delayed first cut can be explained by the differing tissue types present in the regrowth. After the initial cut of the later cut treatments, in which most stems were harvested, regrowth would have had to be preceded by the development of new vegetative tillers. Conversely, the earlier cut plots should have commenced growth sooner as existing vegetative and non-decapitated reproductive tillers could resume reproductive growth immediately. Therefore, although there was a constant 6 week regrowth for all second cuts, the period of active regrowth would have been less the later the date of the initial cut. This may have been the cause of the rise in digestibility observed at the second cuts. The tissue produced at the second cut from those treatments given an initially late first cut, may actually have been actively growing for a shorter time than in the plots which received an earlier first cut treatment and so would have been physiologically younger. As stem quality declines with age this would explain the observed quality rise between the different timing treatments at the second cut. However, in order to explain the rise in leaf quality a similar ageing effect would seem to be implied by these results. The results of this study add further support to existing knowledge, which indicates that there is little justification for the diverse timings of cutting in current farm practice, except when unavoidable due to unsuitable weather conditions. Delayed first cuts or long regrowth periods to subsequent cuts would seem to be a flawed strategy as yield and quality at subsequent cuts are largely predetermined and counterbalanced by the timing of the
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T.J. Gilliland / European Journal of‘ Agronomy 6 (1997) 257-264
first cut and as leaf quality, be it living or dead, has a large influence on total herbage digestibility of reproductively growing swards. It is also evident that even when good early growing conditions support a large production of leaf, giving a higher than usual leaf-to-stem ratio, this cannot be depended upon to maintain a high total sward digestibility over an extended time period. Therefore, as is often erroneously assumed in practice, overall quality will not remain high while additional first cut yield is sought by delayed cutting. Conversely, if leaf/stem ratios are low, digestibility may not necessarily fall more rapidly than usual, as decline in overall quality is not solely due to falling stem digestibility.
Acknowledgment Staff of the Grassland Laboratory, Plant Testing Station, Crossnacreevy, of DAN1 Agriculture and Environmental Science Division and Biometrics Division are thanked for expert technical assistance.
References Alexander, R.H., 1969. The establishment of a laboratory procedure for the in vitro determination of digestibility. The West of Scotland Agricultural College, Res. Bull., No. 42. Camlin, M.S., 1978. Spring growth and management of early, midseason and late varieties of perennial ryegrass. Agric. North. Irel., 52: 303-308. Chestnut& D.M.B., Murdock, J.C., Harrington, F.J. and Binnie, R.C., 1977. The effect of cutting frequency and
applied nitrogen on production and digestibility of perennial ryegrass. J. Br. Grass]. Sot., 32: 177-183. Davies, I., 1972. The pattern of reproductive development and the leafiness of spring and early summer regrowths in two contrasting varieties of ryegrass. J. Agric. Sci., 78: 27-35. Gilliland, T.J. and Camlin, MS., 1984. Varieties and the grassland farmer. 2. Varieties and silage production. Agric. North. Irel., 58: 350-356. Gilliland, T.J., 1993. Defoliation induced changes in leaf/stem ratio and yield/quality balance during reproductive development in two cultivars of perennial ryegrass (Lolium perenne L.). Asp. Appl. Biol., 34: 319-328. Gilliland, T.J., 1995. Production and Flowering of Perennial Ryegrass (L&m perenne L.) in relation to time of cutting. In: D.W. Jeffrey, M.B. Jones and J.H. McAdam (Editors), Irish Grasslands-Their Biology and Management. Royal Irish Academy Press, Dublin, pp. 41-48. Harkess, R.D., 1981. Frequency of cutting and herbage quality for silage. The West of Scotland Agricultural College, Agronomy Publication No. 627. Hornstein, J.S.. Buxton, D.R. and Wedin, W.K., 1985. Structural polysaccharide composition of maturing alfalfa and red clover. Agron. Abstr., p. 126. Jones, M.B., Collett, B. and Brown, S., 1982. Sward growth under cutting and continuous stocking managements: sward structure, tiller density and leaf turnover. Grass Forage Sci., 37: 67-73. Johnson, I.R. and Thornley, J.H.M., 1983. Vegetative crop growth model incorporating leaf area expansion and senescence, applied to grass. Plant Cell Environ., 6: 721-799. Murdock, J.C.. 1989. The conservation of grass. In: W. Holmes (Editor), Grass its Production and Utilization. Blackwell Scientific publications, London, pp. 1733213. Stapleton, J. and Jones, M.B., 1987. Effects of vernalization on the subsequent rates of leaf extension and photosynthesis of perennial ryegrass (Lolium perenne L.). Grass Forage Sci., 42: 27-3 1. Thomas, C., Gibbs, B.G., Aston, K. and Taylor, J.C., 1980. Some factors influencing the performance of beef cattle given silage. In: Forage Conservation in the 80’s. Proc. 11th Occas. Symp. British Grassland Society, Hurley, pp. 383-387. Whittenbury, R., McDonald, P. and Bryan-Jones, D.G., 1967. A short review of some biochemical and microbiological aspects of ensilage. J. Sci. Fed. Agric., 18: 441.
Journal
of
&P--Y
ELSEVIER
European Journal of Agronomy 6 (1997) 257-264
Changes induced by defoliation in the yield and digestibility of leaves and stems of perennial ryegrass (Lolium perenne L.) during reproductive development T.J. Gilliland
*
Department of Agriculture for Northern Ireland, Plant Testing Station, 50 Houston Road, Crossnacreevy, Bevast, BT6 9SH, UK Accepted
25 November
1996
Abstract Ten identical perennial ryegrass plots (cv. Frances) were sequentially harvested for first cut silage at 7 day intervals, with second cuts after 6 weeks regrowth and further cuts until the growing season ended. Total herbage dry matter and digestible organic matter yields increased and digestibility decreased with delayed chtting, as was expected. The opposite and counterbalancing response occurred at the second cut. This pattern of yield change was mirrored by changes in the amount of stem tissue, whereas leaf yield did not change significantly at the first harvest but declined at the second, in response to delayed cutting. Furthermore, stem digestibility declined at the first cycle of harvests
from ca. 75 to 62-64% in the most delayed cutting treatments. In contrast, leaf digestibility remained high (ca. 70%) until after seed-head emergence but then decreased rapidly to ca. 56%. This decline may have been associated with accelerated leaf senescence and redistribution of assimilates, though this needs to be examined. It was concluded that although manipulating first harvest date determined the proportioning of yield and digestibility in the first and second cuts, the observation that the combined yield and digestibility in these two harvests did not vary substantially is an important result for farming practice. 0 1997 Elsevier Science B.V. Keywords:
Conservation;
Development;
Quality;
Ryegrass;
1. Introduction
The technical requirements for converting fresh grass into properly fermented silage are well understood (Whittenbury et al., 1967) and the late timing of the first silage cut to increase yield and the consequent depressed digestibility of the harvested material has also been reported (Harkess,
* E-mail: [email protected]. 1161-0301/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ S1161-0301(96)02053-9
Timing; Yield
1981). Chestnutt et al. (1977) have also reported that an early first cut and a 4-6 weekly regrowth cycle maintained high digestibility with no significant penalty in yield of digestible material in comparison to an 8 weekly regrowth regime. Furthermore, much information is available on leaf turnover and tiller regeneration, though most of the detailed study into these aspects has been conducted on grazed swards (Jones et al., 1982; Johnson and Thornley, 1983). Despite this body of knowledge, farming prac-
258
T. J. Gillilond 1 Europeun Journal of Agronomy 6 i 1997) 257-264
tice still varies greatly. To optimise yield, first cuts are often taken when swards are at the fully headed stage, while early first cutting is often practised in order to achieve high digestibility but at a perceived large yield sacrifice. However, Gilliland (1995) reported that timing of the first silage cut pre-determined the rate of regrowth and herbage digestibility at the second cut. Furthermore, compensatory regrowth at the second and subsequent cuts was found to largely reverse any advantageous effects in terms of yield or quality resulting from manipulating timing of the first cut. It seems likely that these responses were determined by the proportion of reproductive tillers remaining after the first cut, which could potentially limit the digestibility at the following cut. However, as leaf and stem fractions were not separated in that study, the objective of the work reported here was to determine the relative importance of these fractions in controlling compensatory regrowth at the second and subsequent cuts following differently timed initial cuts. In agricultural practice, it is assumed that stem tissue is always of lower digestibility than leaf tissue. However, this is an oversimplification, as the quality of the leaf fraction is unlikely to be a constant as it ages and as stems are initiated as soft, green and highly digestible tissue, which progressively and substantially declines in quality (Harkess, 1981). The purpose of the present study was to provide better knowledge of the dynamic interactions between changing digestibility and yield of each of these fractions, when modified by differently timed harvests. The results were expected to provide a basis for formulating optimal cutting strategies for silage production, particularly when seasonal factors such as very late or very early spring growth induce large changes in the normal leaf-to-stem ratios present in developing silage swards.
2. Materials and methods An early maturing perennial ryegrass (Lolium was autumn sown at
perenne L. cv. Frances)
22 kg ha
’ on a medium loam soil, of boulder clay origin, pH 6.0 at Crossnacreevy, County Down, N. Ireland (54”32’N, Y52W). A fully randomised block design was used comprising four replicates of 10 identical plots ( 5 x 1.5 m) to which 10 different cutting time treatments were applied. In two consecutive years ( 1989, 1990), the plots were sequentially taken for the first silage cut of the growing season at 7 day intervals, producing a range of 63 days between the first and last cutting treatment (Table 1). Second cuts were taken 6 weeks after the first cut of each plot, so maintaining the 63 day range in cutting time, and imposing the same fixed period of regrowth for each plot. Further cuts were taken at varied intervals thereafter, as detailed in Table 1, to synchronise all plots to the same end of season date for the final cut. All cuts were taken at a target height of 60 mm above soil level by a Haldrup plot harvester with a reciprocating cutter-bar. In each harvest year a total of 400 kg ha-’ N, and 516 kg ha-’ K,O was 188 kg ha-’ P,O, applied, split into four dressings to provide N, 47 kg ha-’ P,O, and 129 kg 100 kg ha-’ ha- ’ K,O in spring and after each of the first three harvests. At each cut the total herbage of fresh material was sub-sampled to provide a 300 g sample for DM determination and a 500 g sample for separation into leaf (lamina from ligule to tip) and stem tissues. For the present study, ‘stem’ is defined as comprising true stem and leaf sheaths, but not leaf laminae, as these were removed and included in the ‘leaf’ fraction. These fractions were subsequently dried, weighed and the dry matter (DM) of total shoot tissue and of the leaf and stem fractions was calculated. The dry matter subsamples from the first three harvests of each plot were also subjected to modified acid detergent fibre (MADF) analysis, following exactly the methodology described by Alexander ( 1969). These data were used to assess herbage digestibility (percentage digestible organic matter in the dry matter, DOMD), expressed as g (kg OM)- ‘, and digestible organic matter (DOM) yield in the stem and leaf fractions. Analysis of variance was carried out to compare the yield responses in the various
259
T. J. Gilliland / European Journal of Agronomy 6 (1997) 257-264
Table 1 Schedule for sequential timing of cutting treatments
cut 1
Treatment
Date
1 2 3 4 5 6 7 8 9 10
2 May 9 May 16 May 23 May 30 May 6 June 14 June 19 June 21 June 4 July
cut 2
Treatment
Date
1 2 3 4 5 6 I 8 9 10
6 June 14 June 19 June 21 June 4 July 10 July 17 July 25 July 2 August 8 August
cut 3
cut 4 Cut 5 Cut 6
tissues and parameters attributable to the different cutting time treatments.
3. Results 3.1. Seasonal variation The mean date of ear emergence for cv. Frances, recorded from 60 singly sown spaced plants grown in the same location, was 17 May in 1989 and 5 May in 1990. In the first harvest year, the experimental plots were higher yielding than in the second harvest year, as would be expected. However, this feature resulted in treatment responses being most clearly expressed in the first harvest year, though the same pattern response was observed in both years. 3.2. DM yield Examination of the total DM yield production (Figs. 1(a) and 2(a)) showed the same pattern of accumulation as recorded in a previous study (Gilliland, 1995). Delaying the first cut significantly increased DM yields at that first cut (p
Treatment
Date
1+2 3 4+5 6+1 8 9+lO l-5 6-10 l-5 6-10 l-5
10 July 17 July 25 July 16 August 21 August 29 August 16 August 19 September 19 September 31 October 31 October
the initial cut did not change with delayed cutting date despite a difference of 65 days growth between treatments 1 to 10 (Figs. l(b) and 2(b)). Leaf yields at the second cut declined in line with the delayed initial cutting treatment, though this was more evident in the first harvest year (p ~0.05) than in the second harvest year, when the differences were not significant. Thereafter, leaf production appeared to be determined by the differing lengths of regrowth of each cut rather than any response to the timing of the treatments. As expected, stem production responded profoundly to delayed initial cut treatment (Fig. 1(c)Fig. 2(c)). The later the initial cut, the more stem was produced at that cut (p
260
T. J. Giililand / European Journal qf Agronomy 6 ( 1997) 257-264
Fig. 1. Changes in total, leaf and stem tissue yield in response to date of initial cut, in the first harvest year. Standard errors: (a) cut 1, 0.214; cut 2, 0.157; cut 3,0.062; cut 4,0.041; cut 5 + 6, 0.051; (b) cut 1, 0.055; cut 2, 0.063; cut 3, 0.048; cut 4, 0.033; cut 5+6, 0.040; (c)cut 1. 0.189; cut 2, 0.024: cut 3. 0.009.
treatment, although this change was less than that found in the initial cycle of cuts. The digestibility of both the leaf (Figs. 3(b) and 4(b)) and stem (Figs. 3(c) and 4(c)) fractions declined in the initial cycle of cuts (p
Fig. 2. Changes in total, leaf and stem tissue yield in response to date of initial cut, in the second harvest year. Standard errors: (a) cut I, 0.243; cut 2, 0.121; cut 3.0.051; cut 4,0.036; cut 5+6, 0.049; (b) cut 1, 0.063; cut 2. 0.054; cut 3. 0.061; cut 4, 0.028; cut 5+6. 0.061; (c) cut 1, 0.095; cut 2, 0.031; cut 3, 0.018.
Conversely, the leaves retained their highest digestibility until the end of May, after treatments 4 or 5 had been cut, before declining to ca. 56%. At the second cut, digestibility in both fractions tended to increase with the delayed initial cut treatment, though this response was greater for the leaf fraction (~~0.05) than the stem fraction; differences were not statistically significant. There were no identifiable treatment effects at cut 3.
T. J. Gilliland 1 European Journal of Agronomy 6 (1997) 257-264
Fig. 3. Changes in total, leaf and stem tissue digestibility in response to date of initial cut in the first harvest year. Standard errors: (a) cut 1, 0.469; cut 2, 0.681; cut 3, 0.454; (b) cut 1, 0.541; cut 2, 0.595; cut 3, 0.270; (c) cut 1, 0.541; cut 2, 0.735; cut 3, 0.422.
261
I
I
Fig. 4. Changes in total, leaf and stem tissue digestibility in response to date of initial cut in the second harvest year. Standard errors: (a) cut 1, 0.555; cut 2,0.7735; cut 3,0.267; (b) cut 1, 0.509; cut 2, 0.530; cut 3, 0.349; (c) cut 1, 0.649, cut 2, 0.703; cut 3, 0.324.
3.4. DOM yield
The total digestible yields (DOM) followed exactly the same pattern of responses to timing of the initial cut as the DM yields (Figs. 5(a) and 6(a)) and this pattern was repeated in the stem fraction (Figs. 5(c) and 6(c)). However, in the leaf fraction DOM yield showed a slight though not significant downward trend at the first cut, and at the second cut, a small significant decline (p < 0.05)
in leaf DOM yield was recorded in the first harvest year (Figs. 5(b) and 6(b)).
4. Discussion The observed change in total herbage yield and digestibility with delayed first cut reported here is a well recognized response in forage species
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Fig. 5. Changes in total, leaf and stem tissue digestible yield in response to date of initial cut in the first harvest year. Standard errors: (a) cut 1, 0.187; cut 2, 0.132; cut 3, 0.008; (b) cut 1, 0.017; cut 2, 0.044; cut 3, 0.104; (c) cut 1. 0.125; cut 2, 0.024: cut 3, 0.102.
(Hornstein et al., 1985) and specifically in ryegrasses (Thomas et al., 1980). Furthermore, the dynamics of the compensatory growth induced in the second cut by the timing treatments has also recently been documented (Gilliland, 1993) and was thought to depend on the physiological stage of tillers at defoliation. The sooner a sward was cut, the greater was the number of short or immature reproductive tillers avoiding decapitation and
Fig. 6. Changes in total, leaf and response to date of initial cut Standard errors: (a) cut 1,0.192; cut 1. 0.102; cut 2, 0.057; cut 3, 0.018: cut 3. 0.133.
stem tissue digestible yield in in the second harvest year. cut 2, 0.150; cut 3, 0.016; (b) 0.017; (c) cut 1, 0.111; cut 2,
growing on for the second cut and, more importantly, inhibiting the renewal of vegetative tillers. As reproductive tillers had been shown to grow faster than vegetative ones (Stapleton and Jones, 1987), and as stem tissue is widely regarded as having a lower digestibility than leaf tissue, this explanation seemed a likely cause of these observations. Davies (i972) and Camlin (1978) had also come to similar conclusions. However, with reference to the results of the
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current study, the observation of Murdock (1989) that an early first cut and a 6 week regrowth will produce silage of high digestibility, would seem to oversimplify the interactions involved. Similarly, while Chestnutt et al. (1977) recorded no significant yield penalty between a high digestibility 4-6 week cutting cycle and an 8 week lower quality cycle, this does not conflict with the present study as the timing of the first cut was constant, and this is the important determinant of total yield and quality. While not contradicting previous findings, the present study showed that stem quality may not be the only factor important in determining sward quality in primary growth prior to the first silage cut. The changes in stem weight and digestibility occurred largely as had been predicted, but the absence of any change in leaf DM yield and the degree and speed of decline in leaf quality with delayed cutting was unexpected. Up to the end of May, the results complied with the commonly accepted concept that low quality stem was buffered or counterbalanced by high quality leaf material (Gilliland and Camlin, 1984). Thereafter, however, the large decline in leaf quality to lower digestibility values than the stems, was the primary cause of total herbage quality loss. The observation that the leaf DOMD remained high for a period of time before declining rapidly suggests that some threshold had been reached. This change occurred approximately 7 days later in the first harvest year than in the second harvest year. It was notable that the ear emergence dates for cv. Frances, measured on 60 individual spaced plants during each of these two experimental years, was separated by a similar number of days. Therefore, the most probable explanation of the decline in quality would seem to be leaf senescence and the associated loss of leaf protein to support the developing reproductive structures. However, if this was the complete explanation, it is surprising that the DM yield of leaf did not significantly decrease during the period of delayed cutting as leaf number is finite, being predetermined prior to entry to the reproductive mode, and leaf senescence and, presumably, loss of weight would be progressive. The leaf samples were not separated into green and senescent tissue to allow proper investigation
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of this observation. If the proportion of senescent leaf was rising, this would obviously cause the digestibility of the total leaf fraction to decline rapidly. However, without direct measurements, it is not possible to discount the possibility that the quality of green leaf was also declining with age and contributing, in some degree, to this response. Further experimentation is clearly required to test if these observations are indeed simply a consequence of assimilate repartitioning, or whether, or in addition, leaf quality declines measurably with age over this time. At the second cut, the decline in leaf yield and increase in leaf and stem digestibility in association with delayed first cut can be explained by the differing tissue types present in the regrowth. After the initial cut of the later cut treatments, in which most stems were harvested, regrowth would have had to be preceded by the development of new vegetative tillers. Conversely, the earlier cut plots should have commenced growth sooner as existing vegetative and non-decapitated reproductive tillers could resume reproductive growth immediately. Therefore, although there was a constant 6 week regrowth for all second cuts, the period of active regrowth would have been less the later the date of the initial cut. This may have been the cause of the rise in digestibility observed at the second cuts. The tissue produced at the second cut from those treatments given an initially late first cut, may actually have been actively growing for a shorter time than in the plots which received an earlier first cut treatment and so would have been physiologically younger. As stem quality declines with age this would explain the observed quality rise between the different timing treatments at the second cut. However, in order to explain the rise in leaf quality a similar ageing effect would seem to be implied by these results. The results of this study add further support to existing knowledge, which indicates that there is little justification for the diverse timings of cutting in current farm practice, except when unavoidable due to unsuitable weather conditions. Delayed first cuts or long regrowth periods to subsequent cuts would seem to be a flawed strategy as yield and quality at subsequent cuts are largely predetermined and counterbalanced by the timing of the
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first cut and as leaf quality, be it living or dead, has a large influence on total herbage digestibility of reproductively growing swards. It is also evident that even when good early growing conditions support a large production of leaf, giving a higher than usual leaf-to-stem ratio, this cannot be depended upon to maintain a high total sward digestibility over an extended time period. Therefore, as is often erroneously assumed in practice, overall quality will not remain high while additional first cut yield is sought by delayed cutting. Conversely, if leaf/stem ratios are low, digestibility may not necessarily fall more rapidly than usual, as decline in overall quality is not solely due to falling stem digestibility.
Acknowledgment Staff of the Grassland Laboratory, Plant Testing Station, Crossnacreevy, of DAN1 Agriculture and Environmental Science Division and Biometrics Division are thanked for expert technical assistance.
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applied nitrogen on production and digestibility of perennial ryegrass. J. Br. Grass]. Sot., 32: 177-183. Davies, I., 1972. The pattern of reproductive development and the leafiness of spring and early summer regrowths in two contrasting varieties of ryegrass. J. Agric. Sci., 78: 27-35. Gilliland, T.J. and Camlin, MS., 1984. Varieties and the grassland farmer. 2. Varieties and silage production. Agric. North. Irel., 58: 350-356. Gilliland, T.J., 1993. Defoliation induced changes in leaf/stem ratio and yield/quality balance during reproductive development in two cultivars of perennial ryegrass (Lolium perenne L.). Asp. Appl. Biol., 34: 319-328. Gilliland, T.J., 1995. Production and Flowering of Perennial Ryegrass (L&m perenne L.) in relation to time of cutting. In: D.W. Jeffrey, M.B. Jones and J.H. McAdam (Editors), Irish Grasslands-Their Biology and Management. Royal Irish Academy Press, Dublin, pp. 41-48. Harkess, R.D., 1981. Frequency of cutting and herbage quality for silage. The West of Scotland Agricultural College, Agronomy Publication No. 627. Hornstein, J.S.. Buxton, D.R. and Wedin, W.K., 1985. Structural polysaccharide composition of maturing alfalfa and red clover. Agron. Abstr., p. 126. Jones, M.B., Collett, B. and Brown, S., 1982. Sward growth under cutting and continuous stocking managements: sward structure, tiller density and leaf turnover. Grass Forage Sci., 37: 67-73. Johnson, I.R. and Thornley, J.H.M., 1983. Vegetative crop growth model incorporating leaf area expansion and senescence, applied to grass. Plant Cell Environ., 6: 721-799. Murdock, J.C.. 1989. The conservation of grass. In: W. Holmes (Editor), Grass its Production and Utilization. Blackwell Scientific publications, London, pp. 1733213. Stapleton, J. and Jones, M.B., 1987. Effects of vernalization on the subsequent rates of leaf extension and photosynthesis of perennial ryegrass (Lolium perenne L.). Grass Forage Sci., 42: 27-3 1. Thomas, C., Gibbs, B.G., Aston, K. and Taylor, J.C., 1980. Some factors influencing the performance of beef cattle given silage. In: Forage Conservation in the 80’s. Proc. 11th Occas. Symp. British Grassland Society, Hurley, pp. 383-387. Whittenbury, R., McDonald, P. and Bryan-Jones, D.G., 1967. A short review of some biochemical and microbiological aspects of ensilage. J. Sci. Fed. Agric., 18: 441.