Use of residual leaf area index and light interception as criteria for spring-grazing management of a ryegrass-dominant pasture

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1 New Zealand Journal of Agricultural Research ISSN: (Print) (Online) Journal homepage: Use of residual leaf area index and light interception as criteria for spring-grazing management of a ryegrass-dominant pasture C. J. Korte, B. R. Watkin & W. Harris To cite this article: C. J. Korte, B. R. Watkin & W. Harris (198) Use of residual leaf area index and light interception as criteria for spring-grazing management of a ryegrassdominant pasture, New Zealand Journal of Agricultural Research, 5:3, , DOI: / To link to this article: Published online: 1 Dec 011. Submit your article to this journal Article views: 71 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Agricultural Research, 198, Vol. 5 : Use of residual leaf area index and light interception as criteria for spring-grazing management of a ryegrass-dominant pasture C. J. KORTE B. R. WATKIN Agronomy Department, Massey University Palmerston North, New Zealand W. HARRIS Grasslands Division, DSIR Private Bag, Palmerston North, New Zealand Abstract Detailed measurements of herbage accumulation were made on a 'Grasslands Nui' perennial ryegrass (Lotium perenne)-dominant, 'Grasslands Huia' white clover (Trifolium repens) pasture under late spring grazing intensities, based on residual leaf area index (LAI), and grazing frequencies based on light interception. Lax grazi~g during late spring (residual LAI ) resulted In rank stalky herbage, whereas hard grazing (residual LAI 0.1-D.9) resulted in more leafy herbage and higher ryegrass tiller density. For the 6 weeks of the experiment, 16.5 and 1.6 t DM/ha accumulated with hard and lax spring grazing respectively. During autumn, delaying grazing until weeks after 95% light interception markedly increased green herbage accumulation compared with grazing at 95% light interception, without significantly reducing ryegrass tiller density or white clover content. It is concluded that in spring, in contrast to other seasons, management of ryegrass-dominant pasture to control reproductive development is a considerably more important criterion than management to control leaf area and light interception. Control of reproductive development can be achieved by close grazing, topping, and closing paddocks for conservation, and will provide pasture which is leafy and digestible in summer. Keywords: Grazing management; leaf area index; light interception; Lolium perenne L.; 'Grasslands Nui' perennial ryegrass; reproductive development Received February 198; revision May 198 INTRODUCTION The wide adoption of year-round rotational grazing in New Zealand, particularly with dairy cattle and increasingly with sheep, makes it important to determine the optimum stage of regrowth to graze pasture and the optimum grazing intensity. Brown & Blaser (1968) concluded that although the relationship between leaf area index, light interception, and pasture growth had been oversimplified, this concept was useful for improving the understanding of pasture growth and for developing better management practices. The purpose of this study was to critically examine the use of light interception and leaf area index as criteria for grazing a ryegrass dominant pasture in spring. Previous grazing studies using these criteria have often avoided spring when reproductive development of grasses can greatly influence herbage production. Grazing management to regulate reproductive development, particularly to move leafy growth into late spring and summer, has been proposed by Saxby (198) and Hall (1973). Although several studies have used light interception to define defoliation interval, the optimum stage at which to defoliate pasture remains uncertain. Defoliation before 95% light interception reduces annual herbage production (Wilson & McGuire 1961; Sheard & Winch 1966) but delaying defoliation after 95% light interception has been reported to both increase (Mitamura 197; Terai 1977) and decrease (Tainton 197b) herbage production. Therefore the effect of delaying grazing beyond 95% light interception was investigated. As much of the herbage accumulating in the later stages ofregrowth is dead leaf (Hunt 1970; Tainton 197a), which is largely avoided by grazing animals (Arnold 196; Thomson 1977), production of live and dead herbage was measured. Residual pasture height (Brougham 1960) and residual herbage mass (Baars et al. 1981) have been used to define defoliation intensity in grazing studies, but pastures of similar height or mass can have different leaf areas depending on the ratio of live: dead herbage and the ratio of leaf: non-leaf. A high and low residual leaf area index (LAI) were compared, but, as it was anticipated that repeatedly leaving a high residual LAI would reduce stubble quality (Hunt & Brougham 1967; Jackson 197; Ollerenshaw & Hodgson 1977), treatments with

3 310 New Zealand Journal of Agricultural Research, 198, Vol. 5 Table 1 Defoliation regimes for experiment. Residual leaf area indices are shown in brackets. FHH FHL FLH FLL IHH IHL ILH ILL All plots hard-grazed 10-1 Sep 1975 (0.8) Intensity treatments (5 Oct-ll Jan) Hard (0.9) Hard (0.7) Lax (1.7) Lax (.7) Hard (0.) Hard (0.5) Lax (1.5) Lax (1.6) Hard (0.) Lax (.8) Hard (0.3) Lax (1.5) Hard (0.) Lax (1.9) Hard (0.) Lax (0.9) Hard (0.1) Hard (0.6) Lax (3.0) Lax (.0) Lax (1 January-31 March) Lax (.3) Lax (.5) Lax (1.9) Lax (.) Lax (.) Lax (1.0) Lax (1.8) Lax (1.9) Lax (0.9) Lax (.9) Lax (.) Lax (1.) Lax (0.7) Lax (0.8) Lax (1.0) Lax (1.5) Lax (1.3) Lax (0.9) Lax (1.) Hard (1 April-1 July) Hard (0.) Hard (0.3) Hard (0.3) Hard (0.3) Hard (0.3) Hard (0.) Hard (0.) Hard (0.) Hard (0.5) Hard (0.8) Hard (1.0) Hard (0.3) Hard (0.) Hard (0.5) Hard (0.5) Hard (0.5) Experiment ended 1 July 1976 alternating grazing intensities were included. These latter treatments were compared because the timing of close grazings was expected to interact with reproductive growth of ryegrass. Residual LAI treatments were only applied during late spring to early summer. Since the grazing practice in one season can affect herbage production in subsequent seasons (Brougham 1960), carry-over effects for all treatments were measured under the recommended managements (Brougham 1970) of lax grazing in summer and closer grazing in autumn. Because tiller density and botanical composition can influence pasture production, the effects of the grazing treatments on these sward characteristics were also measured. MATERIALS AND METHODS Site The experiment was conducted at 'Tuapaka' sheep farm, Massey University, 1 km east of Palmerston North. The soil is a Tokomaru silt loam (Cowie 1978), in its natural state of medium fertility and poorly drained because of the presence of a fragipan. The site was adequately drained by a subsurface drainage system. Soil moisture determinations for the top 15 em of soil were made regularly from within the trial area. Except for weeks in late February 1976, soil moisture did not drop below 0.0 of dry soil weight, the point at which pasture growth is restricted on this soil type (Scotter et al. 1979a, b). Climatic data indicated that the summer of was wetter and cooler than average. Soil moisture and climatic data are presented by Korte (1981). Pasture establishment After cultivation out of old pasture, the site was sown on 5 March 1975 with 16 kg/ha 'Grasslands Nui' perennial ryegrass (Lolium perenne L.) and 3 kg/ha 'Grasslands Huia' white clover (Trifolium repens L.). Potassic superphosphate (6% P, 1% K) at a rate of 380 kg/ha was broadcast following sowing and further applications of 5 kg/ha each were made in July 1975 and March Lime at 1300 kg/ha was broadcast over the site in April The pasture was grazed lightly in May, June, and August The experiment began on 1 September Treatments In August 1975 the site was subdivided into fenced paddocks, each 15 m x 16 m. Eight grazing treatments were compared, the combinations of grazing frequencies and grazing intensities, arranged at random within 3 block replicates. Grazing frequencies during the whole experiment were: Frequent (F), grazed when the pasture canopy intercepted 95% of the photosynthetically-active radiation at noon; Infrequent (I), grazed weeks after the pasture intercepted 95% of the photosynthetically-active radiation at noon. Light interception was measured with a 'LI-COR' light meter with a 'Quantum' sensor (Lambda Instrument Corp. Nebraska) placed above and at the base of the pasture at several locations within each paddock at solar noon. Spring/early summer grazing intensities were: repeatedly hard (HH); alternating hard and lax (HL); alternating lax and hard (LH); repeatedly lax (LL). Hard grazing (H) left a residual LAI between 0.1 and 0.9, whereas lax grazing (L) left a residual

4 Korte et al.-criteria for spring-grazing management 311 LAI between 0.9 and 3.0. These grazing intensity treatments were applied between 5 October and 11 January. In late summer (1 January to 31 March) all treatments were lax-grazed, residual LAI From 1 April until the end of the experiment, 1 July 1976, all treatments were hard-grazed, residual LAI Each grazing by Romney sheep lasted 3-5 days. Table 1 shows the actual grazing regime for each treatment. Measurements Immediately before and after grazing, and at approximately weekly intervals between grazings, herbage within at least 3 quadrats (each 0.3 m ) per paddock was cut to ground level. To reduce withinpaddock variation, quadrats were located within each paddock at positions which gave an average capacitance probe reading after taking 5 readings (Jones & Haydock 1970). After cutting, herbage was washed and weighed. A 0Q-6()0 g subsample was dried to measure dry matter (DM) percentage and a second subsample was sorted into grass, white clover, other species, and dead herbage. The area of emerged grass and white clover lamina was measured with an automatic electronic planimeter (Hayshi Denkoh Co. Ltd.). Herbage which was no longer green was classified as dead. Partly dead leaves were separated into greenand dead fractions. The herbage mass (Hodgson 1979) of each fraction, including lamina, and leaf area index were calculated after drying and weighing the components of the second subsample. Using the technique of Mitchell & Glenday (1958) the densities of ryegrass, Poa, and other grass tillers and white clover shoots were determined from 0 or 30 cores (0.3 ern") per paddock taken before each grazing and again 8 days after grazing was completed. Calculations and analysis Net herbage accumulation (Hodgson 1979) was calculated using the method of Campbell (1966). As rest periods for the various treatments did not coincide it was necessary to pool data for seasonal periods to carry out analysis of variance for herbage accumulation of the complete factorial layout. Data were grouped 'into seasonal periods, viz. spring/summer (1 September to 8 February) and autumn/winter (1 March to 8 July), and for the whole experiment (1 September.to 8 July). Where 8 February was part way through a rest period for a treatment, the sum of the seasonal periods does not always exactly equal that for the whole experiment. Analysis of variance was used to partition the effects of grazing frequency and intensity. Unless otherwise stated, the interaction between these factors was not significant at P<0.05, the significance level used throughout this report. RESULTS Herbage mass Fig. 1 provides a detailed description of herbage mass during the experiment. The histograms were obtained by connecting adjoining means in each rest period. Reproductive development of perennial ryegrass was markedly affected by grazing intensity, resulting in swards with different amounts of grass stubble and dead herbage in summer (Fig. 1). For example, on 17 January, LL had higher levels of stubble (1310 kg DM/ha) than the other 3 intensities (50 80 kg DMlha, LSD 5% 380). The stubble died subsequently, as can be seen for FLL in late January (Fig. 1), so that by February there was significantly more dead herbage in LL than the other intensities. In contrast to LL, swards in the other 3 intensity treatments were largely vegetative and leafy in January (Fig. 1). The effectiveness of hard grazing in changing swards from reproductive to vegetative growth depended on timing. The first hard grazing of FHH and FHL in October only removed a fraction of the reproductive meristems and reproductive growth continued until the next hard grazing in December for FHH and for FHL (Fig. 1). Relatively few reproductive tillers developed after these latter grazings. At the first hard grazing of FHL, IHH, IHL, and ILH, ryegrass seedheads were appearing and reproductive herbage was largely eaten so that subsequent growth was vegetative. During autumn, dead herbage disappeared from swards, often at a rate similar to the accumulation of green herbage (Fig. 1). For example, between 16 April and 9 May in FLL, green herbage, mainly leaf, accumulated at 35 kg DM/ha/day whereas dead herbage disappeared at 30 kg DMlhaiday. Herbage accumulation During spring/summer the frequency x intensity interaction was significant for total and green herbage accumulation (Table ). Where grazings were at 95% light interception (F), accumulation was greater in :fih than LL, with HL and LH intermediate. Fig. 1 shows that this difference between FHH and FLL was particularly large in January (9 and - kg green DM/ha/day respectively). By contrast, where grazings were weeks after 95% light interception (I), LL was either greater (total herbage accumulation) or not significantly different (green herbage accumulation) from HH during spring/summer (Table ). In both these comparisons ILH was less than IHH, and IHL

5 31 New Zealand Journal of Agricultural Research, 198, Vol ±7 5± 9±5 7±8-13±7 87±7 80±5-1± 9± ±5 6± 18±3 6±7 3±9 3±8 6±5 TOTAL -15±5 DEAD ± GREEN Accumulation rates o Dead [] Other species 00 Grass lamina o ill] Grass stubble 6 90±1O 10±1 76±3 71±16 9±3 17± 5 l±8 19±11-1 ±16 16±13 90± 89±957±1 7±1 78±11 7±3 7± 0±5-10± -31±3 1± TOTAL DEAD GREEN FHL '" '"E Ql C) '" 0.D Q; 97±5 86± 69±6 7±16 3±9 5±1-3±7 TOTAL I 3±6 -±7 31±11 6±7 37±9 ±3-3±7 DEAD 6 63±8 90±6 38±6 66±3-3± 1±19 9± GREEN FLH 0 ± TOTAL -lo±3 DEAD 6 1±1 GREEN FLL 0 N D Fig. 1 Herbage mass (t DMfha) and average accumulation rates (kg DMlhaiday) during rest periods for each treatment. Green herbage includes grass stubble, grass lamina, and other species (predominantly white clover). Total includes green and dead herbage.

6 Korte et al.-criteria for spring-grazing management ±8 6± 86± 63±6-10±3 73± 81±1 13± 68±11 3± -9±3 1±3 6±6-0± 6±3 TOTAL DEAD GREEN ±7 19±8-1± TOTAL 10±3-0±5-0±3 DEAD 76±11 39±6 19±3 GREEN IHL '" Ol E Ql 0 Cl Ol -e 56±10 ±6 5±1 TOTAL Ql 6± -17±6-17± DEAD I 6 30±8 1±6 ±1 GREEN ILH 0 6 ILL o Fig. 1 Continued

7 31 New Zealand Journal of Agricultural Research, 198, Vol. 5 Table Effect of grazing frequency and grazing intensity treatments on herbage accumulation (t DMlha). Green herbage Dead herbage Total F I F I F Spring/summer HH HL LH LL LSD (P <0.05) Frequency Intensity Interaction Autumn/winter HH HL LH LL LSD (P <0.05) Frequency Intensity Interaction Whole experiment HH HL LH LL LSD (P< 0.05) Frequency Intensity Interaction NS NS NS NS NS NS 1. NS D NS 0.97 NS NS NS NS, non significant intermediate. Grazing frequency only had a significant effect on green and total herbage accumulation in LL, ILL being greater than FLL. Green herbage that accumulated in spring/summer consisted of 83% grass, 15% white clover, and % other species. The interaction described above for green herbage was also significant for grass accumulation in spring/summer. Accumulation of the 'other species' component during spring/summer was greater in HH than LL, with HL and LH intermediate (0, 90, 350, and 10 kg DMlha respectively, LSD 5%, 30). Dead herbage accumulation was greater in LL than HH in spring/ summer (Table ). During autumn/winter, significant frequency x intensity interactions were detected for green and total herbage accumulation (Table ). Accumulation of green herbage was on average 89% greater in I than F for HH, HL, and LH whereas ILL and FLL were not different: However, since considerably more dead herbage (158%) disappeared from swards in I than F, total herbage accumulation was similar in I and F for HH, HL, and LH but greater in FLL than ILL. In F, green herbage accumulation was 1.6 t DMiha greater in LL than LH, with HH and HL intermediate. Grass and white clover accumulation were greater in I than F during autumn/winter (3360 and 570 kg grass DM/ha and 780 and 90 kg clover DM/ha respectively). Since more herbage accumulated in spring/summer than in autumn/winter (13.0 t DMiha in weeks and 1.9 t DM/ha in weeks respectively), the green herbage accumulation interaction described for spring/summer was also evident for green herbage accumulation over the whole experiment (Table ). The greater dead herbage accumulation in F than lover the whole experiment (670 and -580 kg DMiha respectively) reflected the greater disappearance of dead herbage in I than F during autumn/winter. For the whole experiment total herbage accumulation was 1.96 t DMlha or 13% greater in HH than LL, with HL and LH intermediate. Leaf area index The LAI measured at 95% light interception was not significantly affected by grazing frequently in summer, autumn, or winter (Table 3). However, in summer, a higher LAI was measured at 95% light interception where the previous grazing had been hard compared with lax. LAI at 95% light

8 Korte et al.-criteria for spring-grazing management 315 Table 3 LAI at 95% light interception (pooled for all treatments). LSD F (P<0.05) Mean Summer (Dec-Jan): Previous grazing hard NS 6.0 Previous grazing lax NS.8 LSD (P <0.05) 1.0 NS 0.8 Mean NS 5.3 Autumn (Feb-Mar) NS 3. Winter (Jun) NS.9 NS, non-significant Table Correlation coefficients between residual LAI and accumulation rate (kg DMihaiday) for different herbage components measured over the first weeks regrowth in Octover (Frequent) or November (Infrequent). Lamina Stubble Green herbage Dead herbage Total herbage Frequent >.6* 0.711** 0.67 NS NS 0.59 NS Infrequent >.87 NS >.017 NS >.150 NS 0.15 NS NS Correlation coefficient significantly different form 0 at P< 0.01 (**), P< 0.05 (*), or non-significant (NS) Table 5 Effect of grazin treatments on ryegrass tiller density (thousand tillers/m s). 19 Sep 1 Jan 1 Aug Intensity HH HL LH LL LSD (P< 0.05) NS 1.5 NS Frequency F I LSD (P <0.05) NS NS NS interception was lower in autumn and winter than in summer. LA! measured at weekly intervals during rest periods were used to calculate the average LAI in each treatment during November-February. The average LA! was 11% greater in I than F, being 3.7 and 3.3 respectively (LSD 5% 0.3). Average LAI was' not significantly different for HH, HL, LH, and LL. Similar calculations during autumn/winter showed that the average LAI was greater in I than F. For example, the values for FHH and IHH were 1.9 and. respectively during March-May. After the first grazing of the experiment, all plots had received similar pre-treatments so the effects of residual LAI on regrowth could be investigated. Correlation coefficients (Table ) were calculated for net accumulation rate of herbage components and residual LA!. The latter ranged from 0.6 to. in F and from 0.3 to.0 in I. In the weeks following grazing (0 October-3 November in F or 10- November in I) there was no significant correlation between residual LAI and accumulation of green, dead, or total herbage. Following the October grazing of F paddocks, the accumulation rates of the components of green herbage, lamina, and stubble, were significantly correlated with residual LAI. A unit increase in residual LAI was associated with a decreased lamina accumulation rate of 15 kg DMlha/day and an increased stubble accumulation rate of 9 kg DMlha/day. The stubble was mainly ryegrass culm. Although significant, these regressions only accounted for 0-50% of the variation in accumulation rates. Lamina and stubble accumulation were not significantly correlated with residual LAI after the November grazing of I paddocks. Tiller cores Perennial ryegrass, Poa (P. annua L. and P. trivialis L.) and white clover were the main species in tiller cores, occurring in 98%, 69%, and 65% of cores respectively at the end of the experiment (1 August 1976). At the same sampling, other grasses (Bromus mol/is L., Glyceria fluitans (L.) R.Br., Agrostis tenuis Sibth., Cynosurus cristatus L., Holcus lanatus L.) occurred in % of cores and Taraxacum officinale Weber ex Wiggers, the main dicotyledonous weed, occurred in % of cores. Treatment effects on tiller and shoot densities were compared on 3 occasions when all paddocks were sampled within a few days (Table 5). Eight days after the start of the experiment no significant differences were detected in ryegrass tiller density. In January, after the intensity treatments were completed, ryegrass tiller density was lowest in LL and higher in the other 3 intensities. At the end of the experiment, 1 August, differences in ryegrass tiller density was no longer significant. The only other significant effect of treatments on tiller and shoot densities detected was for Poa on 1 August when ILL had twice as many Poa tillers (5700/m ) as other treatments ( /m, LSD 5% 000). DISCUSSION AND CONCLUSIONS Reproductive growth During late spring, reproductive growth by perennial ryegrasses greatly influenced the pattern of herbage accumulation, largely negating the use of light interception as a criterion for grazing, a finding

9 316 New Zealand Journal of Agricultural Research, 198, Vol. 5 similar to that of Sheard & Winch (1966). The most obvious effect of reproductive growth was on accumulation of ryegrass stem and dead herbage. The higher levels of grass stubble and dead herbage in LL than in the other 3 intensity treatments (Fig. 1), was a result of sheep avoiding consumption of grass stems with lax grazing but being forced to eat stems with hard grazing. In LL sheep removed leaf and some seedhead during grazing, and the stem of reproductive tillers accumulated in the sward, then subsequently died. By contrast, leafy vegetative growth was encouraged in HH, HL, and LH where hard grazing restricted growth of reproductive tillers. Using dairy cows, Holmes & McClenaghan (1979) also obtained leafy and rank pastures by hard and lax spring grazings respectively. The higher LAI measured at 95% light interception where the previous grazing had been hard compared with lax (Table 3), reflected different amounts of reproductive growth. For example, a LAI of 8.0 was measured at 95% light interception in FHH on 6 January compared with.7 in FLL on 8 December. Swards in FHH were vegetative with little dead herbage (560 kg DMlha) whereas swards in FLL contained more dead herbage (1780 kg DMlha) and considerable amounts of stubble (330 kg DMlha). Much of the stubble and dead herbage was ryegrass culm and this intercepted light to little photosynthetic effect. When the effect of residual LAI on herbage accumulation was investigated in October and November (Table ) the responses reflected different stages of reproductive development, not differential patterns of light interception. In October, stem elongation had just started in ryegrass, and hard grazing removed some apical meristems whereas few were removed with lax grazing. The positive correlation between stubble accumulation and residual LAI reflected different amounts of stem growth. In November, many seedheads had emerged and little further stem growth could be expected, explaining the lack of correlation between residual LAI and stubble accumulation at this time. Grazing frequency The grazing frequency treatments were designed to result in different levels of light interception and pasture production. As expected, the average LAI, and presumably light interception, was greater in I than F. This was because lamina continued to accumulate after 95% light interception in I whereas the LAI was reduced by grazing in F. However any advantage from delaying grazing after 95% light interception was dependent on whether the sward was vegetative or reproductive. During the reproductive period, despite 11% greater average LAI in I than F, herbage accumulation was only increased by delaying grazing after 95% light interception in LL. The difference between ILL and FLL appeared to be the result of greater secondary reproductive tiller growth in ILL, but this hypothesis could not be confirmed because reproductive tillers were not separated from vegetative tillers. In HH, HL, and LH, where secondary reproductive tiller growth was restricted by hard grazing, no advantage from delaying grazing after 95% light interception was observed during spring/summer. During autumn/winter when swards were vegetative, considerably greater green herbage accumulation was obtained by delaying grazing until weeks after 95% light interception (Table ). This difference could have been caused by either a difference in herbage growth or a difference in herbage losses through death and decay (Morris 1970). Average LAI calculations in autumn/winter indicated that more light was intercepted in I than F, so greater growth in I undoubtedly contributed to the difference in herbage accumulation. As the amount of green herbage after grazing (Fig. 1) and residual LAI (Table 1) was slightly less in F than I during autumn/winter, slightly less herbage death could be expected in I than F (Hodgson et a ). However, the lower dead herbage accumulation in I than F during autumn! winter (-190 and -850 kg DM/ha respectively) was probably influenced more by the amount of herbage available for decay than by death of green herbage. A greater amount of dead herbage was available for decay in I than F, a result of more dead herbage having accumulated during grazing in I than F. During the autumn periods, 30 kg DM/ha of dead herbage accumulated in I, whereas 70 kg DMiha of dead herbage disappeared from F, presumably caused by eating and perhaps greater trampling into the soil. Despite the possible confounding effect of senescence, the difference in green herbage accumulation between F and I in autumn and the results from other experiments support the hypothesis that light interception is a useful criterion for grazing of vegetative swards. Mitamura (197) and Terai (1977) also obtained increased herbage production by delaying defoliation after 95% light interception in vegetative swards. Tainton (197a) and Baars et a1. (1981) showed that laxer grazing of dense vegetative swards during autumn increased herbage production, presumably also because of increased light interception and growth. It is concluded that in autumn and winter, considerably more herbage can be accumulated by increasing light interception, either by laxer grazing or by delaying grazing until weeks after 95% light interception. Although delaying grazing until weeks after 95% light interception did not

10 Korte et al.-criteria for spring-grazing management 317 significantly reduce the tiller density or clover content of the pasture compared with grazing at 95% light interception, even longer regrowths in autumn and winter would reduce both herbage production and the clover content of the sward (Brougham 1970; Baars et al. 1981). Grazing intensity Despite considerable variation in residual leaf area index (LAI) following either hard or lax grazing in late spring, a contrast was maintained between treatments (Table 1). With lax grazing greater variation in residual LAI occurred because this permitted sheep greater opportunity for patch grazing. Intensity treatments were based on residual LAI in an attempt to obtain different levels of light interception. Although differences in residual LAI were achieved, differences in average LAI, and presumably light interception by leaves, were nonsignificant for intensity treatments. This was because lax grazing, although increasing the residual LAI, reduced the LAI at 95% light interception. During spring/summer, 9% more herbage accumulated in FHH than FLL, although both treatments had similar average LAI, and presumably light interception by leaves. Herbage accumulation could have been increased in FHH relative to FLL either because of increased herbage growth or because of reduced herbage losses through death' and decay (Morris 1970). Hodgson et al. (1981) reported greater growth at the same LAI from swards defoliated more closely, perhaps because of increased proportion of young leaves which are photosynthetically more efficient than older leaves (Woledge 1977). Also since FHH had a lower herbage mass than FLL (Fig. 1), less herbage death would be expected in FHH (Hodgson et ai. 1981). The negative correlation between lamina accumulation and residual LAI (Table ) indicates that more of the leaf left after lax grazing died and decayed. Presumably FHL and FLH were intermediate between FHH and FLL because they had an intermediate level of growth and losses. Herbage accumulation during spring/summer was greater in IHH than ILH, a pattern similar to that in F. ILL did not follow this pattern however, accumulating a similar amount to IHH, perhaps, as already mentioned, because the very lax infrequent grazing of ILL encouraged a greater amount of secondary reproductive tiller growth. "The sampling of tiller populations in January was carried out after most reproductive tillers had died so it largely recorded vegetative tillers. Despite the sward being relatively open in early spring, continued lax grazing reduced ryegrass tiller density almost 30% compared with hard grazing. These results are similar to those from other grazing experiments (Tainton 197a; Boswell & Crawford 1978; Baars et al. 1981) and agree with the conclusion of Langer (1963) that close, but not too severe grazing, favoured tiller numbers. Alternating grazing intensities prevented the decline in tiller population; a similar result to that of Ollerenshaw & Hodgson (1977) with mown swards. With grazing intensities similar between January and July, and wet autumn conditions, differences between intensity treatments disappeared. Cores taken before and after each grazing showed that the difference between LL and HH disappeared during autumn (Korte 1981). Except for greater invasion by Poa in ILL between January and August there was no effect of grazing intensities on botanical composition. The reason for the greater ingress of Poa was perhaps a combination of greater seeding by Poa and lack of competition from ryegrass in ILL (Thompson & Grime 1979). Despite a third of tillers being Poa, they contributed relatively little to herbage production since ryegrass tillers were considerably larger. The grazing treatments had no apparent effect on the density or occurrence of white clover, as reported in other studies (Brougham 1960), because of the initially low white clover content of the ryegrass-dominant pastures used in this experiment (only 0% of tiller cores contained white clover at the beginning of the experiment) and because 'Grasslands Nui' is more aggressive than other 'Grasslands' ryegrass cultivars towards white clover (Harris 1977). Practical implications The rate of green herbage accumulation declined with successive lax grazing during early summer, whereas it remained considerably higher and relatively constant in HH (Fig. 1). This confirms the observations of Saxby (198) and Hall (1973) that close grazing in spring will promote vigorous leafy growth compared to pasture that has become rank and stalky. Baars et al. (1981) also obtained considerably greater green herbage production (5%) from ryegrass-dominant pasture by grazing to leave a lower residual herbage mass (750 compared with 1700 kg DM/ha) in spring/summer. It is concluded that in late spring--early summer, green herbage production from ryegrass-dominant pasture will be increased by grazing to leave a low residual herbage mass and to prevent rank growth. A further advantage of preventing rank spring growth is that it results in pastures of higher nutritive value. For example, herbage samples collected on February from FHH and ILL had in vitro organic matter digestibilities of 77% and 53% respectively. Rattray (1978) also showed that an increased proportion of dead herbage in swards was associated with a large reduction in digestibility. The

11 318 New Zealand Journal of Agricultural Research, 198, Vol. 5 nutritive value of herbage may be reduced to such an extent by the presence of dead herbage that animal production is reduced, even with generous herbage allowances which allow scope for selection of high quality herbage (Arnold 196). For example, Lewis & Cullen (1973) and Scales et al. (1981) reported that lamb growth rates were considerably higher on pasture containing a lower proportion of dead material. The hard grazings in late spring that largely prevented ryegrass flowering and rank growth were not particularly severe. Hard grazings seldom removed more than 50% of the herbage and the residual herbage mass was at least 1.5 t DMiha (Fig. 1). In comparison, Jagusch et a1. (1978) reported that during spring in a farmlet experiment at Ruakura, sheep (1 eweslha) removed 5% of herbage offered and the residual herbage mass was 1.6 t DMiha. Such close grazing can undoubtedly reduce dairy cattle performance (Bryant 1980), but this must be balanced against the subsequent improvement in performance resulting from increased pasture quality and production. It is concluded that to obtain optimum herbage production and herbage quality, rotationally grazed ryegrass-dominant pasture should not be too laxly grazed in late spring. Open rank pasture should be avoided and an attempt made to obtain dense leafy pasture by hard grazing. The main effect of hard grazing was to reduce ryegrass flowering and stem growth. The most practical method of forcing livestock to graze pasture more closely during late spring is by closing paddocks for conservation as pasture growth exceeds animal requirements. Mechanical topping can also be used to help prevent accumulation of rank herbage. ACKNOWLEDGMENTS We thank the staff of Grasslands Division Herbage Dissection Laboratory. especially Y. S. Gray, for assistance with herbage dissections and tiller counts; R. N. Barkwith and R. F. Battersby, Agronomy Department for technical assistance; J. A. Raven, Dairy Husbandry Department, for measuring herbage digestibility; and I.B.M. (N.Z.) for financial assistance with computing. REFERENCES Arnold, G. W. 196: Factors within plant association affecting the performance and grazing animals. In: Crisp, D. J. ed. Grazing in terrestrial and marine environments. British Ecological Society symposium No. Baars, J. A.; Jagusch, K. T.; Dyson, C. B.; Farquhar, P. A. 1981: Pasture production sward dynamics under sheep grazing. Proceedings of the New Zealand Society of Animal Production 1: Boswell, C. C.'; Crawford, A. J. M., 1978: Changes in the perennial rye rass component of grazed pastures. Proceedings Of the New Zealand Grassland Association 0: Brougham, R. 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