摘要:

AbstractThe leafage structure of ryegrass canopies and its role in canopy photosynthesis were compared under continuous and rotational grazing by sheep. Under continuous grazing, an increase in the intensity of grazing increased the proportion (by leaf area) of young leaves in the sward. A mechanistic mathematical model was used to demonstrate how this may have arisen, even though it would largely have been the young leaves that were eaten.However, the observations do not confirm the hypothesis that continuously grazed swards have a characteristically greater proportion of young leaves, and so a greater photosynthetic potential, than rotationally grazed ones. The proportion of young leaves increased during regrowth following severe rotational grazing (residual LAI<05) and the photosynthetic potential of the canopy became greater than under continuous grazing.A model of canopy photosynthesis was used to demonstrate that the observed difference in the proportion of young leaves alone was unlikely to account for all the differences in canopy photosynthesis between managements, and further differences in canopy structure were evaluated. Despite the delay in the restoration of leaf area following severe grazing in a rotation, the total photosynthetic uptake of a system involving some 12–13 days regrowth and 3 days grazing exceeded that of a well‐utilized continuously grazed sward. Re growths of longer duration led to progressively greater total photosynthetic uptake, though this was not considered synonymous with greater yi

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02136.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA study was made to characterize the effect of the duration of regrowth on the pattern of changes in the major physiological processes involved in the net accumulation of herbage, and so to provide a rational basis for optimizing production under rotational grazing.During regrowth following a severe defoliation, rates of canopy photosynthesis, and so the rate of production of new leaves, increased rapidly but there was a delay before there was a corresponding increase in the rate of leaf death. Although the amount available for harvest (crop live dry weight,W) continued to increase as the duration of regrowth was extended from ‘short’ (12–13 days), through ‘medium’ (19–23 days) to ‘long’ (30‐34 days), there was not a continued increase in the average growth rate – the increase in the weight of the crop, (W – Wo), divided by the duration of regrowth,t, In this study, the average growth rate (based on changes in the weight of lamina alone) increased as the duration of regrowth was extended from 12‐13 to 19‐23 days but changed little as the duration of regrowth was extended from 19‐23 days to 30‐34 days. In spring and summer, elongating stems increased the average growth rate (of lamina plus stem) up to 30‐34 days but the accumulated stem material could not reliably be harvested by sheep.A mathematical model was used to explain why the average growth rate is characteristically insensitive to the duration of regrowth beyond a given minimum duration. For practical purposes, we suggest from the results of this study that regrowths of at least 14 days but less than 28 days will be effective in achieving not only close to the maximum average growth rate of highly digestible material, but also in sustaining a densely tillered, leafy sward which regrows rapidly from severe defoliation and is more reliabl

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02137.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe possibility of increasing the herbage utilized over a grazing season was investigated in a study comparing continuously stocked steady‐state swards maintained at optimum height (3.5 cm) with intermittently grazed swards. The intermittent systems were designed (a) to allow periodic increase in leaf area and hence growth rate, (b)to ensure that the accumulated herbage was eaten before it senesced, and (c) lo retain high tiller density by alternating periods of herbage accumulation with periods of continuous stocking.Two treatments (no animals or animal numbers reduced to half those on the 3.5 cm steady‐state treatment) were used during the 17‐18‐d periods of herbage accumulation. Grazing down was completed in 3–4 d, after which two treatments (14 d or 28 d) were used for the intervening periods of continuous stocking when sward height was maintained at 3.5 cm.Herbage production was estimated using the tissue turnover technique, with tiller population densities and rates of growth, senescence and net production per tiller measured at frequent intervals.Intermittent grazing treatments where animals were removed during herbage accumulation resulted in changes in tiller size and number, and in growth rates, but not senescence rates, per tiller such that short‐term deviations in the net rate of herbage production occurred compared with the continuously stocked control. The periods of advantage during phases of herbage accumulation were counterbalanced by those of disadvantage during the subsequent steady‐state phases. Where animal numbers were reduced during herbage accumulation, sward conditions differed little from those of the continuously stocked control, implying that intake per individual animal was increased.It was concluded that intermittent grazing systems offered no advantage over simpler continuous stocking systems, provided that a flexible approach to conservation was incorporated to allow control of sward conditions on th

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02138.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe relationship between the rate of canopy gross photosynthesis and LAI was investigated for ryegrass swards released from grazing after a period of continuous stocking to maintain the pasture at LAI 2–3. The regrowing swards were allowed to increase to about LAI 5 over a period of about 2 weeks while the rate of photosynthesis was measured and compared with that for continuously stocked swards maintained at LAI values between 2 and 6. Gross photosynthesis was found to increase more rapidly than LAI on the regrowing swards than on the steady‐state control pastures. However, this response appeared to be influenced by seasonal factors. The potential increase in net COi uptake was estimated to range from 0 to 22%. The response was attributed to an increasing proportion of young photosynthetically‐efficient leaves near the top of the canopy, modified by seasonal changes in the rate of tillering. The agronomic implications of the results are isc

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02139.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA previously described model of grass growth was used to analyse the effect of the severity of defoliation and the duration of the subsequent regrowth on the overall balance between photosynthesis, gross tissue production and leaf death and so on the amount that could be harvested/consumed per hectare under intermittent defoliation. Maximum yield per hectare was shown to be achieved whenever the fluctuations in LAI during regrowth and defoliation led lo the same low average LAI that was previously shown to give maximum yield (amount harvested/consumed per hectare) under continuous grazing. Thus, it is suggested that production under both continuous and intermittent defoliation may best be characterized, and diverse managements may be rationalized, on the basis of the average sward state, the average achieved. The analysis leads us to reconsider some widely held concepts of the growth and utilization of grass applied in both agronomic and ecological theory, and the provision of practical guidelines for management.

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02140.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractHerbage growth was measured in two contrasting years on five farms representing a range of soil types. On fields cut for conservation in 1982 DM production varied between farms from 12.3 tha‐1to 14.1 tha‐1in line with fertilizer N levels, which ranged from 220 to 333 kg ha‐1. In 1983 production was from 10.3 tha‐1to 12.3 tha‐1; this again corresponded with fertilizer N, except on farm B which had the shallowest soils. On fields used for grazing all farmers used rotational management. Herbage accumulation was measured by trimming to 25 mm and harvesting after 4 weeks. As on conservation fields, herbage accumulation was generally greater on farms using more N, at least up to 300 kg ha‐1.In 1982 growth consistently exceeded predictions based on plot experiments. In 1983 growth was on average less than predicted on grazing fields but slightly more than predicted on cutting fields‐We conclude that such predictions of annual production are a valuable aid to farm planning, and need not be reduced to allow for on‐farm conditions, although they are least good at low levels of N. Predictions of individual cuts were not satisfactory, with a strong tendency to overestimate first cut and underestimate second cut.There was no indication that annual herbage production was less on badly drained than on well drained soils. Indeed, in the dry summer of 1983 badly drained land appeared to have an advantage, despite considerable poaching damage in the spring. Furthermore, there was no evidence that production was deficient on swards dominated by Holcus a

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02141.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractUtilized Metabolizable Energy output was calculated and herbage utilization evaluated in two contrasting years on five profitable farms representing a range of soil types.Annual UME output was 72 GJ ha‐1on average, with a range from 47 to 91 GJ ha‐1. Grazed grass provided 66% of the utilized metabolizable energy, and conserved grass 34%. Higher utilized metabolizable energy output was not always obtained at higher fertilizer N inputs, even when soil moisture conditions favoured herbage growth.The efficiency of utilization of herbage conserved (almost always as silage rather than hay) was calculated by expressing utilized metabolizable energy output as a proportion of the quantity of herbage cut, measured by swath weighings; the mean value was 64%, with a range from 55 to 73%. Cellulose analyses indicated that mean dry matter losses via CO2and effluent were 10%; the remaining 26% loss appeared to be due to physical losses in the field, surface waste and wastage at feedout.For grazed herbage the utilized metabolizable energy output was expressed as a proportion of herbage accumulation measured over 28‐day periods. The resultant apparent efficiency of utilization averaged 67%, with a wide range from 51 to 83%. The lowest values were on badly drained farms.It is suggested that:(i) there is considerable potential for increasing output from grazing on dairy farms; higher grazing pressure and more flexible management would be needed. Targets should probably be set lower on badly drained soils;(ii) there is great potential for increasing the efficiency of utilization of conserved forage, by careful application of existing technology;(iii) on the farms studied the utilized metabolizable energy output from grazed and conserved fields appeared to be si

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02142.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractTwo compositionally similar, mature grass hays, of dry matter contents 699 and 836 g kg‐1, were treated with anhydrous ammonia at levels of 10, 20 and 30 kg NH3t DM‐1and 20, 30 and 40 kg NH31 DM‐1, respectively. Treatment at 10 kg t‐1did not inhibit moulding of the damp hay. The remaining treatments increased the nitrogen contents of damp and dry hay, respectively, from 12.5 and 13.8 g kg DM‐1to maxima of 250 and 270 g kg DM‐1and the water‐soluble ammonia‐nitrogen contents of oven‐dried samples from 24 and 23 g kg‐1to maxima of 118 and 88 g kg‐1of total nitrogen. While all treated hays had reduced levels of insoluble hemicellulose, ammoniation increased the insoluble cellulose content of damp hay only.In trials of 37 and 80 days duration, respectively, the untreated and 20 and 30 kg t‐1‘damp'‐ammoniated hays and the untreated and 20, 30 and 40 kg t‐1‘dry'‐ammoniated hays were offered ad libitum, unsupplemented or supplemented with 2 kg d‐1of a barley‐soya compound, to steers of 328 kg and 383 kg mean initial liveweight. Steers offered untreated and 20 and 30 kg t‐1ammoniated damp hays and untreated and 20, 30 and 40 kg t‐1ammoniated dry hays, all ad libitum and alone, consumed 5.50, 6.48 and 6.59 and 6.78, 782, 790 and 8.26 kg d‐1and gained 471,560 and 521 g d‐1and 490,699,794 and 572 g d‐1, respectively. When supplemented, the same hays, again offered ad libitum, produced live‐weight gains of 529, 515 and 736 g d‐1and 725, 705,726 and 895 g d‐1, respectively. The apparent in vivo digestibilities of hay dry matter, organic matter, gross energy, hemiceliulose, cellulose and

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02143.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA mixture of perennial and hybrid ryegrasses(234 g DM kg‐1) was forage harvested and ensiled after a 24‐h wilt in good ensiling conditions in 2‐t capacity silos with no additive application (control) or with the application of eitherLactobacillus plantarum, 4 × 106(g fresh weight of grass)‐1, or of 31 formic acid t‐1. Sufficient 2‐kg capacity laboratory silos were also filled with grass to monitor the changes in chemical composition of the ensiled grass with time. In laboratory silos, inoculation withL. plantarumresulted in a more rapid fall in silage pH (p<0.001) and a more rapid production of lactic acid (P<0.001) than in the control silage. At the end of the storage period (laboratory silos, 80 d; 2‐t silos, 200‐300 d), the inoculated silos had lower pH (pammonia‐N (g kg N1) and acetic acid contents (p<0.01) and higher water soluble carbohydrate (WSC), lactic acid (P<00.1) and ethanol (p<0.05) contents than the control silage. The formic acid‐treated silage had significantly lower contents of ammonia‐N (g kg N‐1,p<0.05), acetic and tactic acids (p<0.01) and higher contents of WSC and ethanol (p<0.01) than the control silage. When fed to wether sheep, the digestibilities of DM, organic matter and gross energy were not altered by additive treatment. The digestibility of modified acid‐detergent fibre was lower for both the inoculated (P<0.01) and formic acid‐treated silages (p<0.05). However, N retention was improved (p<0.05) by both additive treatments. Silage intake was improved (p<0.01) by additive treatment from 53.4 (control) to 58.0 (inoculated) and 60.4 (formic acid) g

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02144.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe effects of cutting height (20 and 44 mm), frequency (once every 2 weeks April‐September inclusive and twice a year) and seed rate (250 and 20 kg ha‐1) on the water use and herbage yield of perennial ryegrass swards, for 3 years after sowing, were compared.Frequent cutting and a low cutting height reduced water use in the first year only. In later years infrequent cutting led to higher yields and higher water‐use efficiencies, but did not affect total water use or the shape of the soil water profile. Seed rate influenced sward composition and the pattern of water use, but not total water use. The swards from the low seed rate had a lower stubble biomass and tiller density, and extracted relatively more water at depths of 75 and 85 cm and less at depths of 15 and 35 cm, than the swards derived from the high seed rate.The ratio of actual: potential evapotranspiration was reduced as the soil water deficit increased and was lowest in the summer after a dry spring. Cutting height and frequency during summer were shown not to be effective methods of either conserving water or manipulating the effective rooting depth of an established

ISSN:0142-5242    

DOI:10.1111/j.1365-2494.1988.tb02145.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY