Effect of defoliation management, based on leaf stage, on perennial ryegrass (Lolium perenne L.), prairie grass (Bromus willdenowii Kunth.) and cocksfoot (Dactylis glomerata L.) under dryland conditions. 1. Regrowth, tillering and water-soluble carbohydrate concentration

A field study was undertaken between April 2003 and May 2004 in southern Tasmania, Australia to quantify and compare changes in herbage productivity and water‐soluble carbohydrate (WSC) concentration of perennial ryegrass (Lolium perenne L.), prairie grass (Bromus willdenowii Kunth.) and cocksfoot (Dactylis glomerata L.) under a defoliation regime based on leaf regrowth stage. Defoliation interval was based on the time taken for two, three or four leaves per tiller to fully expand. Dry‐matter (DM) production and botanical composition were measured at every defoliation event; plant density, DM production per tiller, tiller numbers per plant and WSC concentration were measured bimonthly; and tiller initiation and death rates were monitored every 3 weeks. Species and defoliation interval had a significant effect (P < 0·05) on seasonal DM production. Prairie grass produced significantly more (P < 0·001) DM than cocksfoot and ryegrass (5·7 vs. 4·1 and 4·3 t DM ha^?1 respectively). Plants defoliated at the two‐leaf stage of regrowth produced significantly less DM than plants defoliated at the three‐ and four‐leaf stages, irrespective of species. Defoliation interval had no effect on plant persistence of any species during the first year of establishment, as measured by plant density and tiller number. However, more frequent defoliation was detrimental to the productivity of all species, most likely because of decreased WSC reserves. Results from this study confirmed that to maximize rates of regrowth, the recommended defoliation interval for prairie grass and cocksfoot is the four‐leaf stage, and for perennial ryegrass between the two and three‐leaf stages.     

Effect of defoliation management, based on leaf stage, on perennial ryegrass (Lolium perenne L.), prairie grass (Bromus willdenowii Kunth.) and cocksfoot (Dactylis glomerata L.) under dryland conditions. 2. Nutritive value

A field experiment was undertaken between April 2003 and May 2004 in southern Tasmania, Australia, to quantify and compare changes in the nutritive value of perennial ryegrass (Lolium perenne L.), prairie grass (Bromus willdenowii Kunth.) and cocksfoot (Dactylis glomerata L.) under a defoliation regime based on stage of leaf regrowth. Defoliation interval was based on the time taken for two, three or four leaves per tiller to fully expand. At every defoliation event, samples were collected and analysed for acid‐detergent fibre (ADF), neutral‐detergent fibre (NDF) and total nitrogen (N) concentrations and to estimate metabolizable energy (ME) and digestible dry matter (DDM) concentrations. Amounts of crude protein (CP) and metabolizable energy (MJ) per hectare values were subsequently calculated. There was a significantly lower (P < 0·001) NDF concentration for perennial ryegrass compared with prairie grass and cocksfoot, and a significantly lower (P < 0·001) ADF concentration for cocksfoot compared with prairie grass and perennial ryegrass, regardless of defoliation interval. The CP concentration of cocksfoot was significantly greater (P < 0·001) compared with the CP concentrations of prairie grass and perennial ryegrass. The estimated ME concentrations in cocksfoot were high enough to satisfy the requirements of a lactating dairy cow, with defoliation at or before the four‐leaf stage maintaining ME concentrations between 10·7 and 10·9 MJ kg^?1 DM, and minimizing reproductive plant development. The ME concentrations of prairie grass (10·2–10·4 MJ kg^?1 DM) were significantly lower (P < 0·001) than for cocksfoot (as above) and perennial ryegrass (11·4–11·6 MJ kg^?1 DM) but a higher DM production per hectare resulted in prairie grass providing the greatest amounts of ME ha^?1.     

Priority for allocation of water-soluble carbohydrate reserves during regrowth of Lolium perenne

A glasshouse study was undertaken to determine the priority within the perennial ryegrass (Lolium perenne L.) plant for leaf and root growth and daughter tiller initiation after defoliation, in relation to various levels of water-soluble carbohydrate (WSC) reserves at defoliation. Individual plants were arranged in mini-swards, and underwent varying defoliation frequencies and ambient temperatures before defoliation, and harvest heights at defoliation, to obtain a gradient of WSC content at H₁, the date when all plants were defoliated. Defoliation interval consisted of defoliating either three times at the one new leaf tiller^–1 stage (1-leaf stage) of regrowth, or once only at the 3-leaf stage, up to H₁, while night temperature in the week prior to H₁ was altered from 15°C to either 8 or 20°C. At H₁, plants were defoliated to a stubble height of either 20 or 50 mm. Plants were subsequently destructively harvested at days 4, 6, 8, 12, 18 and 27. Leaf and root extension and tiller dynamics were also measured. On a regrowth timescale, tiller initiation was most sensitive, root regrowth moderately sensitive, and leaf regrowth relatively insensitive to a decrease in WSC. The time of daughter tiller initiation also coincided with replenishment of stubble WSC levels. In contrast to this sequence of regrowth events following defoliation, the quantitative effects on growth were different, with elongation and survival of roots most affected by reduced WSC levels. A 30-fold difference in stubble WSC at H₁ between high and low WSC plants (1·52 vs. 0·05 mg tiller^–1) produced only a 4-fold increase in leaf dry matter (DM) after 27 d (2·2 vs. 0·6 g plant^?1), while tiller number plant^?1 increased 6-fold (138 vs. 23% increase in tiller number from H₁). Root elongation rate was 59 times higher in the high than in the low WSC plants (1·18 vs. 0·02 mm d^?1). From a pasture management perspective, the study confirms that defoliation, coinciding with the 3-leaf stage of regrowth and around a stubble height of 50 mm, optimizes persistence and productivity of perennial ryegrass. By allowing more rapid replenishment of WSC reserves, this optimal defoliation strategy enables a greater proportion of WSC to be allocated to maintain a more active root system, and promotes tillering, compared with more frequent and close defoliation.     

The importance of water-soluble carbohydrate reserves on regrowth and root growth of Lolium perenne (L.)

This glasshouse study aimed to determine the relative importance of water-soluble carbohydrates (WSC) and current photosynthate on root and top regrowth of perennial ryegrass (Lolium perenne L.). Individual plants were arranged in one of two miniswards (Experiments 1 and 2) and underwent varying defoliation frequencies designed to obtain a gradient of WSC content at the final harvest of each treatment (H₁), when all treatments were defoliated. In Experiment 1, the plants were defoliated either three times at the one new leaf per tiller stage of regrowth (treatment 3 × 1), once at the two-leaf and again at the one-leaf stage (treatment 2, 1), once at the one-leaf and again at the two-leaf stage (treatment 1, 2) or once only at the three-leaf stage (treatment 1 × 3), up to H₁. Leaf and root growth and other parameters were assessed over 6 d after H₁ in sunlight, and over a 4-week period in darkness, and related to initial plant WSC content. In Experiment 2, plant defoliation treatments were: 3 × 1, 1, 2 or 1 × 3. Leaf regrowth was assessed for 36 d until the plants had three fully expanded new leaves per tiller. Leaf regrowth in both experiments was significantly related to stubble WSC (below 50 mm height). In Experiment 1, plants were almost fully reliant upon plant reserves for the first 3 d of regrowth, with reliance decreasing up to 6 d. When regrowth of plants was compared after 1 week in light or in darkness, it was estimated that one-third of leaf regrowth was due to plant WSC reserves and the remainder due to photosynthesis. However, the capacity to photosynthesize and to grow roots after H₁ was also significantly related to stubble WSC content. In Experiment 2, there was a significant difference (P<0·01) between defoliation treatments on leaf dry matter (DM) yield at 12 d (×1 leaf tiller^?1) of regrowth, and this was, as in Experiment 1, significantly positively related to WSC content in the stubble. However, after 36 d of regrowth, DM yield of plants defoliated at 2 or 3 leaves tiller^?1 up to H₁ were similar, and both were significantly higher (P<0·01) than regrowth of plants defoliated at the one-leaf stage. After defoliation, the period of reliance on WSC reserves may be substantially increased in situations of shading (canopy competition or cloud cover) or if the new regrowth shoot is removed by regrazing.     

The impact of defoliation frequency and nitrogen fertilizer application in spring on summer survival of perennial ryegrass under grazing in subtropical Australia

Abstract This field study investigated the effect of timing of nitrogen (N) fertilizer application in spring on the survival of grazed perennial ryegrass (Lolium perenne cv. Dobson and Yatsyn) over summer in a subtropical environment. There were five N fertilizer treatments: no applied N, 46 kg N ha^?1 on 22 October or 22 November or 22 December, or on 22 October and again on 22 December. Water‐soluble carbohydrate (WSC) concentration of perennial ryegrass plants entering the summer was altered by varying defoliation frequency, with defoliation interval based on the number of leaves per tiller. Frequent defoliation was set at a regrowth level of one leaf per tiller and less frequent defoliation at a regrowth level of three leaves per tiller, over a total of two by three‐leaf per tiller regrowth periods. Application of N fertilizer was found to have no significant effect (P > 0·05) on survival of perennial ryegrass plants over summer. On the other hand, defoliation had a marked effect on perennial ryegrass persistence, with frequent defoliation decreasing ryegrass plant density (51 vs. 88 plants m^?2; P < 0·001) and increasing the density of tropical weed grasses (99 vs. 73 plants m^?2; P < 0·001) by autumn. Frequently defoliated plants had a lower stubble WSC content on a per plant basis than less frequently defoliated plants in spring (103 vs. 201 mg per plant; P < 0·001) and summer (59 vs. 101 mg per plant; P < 0·001). The lower WSC content was associated with a smaller root system in spring (1·50 vs. 2·14 g per plant; P < 0·001) and autumn (1·79 vs. 2·66 g per plant; P < 0·01), and this was reflected in 0·29 more plants being pulled from the soil by livestock between November 1996 and April 1997. Rhizoctonia fungus was associated with roots of pulled plants, but not with roots of seemingly healthy plants, indicating that this fungus may have a role in a weakened root system, which was more prone to sod pulling. Nitrogen applied in October and November resulted in a reduced WSC concentration, although the effect was restricted to 1 month after N application. The present study indicates that survival of perennial ryegrass plants over the summer in a subtropical region is prejudiced by frequent defoliation, which is associated with a lower WSC concentration and a shallower root system. Under grazing, sod pulling is a reflection of this weaker root system and contributes to plant mortality.     

Carbohydrate and crude protein fractions in perennial ryegrass as affected by defoliation frequency and nitrogen application rate

The objective of this study was to evaluate the effects of defoliation frequency (either at two‐ or three‐leaf stage) and nitrogen (N) application rate (0, 75, 150, 300, 450 kg N ha^?1 year^?1) on herbage carbohydrate and crude protein (CP) fractions, and the water‐soluble carbohydrate‐to‐protein ratio (WSC:CP) in perennial ryegrass swards. Crude protein fractions were analysed according to the Cornell carbohydrate and protein system. Carbohydrate fractions were analysed by ultra‐high‐performance liquid chromatography. Sward defoliation at two‐leaf stage increased the total CP, reduced the buffer‐soluble CP fractions and decreased carbohydrate fractions of herbage (P < 0·001). The effect of defoliation frequency was less marked during early spring and autumn (P < 0·001) than for the rest of the seasons. An increase in N application rate was negatively associated with WSC, fructans and neutral detergent fibre (P < 0·001), and positively associated with CP and nitrate (N‐NO₃) contents of herbage. Nitrogen application rate did not affect CP fractions of herbage (P > 0·05). The fluctuations in CP and WSC contents of herbage resulted in lower WSC:CP ratios during early spring and autumn (0·45:1 and 0·75:1 respectively) than in late spring (1·11:1). The herbage WSC:CP ratio was greater (P < 0·001) at the three‐leaf than the two‐leaf defoliation stage and declined as the N application increased in all seasons (P < 0·001). The results of this study indicate that CP and carbohydrate fractions of herbage can be manipulated by sward defoliation frequency and N application rate. The magnitude of these effects, however, may vary with the season.     

The effect of defoliation practice in Western Australia on tiller development of annual ryegrass (Lolium rigidum) and Italian ryegrass (Lolium multiflorum) and its association with forage quality

The effect of defoliation on the vegetative, early reproductive and inflorescence stages of tiller development, changes in the dry‐matter yield of leaf, stem and inflorescence and the associated changes in forage quality was determined on plants of annual ryegrass (Lolium rigidum Gaud.) and Italian ryegrass (L. multiflorum Lam.). The field study comprised seventy‐two plots of 1 m × 2 m, sown with one annual ryegrass and seven Italian ryegrass cultivars with a range of heading dates from early to late; defoliation commenced 6 weeks after germination. During the vegetative stage of growth, plots were defoliated when the tillers had three fully expanded leaves (three‐leaf stage). During the early reproductive stage of growth, to simulate a cut for silage, plots were defoliated 6–7 weeks after 0·10 of the tillers displayed nodal development. The subsequent regrowth was defoliated every 3 weeks. Assessments of changes in tiller density, yield and quality were made in the growth cycle that followed three contrasting cutting treatments during the winter–spring period (from 10 July). In treatment 1, this growth cycle (following closing‐up before a subsequent conservation cut) commenced on 7 August following two defoliations each taken when the tillers were at the three‐leaf stage. In treatment 2, the growth cycle commenced on 16 October following: for early‐maturing cultivars, two cuts at the three‐leaf stage, a cut for silage and an additional regrowth cut; for medium‐maturing cultivars three cuts at the three‐leaf stage and a cut for silage; and late‐maturing cultivars, five cuts at the three‐leaf stage. In treatment 3, defoliation up to 16 October was as for treatment 2, but the growth cycle studied started on 27 November following two additional regrowth cuts for early‐ and medium‐maturing cultivars and cut for silage for the late‐maturing cultivars. Tiller development for all cultivars was classified into three stages; vegetative, early reproductive and inflorescence. In treatment 1, in vitro dry‐matter digestibility (IVDMD) and crude protein (CP) content were negatively associated with maturation of tillers. IVDMD ranged from 0·85 to 0·60 and CP ranged from 200 to less than 100 g kg^–1 dry matter (DM) during the vegetative and inflorescence stages respectively. This large reduction in forage quality was due to an increase in the proportion of stem, inflorescence and dead material, combined with a reduction in the IVDMD and CP content of the stem. A high level of forage quality was retained for longer with later‐maturing cultivars, and/or when vegetative tillers were initiated from the defoliation of early reproductive tillers (treatments 2 and 3). However, 15 weeks after the closing‐up date in treatment 1, defoliation significantly reduced the density of inflorescences with means (±pooled s.e_m.) of 1560, 1178 and 299 ± 108 tillers m^–2, and DM yield of inflorescence with means of 3·0, 0·6 and 0·1 ± 0·15 t ha^–1 for treatments 1, 2 and 3 respectively. This study supports the recommendation that annual and Italian ryegrass cultivars should be classified according to maturity date based on the onset of inflorescence emergence, and that the judicious defoliation of early reproductive tillers can be used to promote the initiation of new vegetative tillers which in turn will retain forage quality for longer.     

Herbage yield, tiller number and root system activity after defoliation of prairie grass (Bromus catharticus Vahl)

The effects of defoliation upon root and shoot systems of prairie grass (Bromus catharticus Vahl) were examined in both field and pot studies. The varieties used were 78–32 (HY), a high-yielding variety; 79–42 (LY), a low-yielding variety; and the commercial variety Grasslands Matua. In the field, the presence of roots in early and late spring was estimated by measuring uptake of [³²P]phosphate by roots; herbage yields and tiller numbers were recorded. In a pot study, root and shoot dry-matter (DM) yields were analysed. In the field, roots were detected to a depth of 1·2 m. After defoliation to a height of 0·1 m, root presence decreased more than 50% at depths of 0·6 m for LY and 1·0 m for Matua in early spring and at several depths for each variety in late spring. After a second defoliation, the apparent growth rate of shoots decreased by 35% in relation to the first regrowth period. In pots, shoot DM and root DM of control plants (undefoliated) had the following allometric relationship of the form: In (shoot DM) = 0·61 + 1·14 ln (root DM) (r²= 0·81). After defoliation, compared with undefoliated controls, the relative growth rate of shoots and total herbage yields were higher, but root and stubble DM were lower in all three varieties. Pooled root DM means were 10·3 and 6·8 g plant^?1 and pooled stubble DM means were 12·7 and 7·6 g plant^?1 for control and defoliated plants respectively. HY produced heavier tillers than LY, pooled means being 0·94 and 0·53 g DM tiller^?1 (field study) and 3·44 and 2·05 g DM tiller^?1 (pot study) for HY and LY respectively. HY had 5–6 green leaves per tiller, whereas LY had 3–4. Developed green leaves were heavier in HY (58 g m^?2) than in LY (48 g m^?2). It is suggested that differences in both leaf parameters may be related to higher herbage yields for HY than LY.     

Patterns of leaf and root regrowth, and allocation of water-soluble carbohydrate reserves following defoliation of plants of prairie grass (Bromus willdenowii Kunth.)

This study utilized leaf stage‐based defoliation intervals to describe the concentrations and contents of water‐soluble carbohydrate (WSC) and nitrogen (N) in stubble and root reserves and their effect on the regrowth of prairie grass (Bromus willdenowii Kunth.) plants. The priority sequence for allocation of WSC reserves during the regrowth period was also investigated. There were substantially higher concentrations of WSC and N in the stubble compared with the roots following defoliation, confirming the stubble as the primary site for energy storage, with roots playing a lesser role. However, high R² values for the relationships between WSC concentration in roots and regrowth variables suggested that plants of prairie grass were reliant on WSC reserves from the roots in addition to the stubble to meet the energy requirements of plants until adequate photosynthetic tissue had been produced. The sequence of priority for allocation of WSC reserves followed the order of leaf growth, root growth and tillering during the regrowth period. Although WSC reserves were identified as the primary contributor to plant regrowth following defoliation, there was also a strong relationship between stubble N concentration and regrowth variables.     

Response of herbage regrowth and water-soluble carbohydrate concentration of ryegrass species to defoliation practices when grown in a Mediterranean environment

Three experiments were conducted to determine the association between leaf number per tiller at defoliation, water‐soluble carbohydrate (WSC) concentration and herbage mass of juvenile ryegrass plants when grown in a Mediterranean environment. Seedlings of ryegrass were grown in nursery pots arranged side‐by‐side and located outside in the open‐air to simulate a mini‐sward in Experiments 1 and 2, and a mixture of annual ryegrass and subterranean clover (Trifolium subterraneum L.) was grown in a small plot field study in Experiment 3. Swards were defoliated mechanically with the onset of defoliation commencing within 28 d of germination. Frequency of defoliation ranged from one to nine leaves per tiller, whilst defoliation height ranged from 30 mm of pseudostem height that removed all leaf laminae in Experiment 1, to 50 mm of pseudostem height with some leaf laminae remaining post‐defoliation in Experiments 2 and 3. A positive relationship between herbage mass of ryegrass, WSC concentration and leaf number per tiller at defoliation was demonstrated in all experiments. In Experiment 1, the herbage mass of leaf, pseudostem and roots of tillers defoliated at one leaf per tiller was reduced to 0·10, 0·09 and 0·06 of those tillers defoliated less frequently at six leaves per tiller. However, the reduction in herbage mass from frequent defoliation was less severe in Experiment 2 and coincided with a 0·20 reduction in WSC concentration of pseudostem compared with 0·80 measured during Experiment 1. In Experiment 3, the highest harvested herbage mass of ryegrass occurred when defoliation was nine leaves per tiller. Although the harvested herbage from this sward contained senescent herbage, the in vitro dry‐matter digestibility of the harvested herbage did not differ significantly compared with the remaining treatments that had been defoliated more frequently. Leaf numbers of newly germinated ryegrass tillers in a Mediterranean environment were positively associated with WSC concentration of pseudostem and herbage mass. A minimum period of two to three leaf appearances was required to restore WSC concentrations to levels measured prior to defoliation thereby avoiding a significant reduction in herbage mass. However, maximum herbage mass of a mixed sward containing ryegrass and subterranean clover was achieved when defoliation was delayed to nine leaves per tiller.     

Effect of grazing management on herbage protein concentration,milk production and nitrogen excretion of dairy cows in mid‐lactation

The objective of this experiment was to use diurnal and temporal changes in herbage composition to create two pasture diets with contrasting ratios of water‐soluble carbohydrate (WSC) and crude protein (CP) and compare milk production and nitrogen‐use efficiency (NUE) of dairy cows. A grazing experiment using thirty‐six mid‐lactation Friesian x Jersey cows was conducted in late spring in Canterbury, New Zealand. Cows were offered mixed perennial ryegrass and white clover pastures either in the morning after a short 19‐day regrowth interval (SR AM) or in the afternoon after a long 35‐day regrowth interval (LR PM). Pasture treatments resulted in lower pasture mass and greater herbage CP concentration (187 vs. 171 g kg^?1 DM) in the SR AM compared with the LR PM but did not affect WSC (169 g kg^?1 DM) or the ratio of WSC/CP (1·0 g g^?1). Cows had similar apparent DM (17·5 kg DM cow^?1 d^?1) and N (501 g N cow^?1 d^?1) intake for both treatments. Compared with SR AM cows, LR PM cows had lower milk (18·5 vs. 21·2 kg cow^?1 d^?1), milk protein (0·69 vs. 0·81 kg cow^?1 d^?1) and milk solids (1·72 and 1·89 kg cow^?1 d^?1) yield. Urinary N concentration was increased in SR AM, but estimated N excretion and NUE for milk were similar for both treatments. Further studies are required to determine the effect of feeding times on diurnal variation in urine volume and N concentration under grazing to predict urination events with highest leaching risk.     

Interaction between water-soluble carbohydrate reserves and defoliation severity on the regrowth of perennial ryegrass (Lolium perenne L.)-dominant swards

A field-study was undertaken in Hamilton, New Zealand to determine if there was an interaction between water-soluble carbohydrate (WSC) reserve content and defoliation severity on the regrowth of perennial ryegrass-dominant swards during winter. Perennial ryegrass plants with either low or high WSC content were obtained by varying the defoliation frequency. At the third defoliation at the one-leaf stage and at the first defoliation at the three-leaf stage (harvest H₁), swards were mown with a rotary lawnmower to residual stubble heights of 20, 40 or 60 mm. All swards were then allowed to regrow to the three-leaf stage before again defoliating to their treatment residual stubble heights (H₂). Frequently defoliated plants contained proportionately between 0·37 and 0·48 less WSC in the stubble after defoliation, depending on the severity of defoliation. There was no interaction between WSC content and defoliation severity for herbage regrowth between harvests H₁ and H₂. Herbage regrowth was lower from swards containing low WSC plants compared with high WSC plants (2279 vs. 2007 kg DM ha⁻¹). Furthermore, swards defoliated to 20 or 40 mm had greater herbage regrowth compared with those defoliated to 60 mm (2266, 2249 and 1914 kg DM ha⁻¹ for swards defoliated to residual stubble heights of 20, 40 and 60 mm, respectively). Regrowth of perennial ryegrass was positively correlated with post-defoliation stubble WSC content within defoliation severity treatment, implying that WSC contributed to the defoliation frequency-derived difference in herbage yield. However, the effect of defoliation severity on herbage regrowth was not associated with post-defoliation stubble WSC content.     

Chloris gayana Kunth under different defoliation regimes. Morphogenesis,sward structure and leaf area index

Subtropical pastures are an important alternative to increase forage yields to fulfil cattle nutritional requirements. Despite the increasing expansion of these pastures in the semiarid subtropical region of Argentina, there is very little information about their responses to grazing management. The aim of this study was to evaluate the effect of different defoliation regimes on morphogenesis, sward structure and leaf area index of one of the most expanded forage species in this region, Chloris gayana Kunth. A combination of two defoliation frequencies (300 and 500 GDD) and two defoliation intensities (1 and 3 green stubble leaves) was compared by a controlled experiment that comprised 1,500 GDD. Defoliation frequency significantly affected leaf elongation rate (LER) and leaf area index (LAI). Under the high defoliation frequency, LER and LAI resulted almost half than under low defoliation frequency (0.34 ± 0.08 vs. 0.67 ± 0.08 mm·tiller^?1·GDD^?1; 8.31 ± 2.27 m²/m² vs. 13.27 ± 1.59 m²/m², at 300 or 500 GDD respectively), regardless of the intensity. Defoliation frequency or intensity did not affect leaf appearance rate, leaf lifespan, leaf size, number of green leaves per tiller nor tiller density at the end of the experiment. We conclude that to maintain high LER and LAI in Chloris gayana Kunth cv. Épica INTA‐Pemán pastures, defoliation frequency could be of 500 GDD. Since leaf lifespan was 415 ± 110 GDD, under this defoliation frequency, a maximum accumulation of green leaf tissues with very little dead tissues may be achieved.     

Effect of plant age and severity of defoliation on regrowth of sheep's burnet during substrate moisture depletion

Regrowth of 3- and 4-month old (‘young’ and ‘old’ respectively) sheep's burnet [Sanguisorba minor ssp. muricata (Spach) Briq.] was studied under limiting and non-limiting moisture conditions in a glasshouse. Moisture deficits were imposed by using a single cycle of withholding moisture until first wilting. Plants of each age were defoliated severely at three levels which represented approximately 80–100% canopy removal. These levels were based on the proportion retained of the eight most mature leaves on each plant and were referred to as complete [0% residual leaf area (rLA)] and partial [50% rLA (four leaves) and 100% rLA (eight leaves)] defoliation. Vegetative growth and total non-structural carbohydrate (TNC) levels were studied. Leaf number (0-8 leaves), area (0-115 cm²) and dry weight (0-1·0 g) differed (P < 0·05) between defoliation intensities at the start of regrowth, while stubble (1·2 g) and root (12.·6 g) dry weights were similar. Soluble sugars [< 6% dry matter (DM)] and starch (< 1% DM) occurred in leaf, stubble and root. Old plants were morphologically and physiologically more developed than young plants. For example, stubble (2·0 g) and root (21·5 g) dry weights of old plants were greater (P < 0·05) than those of young plants (04 and 3·7 g respectively). Defoliation intensity had a major effect on regrowth, with completely defoliated plants at the final harvest having leaf numbers (forty-nine leaves) and areas (235 cm²) almost twice those of partially defoliated plants. Stubble soluble sugar levels (38% DM) were lower than those of partially defoliated plants (5·5% DM), and it was suggested that these contributed to regrowth. Moisture regime had a negligible influence on plant growth. However, plants in the dry regime had soluble sugar levels 1·4 (stubble) -1·7 (roots) times higher than those watered adequately, which suggested that plants adjusted to the water depletion. The effects of plant age on regrowth were similar for most characters, but the larger and physiologically more mature old plants would probably be more tolerant of successive defoliations.     

The morphological and physiological responses of perennial ryegrass (Lolium perenne L.), cocksfoot (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb.; syn. Schedonorus phoenix Scop.) to variable water availability

There is a growing interest in the use of deficit irrigation and perennial pasture species other than perennial ryegrass (Lolium perenne L.) in temperate agriculture, in response to the decreasing availability of irrigation water. Deficit irrigation requires an understanding of plant responses to drought stress to ensure maximum dry‐matter return on water applied. A glasshouse study was undertaken to investigate some of the morphological and physiological responses of perennial ryegrass, cocksfoot (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb.; syn. Schedonorus phoenix Scop.) to varied moisture availability. One water treatment involved frequent applications of water to maintain a soil water potential of approximately ?10 kPa (100% treatment), and three other treatments involved applications at the same frequency, but using 33, 66 or 133% of the water applied in the 100% treatment. The water treatments continued over two plant regrowth cycles, followed by a ‘recovery’ phase of a single regrowth cycle during which all plants received the same water allocation as the 100% treatment. Depletion and replenishment of stubble water‐soluble carbohydrate (WSC) differed between the three species in response to soil moisture availability. By the second regrowth cycle, stubble WSC concentration and content in moisture‐stressed cocksfoot plants had increased, followed by a decrease during the subsequent recovery phase when the stored WSC reserves were utilized to support regrowth. The changes in stubble WSC reserves corresponded to the maintenance of relatively stable (i.e. the smallest reduction in leaf DM in response to moisture stress), but consistently lower DM production for cocksfoot compared with the other species. In contrast, moisture stress had no effect on the stubble WSC reserves of perennial ryegrass and tall fescue, with the exception of a significant decrease in WSC concentration under the 33% water treatment for perennial ryegrass. Perennial ryegrass achieved an intermediate DM yield and maintained positive growth rates throughout the study, even when watered at 33% of the requirement for optimal soil moisture levels. However, a more pronounced reduction in leaf DM in plants under moisture stress compared with the other species, combined with declining WSC reserves and the death of daughter tillers, highlighted the vulnerability of perennial ryegrass to poor persistence under prolonged drought conditions. Tall fescue appeared to have the greatest scope under moisture stress in terms of maintaining productivity and displaying attributes that contribute to persistence. Its leaf DM was consistently greater than that of the other species, displaying a smaller decline in growth under water stress compared to perennial ryegrass and an ability to recover faster upon re‐watering. This study has expanded the information available that compares and defines the potential of each species under moisture stress and emphasizes the importance of balancing short‐term DM production with long‐term persistence in choice of pasture species.     

In vitro rumen methane output of perennial ryegrass varieties and perennial grass species harvested throughout the growing season

The selection and feeding of perennial ryegrass (Lolium perenne L.) varieties (PRV) or perennial grass species (PGS) may affect enteric methane (CH₄) output because of changes in the fermentation dynamics in the rumen as a result of differences in herbage chemical composition. The objective of this study was to determine the effects of PRV and PGS harvested throughout the growing season on herbage chemical composition, and in vitro rumen fermentation variables and CH₄ output per unit of feed using a batch culture technique. Seven PRV (Experiment 1: Alto, Arrow, Bealey, Dunluce, Greengold, Malone, Tyrella) and six perennial grasses [Experiment 2: perennial ryegrass (Navan), perennial ryegrass (Portstewart), cocksfoot, meadow fescue, tall fescue, timothy; defined as PGS], managed under a simulated grazing regime, were incubated for 24 h with buffered rumen fluid in two separate experiments. The CH₄ output per unit of feed dry‐matter (DM) incubated was not affected (P > 0·05) by PRV (range of mean values across PRV of 23·9–25·3 (SEM 0·41) mL g^?1 DM) or by PGS (25·6–26·6 (SEM 0·37) mL g^?1 DM). The CH₄ output per unit feed DM disappearing during the in vitro rumen incubation was not affected by PRV (33·9–35·1 (SEM 0·70) mL g^?1 DM), and although there was an overall PGS effect (P < 0·05; 37·2–40·3 (SEM 0·71) mL g^?1 DM), none of the paired contrasts between PGS were significant when analysed using Tukey adjusted comparisons. This outcome reflected either small‐scale or a lack of treatment effects on individual herbage chemical composition (e.g. 454–483 g NDF kg^?1 DM, 215–224 g CP kg^?1 DM and 94–122 g water‐soluble carbohydrate (WSC) kg^?1 DM across PRV; 452–506 g NDF kg^?1 DM, 208–243 g CP kg^?1 DM and 73–131 g WSC kg^?1 DM across PGS) and in vitro rumen fermentation variables. Hence, these results provide no encouragement that choices among the grasses examined, produced within the management regimes operated, would reduce enteric CH₄ output per unit of feed in vivo. However, the technique utilized did not take account of animal × PRV or PGS interactions, such as potential differences in intake between animals, that may occur under farm conditions.     

Leaf number as a criterion for determining defoliation time for Lolium perenne: 2. Effect of defoliation frequency and height

The object of this study was to determine the importance of frequency and height of defoliation on regrowth potential of Lolium perenne. Defoliation interval was based on stage of the regrowth cycle, as indicated by leaves per tiller.
Simulated swards of Lolium perenne cv Yatsyn were grown as individual plants in a glasshouse kept at a day/night temperature of 25°C/15°C.
Treatments imposed were defoliation at 2, 5 or 12 cm residual height, and low and high water soluble carbohydrate (WSC) level obtained by varying defoliation interval, i.e. defoliating at the 1-leaf or 3-leaf stage of the regrowth cycle. Regrowth after frequent short defoliations was only 65% of the less frequently defoliated plants taken over the full regrowth cycle. This was associated with a lower stubble WSC content (2·15 vs 17·5% in stubble) and a twenty-seven-fold difference in the amount of WSC in the stubble per plant. This difference in total WSC was a combined effect of more and heavier tillers and higher WSC content in stubble of plants defoliated less frequently at the end of the regrowth cycle. The regrowth of plants with WSC levels depleted by frequent defoliation when defoliated at 2 cm was significantly below that of those defoliated at 5 and 12 cm.
The results indicate the desirability of defoliating plants at the 3-leaf stage of the regrowth cycle. This not only allows the full regrowth potential to be expressed in that growth cycle, but also in the next cycle, by allowing the replenishment of WSC reserves and optimizing tiller status. The potential to regrow appears then to be based more on the total amount of WSC than the proportion of WSC in stubble.     

Production, survival and nutritive value of the perennial legumes Dorycnium hirsutum and D. rectum subjected to different cutting heights

Dorycnium hirsutum and D. rectum are perennial legumes which may have potential for use as pastures for the control of groundwater recharge in southern Australia. Little is known about the quality of the forage of Dorycnium species for grazing livestock or how these species respond to cutting. The effect of cutting height on plant survival, production of dry matter (DM), the proportion of leaf, edible stem (approximately <5 mm diameter) and woody stem in the DM and the nutritive value of the edible components was investigated. Biomass above five cutting‐height treatments (uncut, ground level, 5–8 cm, 10–15 cm and 15–30 cm above ground level) was removed at 8‐week intervals from plots of D. hirsutum and D. rectum from September 2002 to July 2003. In both species, plants subjected to lower cutting height treatments produced less DM above the height of the cut than those cut at higher heights. DM production declined over time in all treatments. Plants cut to ground level failed to regrow after the second harvest in D. hirsutum and the fourth harvest in D. rectum. Thus, these Dorycnium species were susceptible to high severity defoliations at 8‐week intervals. Negligible inedible woody stem was present in regrowth of both species after 8 weeks but D. hirsutum regrowth had a higher proportion of leaf (0·72) than D. rectum (0·56). Plants left uncut accumulated a large proportion of inedible woody stem in the DM (0·69 in both species) by July 2003, particularly at the base of the plant. Edible DM from regrowth of D. hirsutum and D. rectum had crude protein (CP) concentrations of 120 and 150 g kg^?1 DM; dry matter digestibility (DMD) values of 0·45 and 0·58; organic matter digestibility (OMD) values of 0·50 and 0·64; neutral‐detergent fibre (NDF) concentrations of 370 and 290 g kg^?1 DM; and acid‐detergent fibre (ADF) concentrations of 260 and 210 g kg^?1 DM, respectively. Medicago sativa, grown under similar conditions, had higher digestibility values (0·63 DMD and 0·66 OMD) and similar CP concentrations to D. rectum (140 g kg^?1 DM), but higher concentrations of NDF and ADF (410 and 290 g kg^?1 DM). Leaf material from both Dorycnium species had a higher nutritive value than edible stems, with DMD and OMD values of leaf of D. rectum being 0·68 and 0·74 respectively. Uncut plants had a much lower nutritive value of edible DM than the regrowth from cut treatments; older material was also of a lower nutritive value. The relatively low nutritive value of even the young regrowth of Dorycnium species suggests that forage quality is a major limitation to its use. Forage of Dorycnium species could be used during periods when other sources of forage are in short supply but infrequent grazing it is likely to produce forage of a low nutritive value.     

A canopy photosynthesis model to predict the dry matter production of cocksfoot pastures under varying temperature, nitrogen and water regimes

Daily net canopy photosynthesis (P_n) of cocksfoot (Dactylis glomerata L.) was predicted for combinations of temperature, herbage nitrogen (N) concentration and water status from the integration of models of leaf photosynthesis of the light‐saturated photosynthetic rate (P_max), photosynthetic efficiency (α) and the degree of curvature (θ) of leaf light‐response curves. The effect on P_n, maximum P_n (P_{n max}) and the optimum leaf area index (LAI at P_{n max}) was examined when any one of these factors was limiting. The ranges that gave the optimum values of P_n (P_{n max} = 30·8–33·5 g CO₂m^?2 d^?1) for temperature (19–22°C) and N concentration (40–50 g N kg^?1 DM) were smaller than those for net leaf photosynthesis. Also, P_n fell to 0 at a lower level of water stress (pre‐dawn leaf water potential, ψ_lp = ?12·5 bar) than for P_max. The canopy photosynthesis model was then used to compare predicted and measured dry matter (DM) production for cocksfoot pastures grown under a diverse range of environmental conditions with field data from New Zealand and Argentina. To predict DM production leaf area index and leaf canopy angle were included from field measurements. The model explained about 0·85 of the variation in cocksfoot DM production for the range of 6·5–134 kg DM ha^?1 d^?1. The canopy model overestimated the DM production by 0·10 which indicates that a further P_max function for leaves of different ages and a partitioning sub‐model may be needed to improve predictions of DM production.     

Inheritance of the concentration of water-soluble carbohydrates and its relationship with the concentrations of fibre and crude protein in herbage of cocksfoot (Dactylis glomerata L.)

The concentration of water‐soluble carbohydrates (WSC) of cocksfoot is lower than that of other temperate grasses. Increasing the WSC concentration in cocksfoot is important in increasing its digestibility and preference by ruminants. The genetic variation in mono‐ and disaccharides, fructans and total WSC concentrations and their inter‐relationships with crude protein (CP) and fibre concentrations of cocksfoot (Dactylis glomerata L.) at the vegetative growth stage in half‐sib cross populations were assessed in two experiments conducted under spaced planting and sward conditions. There was a wide range in the means for concentrations of fructan, WSC and neutral‐detergent fibre (NDF) in parents and progeny whereas there was a narrow range of the means for concentrations of mono‐ and disaccharides, CP and acid‐detergent fibre (ADF). Mean concentrations of mono‐ and disaccharides showed the smallest range. Mean squares of entries in parents and progenies were significant for mono‐ and disaccharides, fructan and total WSC concentrations in all populations. The entry × year interactions for fructan and total WSC concentrations were significant in the parents and the half‐sib progeny. However, the entry × year interactions for mono‐ and disaccharide concentrations in progeny were not significant. Concentration of WSC was under genetic control as mean squares of the concentration values were significant and variance components for all traits were significantly larger than zero. There were negative genetic correlations between WSC and ADF and NDF concentrations. Narrow‐sense heritabilities (h_n) estimated from variance components of progeny for total WSC concentrations were 0·59 and 0·53 in sward and spaced planting conditions respectively. The h_n of fructan and WSC concentrations were similar in both sward and spaced planting conditions, whereas that of mono‐ and disaccharide concentrations varied from 0·20 to 0·69. It was found that the genetic variation of the total WSC concentration in cocksfoot depended mainly on genetic variation in fructan concentration. These results suggested that the forage quality of cocksfoot at the vegetative growth stage was influenced by an additive gene effect and could be improved genetically by recurrent selection.