Growing Point Density Research Articles

Effect of herbage rejection by steers on white clover (Trifolium repens) branching and development in continuously stocked grass–clover swards

SUMMARYThe proportion of sward area rejected for grazing under different stocking intensities in grass-white clover (Trifolium repens L.) swards continuously grazed by steers toconstant sward height was determined and the effect of these areas on clover stolon developmentwas investigated.In a randomized block field experiment, grazing treatments imposed from 1988 to 1991 comprised target sward surface heights (SSH) of 5, 7 or 9 cm from April to July followed by 7 or 9 cm from July to October (i.e. six treatments). The percentage area rejected was higher in 9 cm than in the shorter swards, the proportion of the total areas rejected increasing generally over the year. Within the rejected areas in spring the number of branches on the main axes of white clover plants was only 17% and the mean number of nodes per branch was 14% of those in grazed areas. Corresponding percentages in summer in rejected areas were 56 and 46% compared with grazed areas.In a glasshouse experiment involving stolons grown for 18–21 days, the difference between those from grazed and rejected areas in branch number per stolon and mean node number per branch was much reduced. Branching and rate of branch development (node number) were slower in stolons maintained under dark conditions.Mean growing point density over six sampling dates from summer 1990 to autumn 1991 was significantly higher in the 5 cm than in the 9 cm spring SSH treatment. Although rejected areashad only 34% of the growing point density of grazed areas in autumn, the latter comprised the majority of the total area in most sward height treatments and so reduced the overall effect of rejected areas on growing point density.

The regulation of clover shoot growing points density and morphology during short-term clover decline in mixed swards

The spring growth of infrequently cut mixed swards, with or without inorganic fertilizer nitrogen applied, was studied at two sites. During regrowth, a decline in the sward clover content to 20–30 per cent of the initial clover content was observed in some situations (in N-fertilized swards, or during early spring growth). Changes in the density and in the morphology of clover shoot growing points were analysed during this decline in clover content. After 40 to 50 days of regrowth, the mean density of clover shoot growing points declined by 35 to 67 per cent. The mean number of growing points per branch was reduced by 17 to 69 per cent and the mean branch density by 21 to 55 per cent. The mean biomass per a clover shoot growing point was negatively correlated with growing point density and the decline in the mean growing point density was accompanied by a 200 to 300 per cent increase in the mean biomass per growing point, which indicated an asymmetrical mortality of clover shoot growing points. Clover leaf area per unit shoot weight was negatively correlated with the mean size of a growing point and, accordingly, clover leaf area ratio declined after approximately 30 days of regrowth.The links between the asymmetrical mortality of clover growing points, competition for light and the short-term decline of white clover within mixed swards allowed to regrow to a high leaf area index, are discussed.

Effect of winter pasture residuals' and grazing- off on subsequefit milk production and pasture performance

Concern at the effect of winter pasture residuals on pasture productivity led to the comparison of different wintering systems at the DRC No. 3 Dairy from May-December 1993. Three farmlets were designed to have 2000 kg DM/ha average farm cover at calving, but with pastures grazed to either 900, 1400 or 1800 kg DM/ha at least once during the winter. A fourth farmlet was spelled from grazing from 25 May-4 July to give an average farm cover at calving of 2900 kg DM/ha. Pasture regrowth, composition and structure, milk yield, liveweight and reproductive performance were measured. A simulation model UDDER was used to generalise from the specific experimental results. Pasture regrowth in July-August was greater on the wintering-off treatment than those grazed during winter, but treatment differences in late spring were inconsistent. Wintering-off decreased ryegrass tiller and white clover growing point densities in August but differences had disappeared by November. Winter grazing treatment had no effect on any component of milk production from calving to late October. From late October until m&December-milk-protein-and.milksolids y i e l d were less on the wintering-off treatment than the mean of the other three treatments (0.62 vs 0.68 kg/cow/day) and (1.49 vs 1.59 kg/cow/day), respectively. Grazing to different winter pasture residuals had no consistent effects on subsequent pasture productivity, composition or milk yield. Simulation showed that wintering-off can lead to increased milksolids production when average farm cover in September is predicted to fall below 1750 kg DM/ha. Timing and amount of conservation become c&i&i1 slfterwintering;off if pasture quality and hence milk production are to be maintained in late spring. Keywords: dairy cow, milksolids, milk yield, pasture regrowth, pasture residual, pasture structure

White Clover Response to Grazing Method

AbstractGrazing management is a major factor influencing white clover (Trifolium repens L.) growth. Our objective was to determine the response of white clover cultivars differing in leaf size to grazing method (continuous vs. rotational stocking) using cattle (Bos spp.). In each of 2 yr, a predominantly tall fescue (Festuca arundinacea Schreb.) sod on a Savannah fine sandy loam (fine‐loamy siliceous, thermic Typic Fragiudult) was oversown in September with ‘Grasslands Huia’ (medium‐small leaf), ‘Louisiana S‐1’ (medium‐large leaf), and ‘Regal’ (large leaf) white clover. From March to August of the following year, plots of each cultivar were stocked continuously (3‐ to 5‐cm stubble) on rotationally (grazed to 5‐cm stubble every 35 to 38 d). Clover growth was measured prior to rotational grazing and stolon survival was determined in November. When precipitation during the grazing season was 59% above normal, grazing method had no influence on mean single leaf area, stolon dry weight, and stolon growing point density of white clover. In contrast, continuous stocking reduced these responses when precipitation was 32% below normal the following year. Cultivar ranking for mean single leaf area was generally the same as that for leaf size category: Grasslands Huia < Louisiana S‐1 < Regal. Although stolon length and growing point density were frequently greatest for Grasslands Huia, stolon survival of Grasslands Huia was no greater than that of the larger‐leafed cultivars. Despite varying effects of grazing method on growth and morphology, stolon survival of white clover was always greater under rotational stocking.

Out‐of‐season management of grass/clover swards to manipulate clover content

AbstractTwo experiments are described in which the effect of grazing or defoliating mixed swards at different times over winter and spring on clover content and development was investigated.In the first experiment swards were grazed with sheep (to about 3 cm) for a short period in (a) November, (b) November, January and March, (c) March or (d) not at all, in three consecutive years. All swards were grazed intermittently during the grazing season with cattle and cut for silage once each year. Each plot received either 0 or 50 kg N ha−1 in March.The effect of N fertilizer was to reduce clover content in each summer and clover growing point density in the third year.In two of the three years, treatments involving grazing in March had lower subsequent net annual herbage accumulation compared with the other two treatments and higher clover content in summer of the third year. Reduction in growing point density in all plots during the grazing season was associated with cattle grazing when conditions were wet, suggesting that stolon burial was implicated. Grazing with sheep in November, January and March resulted in significantly more visible (when counted in situ) clover growing points in April in year 2 and more total growing points (counted after dissection of turves) in the third year than the November grazed and ungrazed treatments which had, on occasions, higher grass tiller density.In a microplot experiment, high herbage mass standing over winter was associated with lower potential photosynthesis per unit clover lamina area and lower growing point density in March. Cutting herbage in March to 2‐3 cm resulted in higher clover content and higher growing point number per unit stolon length. The latter was significantly correlated with total irradiance and red: far red at the canopy base. Potential photosynthesis of clover was not affected by cutting in March.It is concluded that growing point density can be increased by grazing or cutting during winter or spring. However, in order for these new stolons to contribute to clover yield during the summer, they have to be maintained until then by ensuring that competition from grass is minimized by keeping the sward short in winter and spring and avoiding the burial of stolons during grazing.

Variability of stolon characteristics and response to shading in two cultivars of white clover (Trifolium repensL.)

Abstract ‘Grasslands Tahora’ and ‘Grasslands Kopu’ white clover cultivars were compared for several shoot characters. Selections were made for high and low proportions of nodes branching, long and short internodes, and large and small leaves, and position of the first branch on the stolon within each cultivar. These selections plus a random sample of genotypes from each cultivar were regrown in full sunlight and in 50% full sunlight. Tahora and Kopu differed in all characters measured except stolon elongation rate and proportion of nodes active (i.e., the total producing branches and flowers). Tahora had branches occurring earlier on a stolon, a higher node production rate, and proportion of nodes branching, but smaller leaves, shorter petioles, thinner stolons, shorter internodes, and lower proportion of nodes flowering than Kopu. The two cultivars differed in skewness of frequency distributions for first branching, node, leaf size, proportion of nodes branching and flowering. Shading greatly increased petiole length and decreased proportion of nodes branching. However, there were significant cultivar × light intensity interactions for stolon elongation rate, leaf size, petiole length, internode length, and proportion of nodes branching. Genotypes selected originally for large leaves had larger leaves in the subsequent shading experiment than genotypes selected originally for small leaves. Similarly, the long internode selection subsequently had longer internodes than the short internode selection. However, genotypes selected originally for a high proportion of nodes branching were not subsequently different from those selected for a low proportion of nodes branching. The same applied to selections for earliness of nodal branching. In attempts to improve stolon growing point density in pastures, selection for internode length is likely to be more successful than selections for proportion of nodes branching and earliness of branching.

Performance of white clover cultivars and breeding lines in a mixed species sward

Abstract In grazing trials, 158 different cultivars and breeding lines of white clover (Trifolium repens L.) were evaluated in mixed species swards for yield and persistence over 3 or 4 years. Plant type and flower abundance were also recorded. Cultivars and breeding lines were clustered into groups on the basis of proportion of clover in the sward. A consistently high proportion of clover was observed for New Zealand material (cultivar Grasslands Huia) crossed with predominantly Mediterranean germ plasm, for germplasm originating from Crau Plains in France, an ecotype from Syria, and a ladino line from Japan. Breeding lines were also identified that combined high yield with relatively high stolon growing point density which ensured improved persistence for such large-leaved material.

Performance of white clover cultivars and breeding lines in a mixed species sward 2. Plant characters contributing to differences in clover proportion in swards

Abstract One hundred and fifty-eight cultivars and breeding lines of white clover were grown in four trials to determine which leaf and stolon characters gave the best prediction of clover content in a mixed species sward. Stepwise regression analysis was used and showed that in a rotationally grazed sward leaf number and leaf size, rather than stolon growing point density, were the best predictors of proportion of clover in swards.

Evaluation of elite white clover germplasm under rotational cattle and sheep grazing

Four white clover (Trifolium repens L.) cultivars, 4 pre-release cultivars and 24 breeding lines were evaluated in small plots in mixed species swards under rotational cattle and sheep grazing for 2 years. There was no significant stock class x line interaction for proportion of clover in the sward. There was no evidence that medium- and smallleaved types yielded better under sheep grazing while larger-leaved types yielded better under cattle grazing. Lines with the highest clover content tended to be large leaved and upright irrespective of stock class. They also had moderate to high stolon growing point densities. Breeding programmes have developed lines with high stolon growing point densities, ensuring vegetative persistence, while still maintaining a high proportion of clover in the sward. These pre-release cultivars and breeding lines gave significantly better clover contents than existing cultivars. Keywords cultivars, selections, plant habit, cyanogenesis, stolon growing point density, clover content, cattle grazing, sheep grazing

Variation in spring growth within white clover

In a spaced plant trial, plants of 27 cultivars in their second year of growth were undefoliated from late winter through spring and their growth rate determined relative to that of 'Grasslands Huia'. Variation between cultivars was large. Cultivars which had higher relative growth rates than Huia during this period included some small-leaved frost tolerant germplasm, although all of these had lower absolute growth scores than Huia. The second study measured proportion of clover, leaf size and stolon growing point densities of 27 white clover cultivars and breeding lines grown in mixed species grazed swards during late winter and spring in two successive years. Once again, variation between cultivars in spring performance relative to Huia was large. It was concluded that there is sufficient variation available within white clover for improvements in spring growth relative to Huia to be achieved by breeding. Nematode Resistant Pitau consistently had high growth rates relative to Huia in late winter and early spring,but this did not carry through into late spring. Cultivars and lines with better late winter growth than Huia had higher stolon growing point densities for their respective leaf sizes than poorer, later winter growers. Keywords evaluation, mixed swards, spaced plants, later winter growth, stolon density

Seasonal variation in stolon growing point density of a world collection of white clover (Trifolium repensL.) cultivars

Abstract The seasonal variation in density of aerial and surface stolon growing points of 109 cultivars and 16 breeding lines of white clover was examined in a rotationally grazed sward over 3 years. Aerial and surface stolon growing points numbers were higher in summer and autumn than in winter and spring. There was, however, a large cultivar × season interaction for stolon growing point density. Very small-leaved cultivars had a greater change in stolon growingpoint density from summer orautumn to winter or spring than larger-leaved cultivars. Earlier-flowering Unes also had a more significant seasonal drop in stolon growing point density from autumn to winter or spring than later-flowering lines.

Turnover of grass laminae and white clover leaves in mixed swards continuously grazed with steers at a high‐and low‐N fertilizer level

AbstractTurnover rates of grass laminae and clover leaf tissue were estimated over a range of intervals within three periods each year in the second to fourth years (1983‐85) of a trial involving swards continuously grazed by steers and receiving either 60 kg N ha‐1 in spring (60N) or 360 kg N ha−1 throughout the year (360N). Within the 60N swards initial stocking rates at turnout were low (60N LS) at 7‐2 steers ha−1 and high (60N HS) at 90 steers ha−1 in 1983, and in 1984 and 1985 corresponding rates were 10‐8 and 13‐5 ha−1. The 360N swards were initially stocked at turnout at 96 (360N LS) and 120 (360N HS) steers ha−1. Stocking rates were reduced by 33% in midsummer except for 60N in 1984 and 1985 when they were reduced by 50%.Meaned over 3 years, 360N HS had lower herbage mass than 60N LS. Tiller density in 360N was almost 50% higher than in 60N and clover growing point density was only one quarter that of 60N with the 60N LS having lower clover densities than 60N HS in 1985.Generally, leaf extension rate per tiller was higher in 360N than 60N and, when significant, 60N LS had higher senescence rates per tiller than 360N HS.Rate of increase in new clover lamina tissue per stolon was not affected by treatments, whereas in 1983 LS had higher senescence rates of clover laminae than HS. Petiole growth per stolon was higher in LS than HS in 1983 and 1984, the mean over these years for 360N HS being 77% that of 60N LS. Petiole senescence per stolon was lower in 360N HS than 60N LS only in 1983.When comparing 60N HS and 360N LS (representing similar levels of grazing intensity, having similar herbage mass) the gross growth of leaf material in the former was 75% of the latter, in contrast to 57% for net growth. Clover contributed 18% to the estimated growth of leaves compared to a mean of 7% in herbage mass. Taking inflorescence and pseudostem into account in 1984 and 1985,60N HS had 7% clover in standing herbage and 14% in net growth. Therefore, the contribution of clover to growth is considerably higher than its presence in herbage mass would suggest in continuously grazed swards. It is concluded that low‐N swards, owing to their lower tiller density and slower grass leaf extension rate, will be less efficiently grazed than swards at higher N levels at a given herbage mass, but the presence of clover will partly offset that disadvantage.

Density ofTrifolium repensplants in mixed swards under intensive grazing by sheep

SUMMARYDensities of physiologically independent plants of white clover were studied in New Zealand in pastures stocked at 22·5 ewes plus lambs/ha by set stocking, rotational grazing or a combination of both systems. Plants were sampled once a month for 1 year (1984/85) by taking turves and washing out the plants. Numbers of growing points and stolon dry weight per plant were obtained. At each sampling fifty, 50 mm diameter pasture plugs were taken from each sward and growing point density and stolon mass/m2of white clover were measured. The density of white clover plants in the swards was estimated on the basis of both stolon dry weight and number of growing points.The two estimates gave similar results. There was a trend of lowest densities in set-stocked pastures (334/m2), intermediate densities in combination management pastures (431/m2) and highest densities in the rotationally grazed pastures (553/m2). The overall mean density of white clover plants was 439/m2and the range was 193–811/m2.The structure of swards under the three systems of grazing differed and this was considered to contribute towards the variation in density of white clover plants in the various swards.

Effect of sheep grazing management on distribution of white clover stolons among 3 horizontal strata in ryegrass/white clover swards

Abstract White clover stolons were dissected from core samples taken from pastures with 3 different management backgrounds; set stocked (S), rotationally grazed (R), or set stocked from lambing until drafting, then rotationally grazed for the rest of the year (C). Stolons were classified as aerial, surface, or buried. Stolon dry weight and stolon length for each stolon class, and the number of growing points were measured once a month for 1 year. Despite causing large differences in pasture availability and structure, sheep grazing management had little effect on the amount of stolon in the sward or the distribution of stolon in the 3 classes. However, management did affect stolon length per unit stolon dry weight and the degree of stolon branching (measured as growing point density) in the order S>C>R. The seasonal change in stolon distribution among the 3 classes was large, but similar for all grazing managements. Aerial and surface stolons were at a maximum in late summer-early autumn and at a minimum ...