Responses to the renovation of an irrigated perennial pasture in northern Victoria. 1. Pasture consumption and nutritive characteristics

A field experiment was established in northern Victoria in the autumn of 1999 to quantify the effects of renovating a 15-year-old irrigated perennial pasture which had a high paspalum content [>40% dry matter (DM)] in summer. The treatments were: (i) control, the existing pasture; (ii) oversown, in which the existing pasture was grazed, topped and direct drilled; and (iii) resown, in which the existing pasture was sprayed, cultivated and sown with a new pasture. The grass species used in both renovation treatments were perennial ryegrass, Italian ryegrass and tall fescue. The treatments were grazed by dairy cows when the perennial ryegrass had reached the 2.5–3 leaf stage. Grazing of the resown tall fescue coincided with the resown ryegrass in years 1 and 2, but in subsequent years, resown tall fescue was grazed at a rising plate meter height of 80 mm. All treatments were grazed to a residual pasture height of 40–45 mm, as measured with a rising plate meter. Pasture consumption (measured as DM removed by dairy cows), in vitro DM digestibility (in vitro DMD) and crude protein (CP) contents were measured. Oversowing increased pasture consumption over the 4-year period, compared with the control, by an average of 1.1 t DM/ha.year when oversown once with perennial ryegrass and by 1.6 t DM/ha.year when oversown annually with Italian ryegrass. This increase occurred principally during the winter–spring period for pastures oversown with both perennial (0.7 t DM/ha) and Italian (1.6 t DM/ha) ryegrass. Oversowing with perennial or Italian ryegrass did not affect the in vitro DMD or CP content of the pasture on offer. These results show that oversowing with either perennial or Italian ryegrass is a viable means of increasing pasture availability over winter and spring from perennial pastures consisting of a mixture of perennial ryegrass, white clover and paspalum. Pasture consumption in the first 12 months after resowing was 3.5–4.1 t DM/ha lower from the resown than from the control pasture. This was because of two fewer grazings in autumn–winter and to reductions in pasture consumption of 20% in spring and of 40% in summer. These reductions would add considerably to the cost of resowing through increasing the need for supplementary feeding. Pasture consumption from the resown perennial ryegrass pasture in years 2–4 was, on average, the same as the control, although it was higher during winter and spring and lower during summer. Pasture consumption from the resown tall fescue pasture in years 2–4 was, on average, 2.5 t DM/ha.year higher than that of the resown perennial ryegrass pasture, with most of this increase occurring in summer and autumn. The resown pastures had higher in vitro DMD and CP contents than the control with little difference between the resown perennial ryegrass and tall fescue pastures. These findings show that tall fescue is a viable alternative to perennial ryegrass when resowing pastures. The use of nitrogen fertiliser did not affect the in vitro DMD or CP contents of the pasture on offer but allowed an increase in DM consumption, with this increase being greater for the control and oversown pastures than for the resown pasture.

Influence of regrowth time on the forage quality of prairie grass, perennial ryegrass and tall fescue under non-limiting soil nutrient and moisture conditions

The influence of regrowth time on the forage quality of prairie grass (Bromus willdenowii Kunth. cv. Matua), perennial ryegrass (Lolium perenne L. cv. Dobson) and tall fescue (Festuca arundinacea Schreb. cv. Dovey) was determined under non-limiting soil nutrient and moisture growth conditions. In a glasshouse, individual plants of each species were arranged in separate mini-swards and were defoliated at 6, 10 and 14 weeks after sowing to a stubble height of 60 mm for perennial ryegrass and tall fescue and 90 mm for prairie grass. Following defoliation at 14 weeks, selected individual plants were cut to the previous stubble height as each new leaf per tiller was fully expanded, to provide leaf material for nutrient analysis, until prairie grass, perennial ryegrass and tall fescue had attained 6–8, 5 and 3 leaves/tiller, respectively. The concentration of leaf phosphorus (P) decreased from 6.6 to 5.9 g/kg dry matter (DM) in prairie grass, increased from 5.9 to 6.9 g/kg DM in perennial ryegrass, and initially increased to 8.8 g/kg DM and then decreased to 8.4 g/kg DM in tall fescue. The mean potassium (K) content in perennial ryegrass was 29.6 g/kg DM and was not significantly affected by duration of regrowth, whereas K content in prairie grass and tall fescue fell from 51.7 to 43.6 g/kg DM and from 55.5 to 47.9 g/kg DM, respectively, after the first leaf per tiller formed. Calcium levels increased with regrowth in all species and at the completion of regrowth were 5.8, 3.8 and 3.4 g/kg DM in prairie grass, perennial ryegrass and tall fescue, respectively. The magnesium (Mg) and sodium (Na) content of perennial ryegrass showed no change throughout the regrowth period and had measured values of 2.5 and 2.8 g/kg DM, respectively. For tall fescue, the concentration of leaf Mg decreased from 0.30 to 0.24 g/kg DM, whereas the Na concentration increased from 1.2 to 2.1 g/kg DM. The Mg content of prairie grass remained constant at 2.0 g/kg DM, whereas the Na content increased from 2.7 to 4.3 (g/kg DM). While the crude protein content of all grasses declined over the regrowth period, values remained over 200 g/kg DM, well above the recommended content for lactating cows. The leaf water-soluble carbohydrate (WSC) of prairie grass and perennial ryegrass increased over the regrowth period from 29.7 to 43.9 g/kg DM and from 25.9 to 72.5 g/kg DM, respectively, whereas tall fescue showed no change at 55.6 g/kg DM. The change in in vitro organic matter digestibility (OMD) with age was 125 and 44 (g/kg DM) for tall fescue and perennial ryegrass, respectively. The OMD of prairie grass decreased following the onset of stem elongation at the 5-leaves/tiller stage of regrowth from 824 to 756 g/kg DM. In this glasshouse study, the pattern of change in K and Ca content was the same as observed in the field but the absolute content, including that of Na, was greatly elevated, particularly in prairie grass. In terms of nutrient content capability, N, P and K were readily taken up by these C3 grasses, while the uptake of Mg and Na appear to reflect genetic differences between species. The differences in forage quality as determined under optimal growth conditions in this study, as compared with field grown forage, are presumed to indicate possible soil nutrient deficiencies in field situations.

Effects of spring grazing on dryland perennial ryegrass/white clover dairy pastures. 1. Pasture accumulation rates, dry matter consumed yield, and nutritive characteristics

A 3-year experiment (September 1999–August 2002) was conducted in south-western Victoria to determine the impact of spring grazing on pasture accumulation rates, dry matter (DM) consumed yield (estimate of DM yield), and pasture nutritive characteristics [metabolisable energy (ME), crude protein (CP), neutral detergent fibre (NDF), and water-soluble carbohydrates (WSC)] of a perennial ryegrass (Lolium perenne L.)–white clover (Trifolium repens L.) pasture. Spring grazing treatments, applied annually from September to November, were based on ryegrass leaf development stage with high (HF), medium (MF), and low (LF) grazing frequency being 2-, 3-, and 4-leaf stage, respectively, and post-grazing height as the grazing intensity with high (HI), medium (MI), and low (LI) grazing intensity being 3, 5, and 8 cm, respectively. Five combinations were used: HFHI, LFHI, MFMI, HFLI, and LFLI. A sixth treatment, rapid grazing (RG), maintained pasture between 1500 and 1800 kg DM/ha by grazing weekly during spring, and a seventh and eighth treatment, simulating forage conservation for early-cut silage (lock-up for 6–7 weeks; SIL) and late-cut hay (lock-up for 11–12 weeks; HAY), were also included. For the remainder of the year, all plots were grazed at the perennial ryegrass 3-leaf stage of growth, or when pasture mass had reached 2800 kg DM/ha, and grazed to a residual height of 5 cm. On average, pasture accumulation rates ranged from <5 (February–March) to 100–110 kg DM/ha.day (September–October). Overall, SIL resulted in a lower accumulation rate than all other treatments. High spring grazing frequency (including RG) treatments led to more grazing events than medium and low spring grazing frequency treatments. In Years 1, 2, and 3, DM consumed ranged from 9.7 (HAY) to 16.3 (RG), 4.2 (HAY) to 10.1 (HFHI), and 7.3 (SIL) to 10.9 t DM/ha.year (HAY), respectively. HAY resulted in a lower pasture ME content than SIL, HFHI, and LFHI spring grazing, and LFLI spring grazing resulted in a lower pasture ME content than all other treatments except HAY. HFHI grazing resulted in an increase in ME content over time, whereas the rate of increase in ME content over time was higher for LFLI spring grazing than for HAY, RG, and HFLI spring grazing. For all treatments, average pasture ME content ranged from 9.4 (January–February) to 11.4 MJ/kg DM (September). HAY resulted in a lower CP content than all treatments except LFLI grazing. RG resulted in no change in CP content over time. For all treatments, average pasture CP content ranged from a low of 11–14 (January–February) to a high of 24–28% DM (August–September). LFLI grazing resulted in a higher NDF content than HFHI, LFHI, MFMI, and HFLI grazing, while RG resulted in a lower NDF content than LFHI, MFMI, and HFLI. For all treatments, average pasture NDF content ranged from a low of 48–55 (August–September) to a high of 58–62% DM (January–February). All treatments resulted in an increase in pasture WSC content over time. The results demonstrate that frequent and intense grazing management (e.g. HFHI and RG) during spring is important in maintaining high pasture DM yields. Results also indicate positive pasture nutritive characteristic (ME, CP, and NDF) gains with more frequent spring grazing than with infrequent spring grazing. No treatment effect was observed for WSC content.