DairyNZ scientist Cathal Wims explains why.
- Spring grazing management influences the amount and quality of pasture grown later in the season.
- Pasture quality is optimised when pastures are grazed between the 2 and 3-leaf stage of regrowth, and grazed to residuals of 3.5-4cm (or 7-8 clicks on RPM).
- Good pasture management in spring increases tillering in perennial ryegrass
Optimal time to graze
The timing of grazing during the regrowth cycle has a significant impact on the amount and quality of pasture grown. Pasture growth generally follows an ‘S’ shaped curve, beginning slowly (known as a lag phase), then accelerating before levelling-off. Understanding this growth pattern, and the factors that influence it, helps in determining the optimum time to graze.
Plant leaves capture light energy for photosynthesis, which provides energy for plant growth. Grazing or harvesting the pasture removes leaves and significantly reduces the ability of plants to photosynthesise as it deprives plants of their primary food source, light energy. With much less energy available from photosynthesis, the plant is reliant on ‘reserves’ stored in the tiller stubble immediately after grazing for maintenance and to grow new leaf. This reduced energy supply results in the first leaf produced after grazing being relatively small. As the first leaf expands and leaf area increases, an increasing amount of light energy is captured and the rates of photosynthesis and pasture growth increase. This in turn results in more energy for the next leaf, and it will be a bit bigger. This pattern continues until the plant regains it full energy status.
Each leaf produced has a limited lifespan. Perennial ryegrass is called a ‘3-leaf’ plant as it only maintains about three live leaves per tiller (Figure 1). Once the third new leaf has been produced, the first leaf begins to die.
Eventually a ceiling yield will be reached, where the plant is still producing new leaves but the amount produced is cancelled by the rate of death and decay by older leaves. At this point there is no net gain in pasture growth (Figure 2).
From a grazing management perspective, this means that grazing too late in the regrowth cycle, i.e. after the third new leaf is produced leads to leaf death and pasture wastage, but grazing too early reduces pasture yield.
How do I easily identify the optimal time to graze?
Maximum average growth rate occurs at approximately the 3-leaf stage after grazing3. Monitoring leaf stage is an effective indicator of when a paddock is ready to graze. Current recommendations are to graze pastures between the 2 and 3-leaf stage of regrowth. The first, second and third successive leaves produced after grazing contribute 25 percent, 35 percent and 40 percent, respectively, of the total available pasture mass at the next grazing4. As the contribution of the first leaf to total pasture mass is relatively small, fast rotations that consistently graze pastures before the 2-leaf stage of regrowth will significantly reduce pasture growth. Increasing rotation length to consistently graze pastures at the 3-leaf stage results in a yield advantage compared with grazing at the 2-leaf stage. Chapman et al5 calculated this yield advantage to be 1.1 t DM/ha/year for Canterbury irrigated pastures (Table 1).
Managing regrowth interval in spring
The contribution of each successive leaf to total available pasture mass varies seasonally (Table 1). When reproductive growth is present and growth rates are high in spring (October – November) the difference between the contribution of the second and third leaf to the available yield at grazing largely disappears4 and there is little additional yield benefit in delaying grazing from the 2-leaf stage to the 3-leaf stage5. There is always a yield penalty if pastures are grazed before the 2-leaf stage of regrowth. Leaf stage can therefore be viewed as a flexible grazing management tool with a grazing window between the 2 and 3-leaf stage5. Rather than rigid adherence to a single leaf stage grazing target, grazing management must also consider system needs such as pasture cover targets, feed demand requirements and pasture quality. For example, during periods of high growth rates in spring, lower stocked-farms may graze closer to the 2-leaf stage to control pastures covers and maintain pasture quality.
Table 1: An example of the seasonal contribution of successive perennial ryegrass leaves to total dry matter yield based on typical growth rates and leaf appearance intervals for Canterbury irrigated pastures9.
|Growth rate (kg/DM/ha/d)||Leaf appearance interval||Contribution of 1st:2nd:3rd leaf total DM (%)||DM grown (kg/ha) when grazed at:||Difference|
Perennial ryegrass moves from vegetative to reproductive growth during spring which results in significant changes in pasture composition. Reproductive growth leads to stem elongation and an increase in the proportion of stem to green leaf which tends to lower overall pasture quality, as stem has a lower nutritive value6. More frequent grazing during spring removes stems before they are fully elongated resulting in pastures with lower stem content, less dead material, more green leaf and higher ME . Current recommendations are to graze pastures at a pre-grazing mass of 2600-3200kg DM/ha8, depending on stocking rate.
The pasture regrowth recommendations are based on pastures grazed to an optimal residual of 3.5-4.0 cm (7-8 clicks on RPM). However, on farm research has identified that optimal post-grazing residuals are achieved only 50% of the time7.
High post-grazing residuals result in reduced growth rates during the subsequent regrowth cycle2. Why is this? The greater residual leaf area following high post-grazing residuals actually results in higher growth rates initially. However, the rates of leaf death are also high (as there is more residual leaf and residual leaves tend to be older) and reach a maximum and cancel the rate of new leaf production earlier in the regrowth cycle compared with optimal residuals. As a result, the maximum average growth rate is reached earlier in the regrowth cycle and is a lower value than for well-managed pastures.
The post-grazing residual from which a pasture regrows also has a significant impact on pasture composition and quality at subsequent grazings, and subsequent milk production. Research both in New Zealand and overseas has demonstrated that lax grazing during spring results in pastures with greater stem content, higher amounts of dead material and of lower digestibility at subsequent grazings. As a result, laxly grazed pastures support lower levels of milk production at subsequent grazings10.
Achieving optimal post-grazing residuals also stimulates the production of new or daughter tillers which keeps tiller density high. Daughter tillers are produced from buds located at the base of the parent tiller (Figure 3) and in order to maintain pasture productivity and persistence, each tiller must leave behind at least one offspring. Consistent post-grazing residuals of 3.5-4cm (or 7-8 clicks on the RPM) increases the quantity of light reaching the base of the pasture which stimulates tiller production and aids the survival of newly emerged tillers. Studies in New Zealand have shown that pastures that are grazed to optimal post-grazing residuals have a higher tiller density compared with laxly grazed pastures12.
1. Lee, J. M., P. Hedley, and J. R. Roche. 2011. Grazing management guidelines for optimal pasture growth and quality. DairyNZ Technical Series p. 6-10.
2. Parsons, A. J., I. R. Johnson, and A. Harvey. 1988. Use of a model to optimize the interaction between frequency and severity of intermittent defoliation and to provide a fundamental comparison of the continuous and intermittent defoliation of grass. Grass and Forage Science 43: 49-59.
3. Parsons, A. J. and D. F. Chapman. 2000. The Principles of Pasture Growth and Utilization, in ‘Grass: Its Production and Utilization’ (Ed. A. Hopkins), Blackwell Science, London. p. 31-89.
4. Chapman, D. F., J. Tharmaraj, M. Agnusdei, and J. Hill. 2012. Regrowth dynamics and grazing decision rules: further analysis for dairy production systems based on perennial ryegrass (Lolium perenne L.) pastures. Grass and Forage Science 67: 77-95.
5. Fulkerson, W. J. and D. J. Donaghy. 2001. Plant-soluble carbohydrate reserves and senescence-key criteria for developing an effective grazing management system for ryegrass-based pastures: a review. Australian Journal of Experimental Agriculture 41: 261-275.
6. Terry, R. A., and J. M. A. Tilley. 1964. The digestibility of the leaves and stems of perennial ryegrass, cocksfoot, timothy, tall fescue, lucerne and sainfoin, as measured by an in vitro procedure. Grass and Forage Science 19: 363-372.
7. McCarthy, S., C. Hirst, D. Donaghy, D. Gray, and B. Wood. 2014. Opportunities to improve grazing management. Proceedings of the New Zealand Grassland Association 76: 75-80.
8. DairyNZ. 2008. Principles of Grazing Management. Farm Fact 1-2.
9. Chapman, D. F., S. McCarthy, and J. Kay. 2014. Hidden dollars in grazing management: Getting the most profit from your pastures. South Island Dairy Event, Invercargill, p. 21-36.
10. Hoogendoorn, C. J., C. W. Holmes, and A. C. P. Chu. 1992. Some effects of herbage composition, as influenced by previous grazing management, on milk production by cows grazing on ryegrass/white clover pastures. 2. Milk production in late spring/summer: effects of grazing intensity during the preceding spring period. Grass and Forage Science 47: 316-325.
This article was originally published in Technical Series September 2016