Plant-soluble carbohydrate reserves and senescence - key criteria for developing an effective grazing management system for ryegrass-based pastures: a review

This review examines the use of changes in soluble carbohydrate reserves, and the onset of senescence in ryegrass (Lolium spp.), as key criteria for successfully managing an intermittent grazing system for dairy cattle. Ryegrass is a '3-leaf' plant; that is, only about 3 green leaves/tiller exist at any one time with the initiation of a new leaf coinciding with senescence of the oldest fourth leaf. Thus, grazing pasture older than 3 leaves/tiller will not only lead to wastage of pasture but also the senescent material will reduce overall quality of herbage. Based on this, the time taken for 3 new leaves/tiller to regrow sets the maximum grazing interval. On the other hand, in a well-utilised dairy pasture, most ryegrass leaf has been removed and the plant relies on stored water-soluble carbohydrate reserves to grow new shoots and hence regain photosynthetic capacity. If the concentration of water-soluble carbohydrates is inadequate, because there has been insufficient time to replenish in the previous inter-grazing period, regrowth will be suppressed and this may also affect persistence in the longer term. Immediately after grazing, water-soluble carbohydrate reserves decline as they are used to regrow new shoots, and root growth stops. It is not until about 3/4 of a new leaf/tiller has regrown that the plant has adequate photosynthetic capacity for growth and maintenance and only then does water-soluble carbohydrate replenishment and root growth commence. Studies have shown that subsequent regrowth is suppressed if plants are redefoliated before the 2 leaves/tiller stage of regrowth. Also, the levels of potassium and nitrogen (as nitrates and other non-protein nitrogen products) may be very high and cause metabolic problems in stock grazing such pasture. Thus, replenishment of water-soluble carbohydrate reserves sets the minimum grazing interval at 2 leaves/tiller. The rate of accumulation of water-soluble carbohydrates in the plant is a function of input through photosynthesis (source) and output to growth and respiration (sinks). Thus, apart from grazing interval (which sets the time to replenish water-soluble carbohydrate plant reserves), water-soluble carbohydrate storage will be influenced by incoming solar radiation (cloud cover, day length, pasture canopy density) and energy needs of the plant through respiration (temperature, canopy mass) and growth. Relating grazing interval to leaf number places the emphasis on the readiness of plants to be grazed rather than on the animals' requirements, with leaf appearance interval depending primarily on ambient temperature. This allows grazing interval to be expressed in a similar morphological stage of growth, irrespective of season or location. Setting grazing interval on these 2 criteria has been shown to maximise growth and persistence of ryegrass and optimise the levels of most nutrients in pasture required by dairy cattle including protein, water-soluble carbohydrates, calcium, potassium and magnesium. Metabolisable energy and fibre do not change appreciably up to the 3 leaves/tiller stage of regrowth. On the other hand, grazing pasture before 2 leaves/tiller not only retards regrowth and reduces persistence, it provides forage too high in potassium and protein (nitrates) and too low in water-soluble carbohydrates for dairy cattle.