GRAZING MANAGEMENT OF IRRIGATED RHODES GRASS

 Mowanjum Irrigation Trial

The harvesting of a tropical grass by direct grazing is fundamentally different to that by mechanical equipment. The nuances of grazing management, and the quality and quantity of pasture eaten and converted to beef need to be understood to guide effective management.

The following principles have been established from well-managed field research, mostly in Queensland and relating to dairy cattle, and observed to be just as relevant in managing beef cattle grazing irrigated Rhodes grass in the Kimberley and Pilbara.

Regional research continues to refine understanding relating to local Bos indicus cattle and crosses, climate and soil differences and tools for decision making. Local observations and measurements to date indicate that an example of a well-managed and maintained Rhodes grass pasture has the following leaf quality measurements:

Dry Matter Digestibility% (DMD %) 57.4 Metabolizable Energy, Megajoules/kg (ME MJ/kg) 8.2 Crude Protein% (CP%) 15.5

Presuming cattle have unrestricted access to this leaf, the following is an example of performance linked to current understanding. Growing cattleat 200kg live weight have a maximum predicted intake of leaf of 4.6kg and possible energy intake of 8.2 MJ ME/kg, giving a total energy intake of 37.7MJ.

Cattle Energy Requirements to improve growth (200kg liveweight): Maintain weight 28.0 MJ 0.25kg average daily gain 33.0 MJ 0.5kg average daily gain 41.5 MJ

Note: 1. Protein is not limiting growth. 2. Expected growth rates are around 0.4kg/day. This is what is commonly measured. 3. In this example, cattle are eating 2.3% of their liveweight. The more digestible the pasture, the more can be eaten and the more weight gain possible.

Pasture at Mowanjum has ranged from 8.2MJ/ME to 9.8 MJ/ME with steady improvement and consistency as system management has been refined. This has equated to an average daily weight gain of between 0.5 to 0.7kg.

The physical and chemical properties of the Rhodes grass are responsible for the upper limit on cattle performance. As the cattle are compelled to eat more stem, or the sward structure impedes leaf gathering or otherwise deteriorates, performance reduces.

Rank Rhodes grass limits animal production

Good grazing Rhodes grass pasture

Providing a supplementary feed with Rhodes grass will increase average daily weight gain and alternative pasture options may also increase weight gain.

Principles and Observations

Stocking rate and grazing interval must be flexible to respond to pasture growth rate and characteristics The high growth rates of well-watered and well-fertilised tropical pastures require rapid responses to maintain the required sward characteristics. As little as a one day delay can quickly have a compounding effect. A dense leafy sward is associated with greater cattle bite size, the main driver of intake. Grazing between 30cm and 10cm is a guide to maximise leaf intake and restrict stem elongation. Cattle select heavily for leaf rather than stem, even though there may be more stem by weight in the sward. Grazing prior to the four leaf stage on a tiller is a useful guide. The first half of the uppermost leaves is sought first, followed by leaf-bearing stem. If no other options then the almost leafless stem will be consumed. This slows down intake as bites become smaller and at a higher frequency however this does not compensate for the leaf deficiency. Towards the end of this sequencecattle are inevitably losing weight. Stem is nearly always less digestible than leaf as a result of its higher lignin and other structural carbohydrate composition and has a significantly longer rumen retention time. Appetite is reduced as a consequence, daily intake is restricted and, with it, the energy and protein required for maintenance and weight gain. If reproductive flowering stem is allowed to proliferate it will be largely rejected by cattle, increase in amount and become dominant in the sward. The consequences of this are far reaching; - Leaf acquisition is impeded. - Pasture is rejected or trampled. - Slower regrowth of reproductive tillers compared with leafy, vegetative tillers. - Reduction in sward density resulting in smaller bite size. - A stem-dominant pasture sward base having the character of a horizontal barrier to grazing. - Mulching/topping to 10cm may aid stem suppression; however minimal material is to be left as it acts to inhibit pasture growth if left in clumps. - Cutting to around 7cm combined with removal of material, whatever the quality, is necessary to restore a degraded pasture.

Contact:

Kevin Bell

Pardoo Beef Corporation

kevin.bell@pardoo.com.au

For grazing, a dense, leafy sward needs to be generated and maintained as an absolute priority.