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4.1 A guide to animal nutrition and expected growth rates

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Animal growth rate is a function of energy intake (voluntary feed intake x feed metabolizable energy content), dietary protein, animal size, physiological state and animal activity. Pasture or forage intake, measured in DM, is directly related to pasture or forage digestibility (Figure 1). If intake is estimated as % of live weight, it decreases with increasing animal weight.

Ruminants such as cattle, sheep and goats have a specialized digestive system with a four-chamber stomach - rumen, reticulum, omasum and abomasum. The CP required for growth depends mainly on the live weight of the animal and its growth rate. What happens is that there is more CP degraded to ammonia N than the microbes can use, given the ME content of the pasture or forage.

Expected growth of cattle can be estimated based on live weight and forage quality on pasture or forage. The upper limit for cattle growth rates is determined by the intake of metabolizable energy in the feed. Cattle walking distance affects growth rate when LWG is limited by pasture digestibility (Table 3).

Example of effect of high energy supplementation (grains or high energy pellets) on pasture intake (%) at 3 levels of supplementation as % live weight per day (% LW/day) compared to grazing alone (control).

Figure 1.  Predicted dry matter intake of forage by 200, 400 and 600kg  steers as a % of their liveweight across a range of pasture digestibility
Figure 1. Predicted dry matter intake of forage by 200, 400 and 600kg steers as a % of their liveweight across a range of pasture digestibility

Further information

Grazing Rhodes grass pastures

Key messages

Characteristics of grazed

Rhodes grass and other tropical grass pastures

High growth rates in 4-5 per week (e.g. in summer 150 to >200 kg dry matter/ha per day) are associated with a higher proportion of stem. A common feature of most pastures after one or more grazing seasons is variable pasture height and density. Clumps of pasture stand out - higher, more mature areas (areas with 'EA' faeces or areas affected by manure).

These typically take up 15-25% of a pasture, with the proportion increasing with time since the pasture was 'reset' (see Tools, page 188). The pasture between these laxly grazed groves, or interstitial areas (IA), is shorter and is the preferred area of ​​livestock. Excreta Area (EA) - the clumps of tall grass in a pasture that have not been grazed or are lax, mainly associated with manure and urine patches.

Forage on offer (FOO) – describes the amount of pasture dry matter available to grazing animals at any time and is assessed by estimating the pasture dry weight/ha. Feed budget – an assessment of the FET and number of grazing days available for a certain class of livestock.

Terminology

With a Rhodes grass meadow growing under favorable conditions, there is an exponential increase in stem growth after about three weeks of regrowth74. A paddock or feed budget can make better decisions about allocating livestock to pasture and when to move them.

Characteristics of cattle

Leaf canopy height and FOO will be variable with seasonal growth, but can typically be 20-30 cm and 2000-2500 kg/ha FOO respectively. A grazing period of one day is ideal but rarely achievable in practice; it is recommended that 3 days be considered the maximum. The period of regrowth from controlled grazing (or restoration) to the desired FOO will vary greatly with the season.

For example, in the low rainfall coastal zone of the Pilbara it can vary from 12 days over summer to 45 days in winter.

Grazing guidelines

Tools

A convenient paddock pasture DM estimation

To complete the calculation, you will need to know the current FOO, desired FOO when the animals are removed, the pasture growth rate, daily consumption of the animals, the number of grazing animals and an estimate of wastage. Recovery refers to mechanical removal of the clods, which becomes essential after two or three grazing cycles, by mowing or mulching to about 10 cm. If the cut material is small, it may not be necessary to remove it.

However, if this is judged to be excessive, to the extent that too much pasture is 'covered', the dead material must be removed, for example by raking and baling, and discarded if of poor quality, as is often the case. In the case of mulch on the grazed pasture, where the material is cut into small lengths and deposited back, it is important that the quantity is not excessive as the resulting litter will act as a physical and nutrient barrier.

Identifying problems with the pasture

Understanding feed quality

Clinton Revell

Introduction

Feed quality testing and interpretation

Feed analysis report

In general, fiber (NDF and ADF) typically increases, and water-soluble carbohydrates, digestibility, and metabolizable energy decrease as plants mature. The highest ME in Rhodes grass is found during the first 14 days of regrowth when the lawn is dominated by new foliage (Section 3.3). As the stand matures, the proportion of stem, which is more hardened and less digestible, increases (reflected in an increase in ADF and NDF).

Legumes are generally higher in protein content, but CP in grasses is strongly influenced by nitrogen supply. In temperate grasses, the fructans (sugars) are the main storage carbohydrates, while C4 grasses store starch rather than sugar and consequently have low WSC. Temperate grasses can have high WSC, and these soluble carbohydrates are highest in stems rather than leaves and are at their highest concentrations just before flowering.

This contrasts with C4 grasses where the stems are significantly lower in digestibility (energy) and protein and higher in fiber than the leaves. However, although the forage quality of warm-season C4 grasses improves at moderate temperatures, it is still significantly lower than temperate grasses. However, tropical grass pastures often vary in composition and nutritional value with higher quality leaf and lower quality stem.

Livestock grazing tropical pastures can partially compensate for the lower overall forage quality by selectively grazing the higher quality areas of the turf. However, the upper limit of production is limited by the nutritional value of the highest quality food that can be chosen89. When a pasture is grazed, the animals are forced to eat lower quality grassland with a higher stem content that contains less energy and protein.

Figure 1.  Cell wall composition showing components of neutral detergent fibre  (NDF) and acid detergent fibre (ADF)
Figure 1. Cell wall composition showing components of neutral detergent fibre (NDF) and acid detergent fibre (ADF)

Feed quality of C4 grasses compared with C3 grasses

3 Galloway P, van Gool D, Laycock J, Holmes K, Rowe D (2018) 'The Shay Gap Landscaping: investigating the suitability of soils and landforms for irrigated agriculture in the Western Conservation Basin', Management Technical Report of Resources 411, Department of Primary Industries and Regional Development, Perth. 4 Smolinski HJ (2019) 'Investigations of the potential for irrigated agriculture in the Bonaparte Plains: soil capability assessment report', Resource Management Technical Report 410, Department of Primary Industries and Regional Development, Perth. 5 Smolinski H, Pathan S, Galloway P, Kuswardiyanto K, Laycock J (2015) 'Cockatoo Sands in the Victoria Highway and Carlton Hill areas, East Kimberley: assessment of soil capability for the development of irrigated agriculture', Management Technical Report of Resources 391, Department of Agriculture and Food, Western Australia, Perth.

6 Smolinski H, Galloway P, Laycock J (2016) 'Pindan soils in the La Grange area, West Kimberley: land capacity assessment for irrigated agriculture', Resource Management Technical Report 396, Department of Agriculture and Food, Western Australia, Perth. 8 Bennett D (2019) Exploring the potential for irrigated agriculture on the Bonaparte Plains: Drilling Completion Report, 2nd edn, Resource Management Technical Report 414, Department of Primary Industries and Regional Development, Western Australia. 22 Collins R and Grundy T (2005) The Butterfly Pea Book: A guide to establishing and managing butterfly pea pastures in central Queensland, Queensland Department of Primary Industries and Fisheries, Brisbane, 60p.

26 DoW (Department of Water) (2014) Water resources inventory: water availability, quality and trends, Department of Water, Western Australia. 39 Haig T (2009) Pilbara Coastal Water Study, hydrological data series, report HG34, Department of Water, Western Australia. 44 Hills A and Penny SA (2005) Guide to growing summer cereals and forages in the south coast region, Western Australia, report 20/04, Department of Agriculture and Food, Western Australia, Perth.

51 Jones R and Wright D (2006) 'Perennial pastures, plant diseases and the 'green bridge'', in Perennial Grasslands for Western Australia (eds GA Moore, P Sanford and T Wiley), Bulletin 4690, Department of Agriculture and Food, Western Australia, Perth. 52 Kaiser AG, Pilz JW, Burns HM and Griffiths NW (2004) Successful silage, Topfodder, NSW Department of Primary Industries and Dairy Australia. 66 Mullen C (2020) 'Summer legume forage crops: peas, lablab, soybeans', NSW Department of Primary Industries, accessed March 2020.

68 NSW DPI (Department of Primary Industries) (2016) Interpretation of Water Quality Test Results, Primefact 1344, 1st Edition, Department of Agricultural Water for Primary Industries, New South Wales, Australia. 73 Paul RJ, Raper GP, George RJ, Wright NJ, Lillicrap AM and Gardiner PS (2019) Groundwater surveys to support irrigated agriculture at La Grange, Western Australia: results 2013–18, Resource Management Technical Report 398, Department of Primary of Industry and Regional Development, Perth. Insect pests of vegetables in the Ord River Irrigation Area, Bulletin 3423, Western Australian Department of Agriculture.

86 Schoknecht N og Pathan S (2012) Soil groups of Western Australia: a simple guide to the main soils of Western Australia, 4th edn, Resource Management Technical Report 380, Department of Agriculture and Food, Western Australia, Perth. West Kimberley: vurdering af jordkapacitet for kunstvandet landbrug, Resource Management Technical Report 396, Department of Agriculture and Food, Western Australia, Perth.

Table 1.  Typical range in nutritive value components of forages
Table 1. Typical range in nutritive value components of forages

Figure

Figure 1.  Predicted dry matter intake of forage by 200, 400 and 600kg  steers as a % of their liveweight across a range of pasture digestibility
Figure 2.  Basic outline of  the digestive tract of a cow
Table 1.  The threshold dietary crude protein% (CP%) above which there is loss of N from the rumen for pastures  varying in feed quality (DM digestibility%, metabolisable energy)
Table 2.  Indicative cattle growth rates based on MLA feed intake, liveweight and feed  quality relationships and assuming minimum protein requirements are met and that the  cattle are walking 2km per day*
+7

References

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