Grazing management following rain

18 Aug 2018

Callen Thompson, Senior Land Services Officer Mixed Farming Coonabarabran

Across the region landholders are dealing with very limited soil moisture and dry conditions. Many producers are supplementary feeding, some are complete feeding in confinement or sacrifice paddocks and some producers have completely destocked. The next rainfall event may be a drought breaker or it may just grow some short green feed. It will be tempting to put livestock into green paddocks as soon as possible to reduce feeding costs and get back into production. This article aims to provide guidance on re-introducing livestock to your pasture paddocks.

Following rain, annual plants will germinate and surviving perennial plants will start producing leaves and tillers. This early growth or “green pick” is high in water content and low in dry matter. Dry matter is the important component of the plant because it contains the nutrients livestock need for maintenance and growth. Grazing paddocks too early will reduce the pastures ability to grow as well as having a negative effect on livestock performance. It is important to keep feeding/supplementing until there is enough dry matter in the pasture to maintain livestock.

When should livestock return to the paddock?

A dry sheep needs at least 400 kilograms of dry matter per hectare (kg DM/ha) per year to maintain body weight. Where dry matter is lower, the sheep will be unable to eat enough grass to acquire the nutrients it needs. For a dry cow approximately 900kg DM/ha is needed for maintenance. Table 1 indicates the required KG DM/ha of tropical grasses required for different classes of livestock. Plants at younger growth stages will have higher digestibility than plants at older growth stages.
As long as a pasture is vegetative, grazing at 1500kg DM/ha for sheep and 2500 kg DM/ha for cattle should maximize growth rates and production.

If the rainfall event is not sufficient or there has not been follow-up rain, resist the temptation to introduce stock onto pasture paddocks with less dry matter than is needed by your class of livestock. Your stock and pastures will thank you in the long run.

Figure 1 (above): Premier digit grass dry matter per hectare

Table 1: Minimum kg DM/ha to maintain satisfactory production levels for stock grazing tropical grasses.
NS = not suitable (Adapted from PROGRAZE Manual 8^th Edition)

The growth stage or phase of the plant has a significant impact on the plants ability to grow.

Phase 1 plants (often less than 1000kg DM/ha) generally slow growing as they don’t have the leaf area to produce the energy needed for the plant to grow rapidly. Energy is generally high but DM is too low for livestock production.

Phase 2 or vegetative plants are at the optimum stage for plant growth and have high quality and dry matter production. They will have 6-8 leaves and be greater than 1000kg DM/ha. Ideally this is the best time to graze a pasture in a normal season and the earliest a paddock should be grazed following drought. Depending on the time of season and the pasture composition, this may take four to six weeks.

Figure2: Simplified growth curve of pasture (Source: The PROGRAZE Manual 8^th Edition)

Phase 3 plants Once the plant hits phase three it has become reproductive. It has a high level of DM, but quality is reduced.

Where possible following drought, it is good management to allow pastures to reach phase three prior to grazing to let them replenish root energy reserves and set seed. This is particularly important for phalaris pastures in spring.

Figure 2 shows the different growth phases of pasture plants.

How do I measure dry matter per hectare?

Find an area of the paddock that representative of the whole paddock
Using a 50cm by 50cm quadrant, cut herbage and place in a paper bag
The more samples you take the more accurate the result will be
Dry samples in an oven at 70-80 degrees (can take 12-48 hours) or using a microwave(always put a separate container with 100mls of water in the microwave with the sample to reduce the chance of fire)
Weigh samples
Use the formula below to calculate kg/DM/ha

DM(g) x 4 = g/square m
g/square m x 10000 = g/ha
g/ha / 1000 = kg/ha

For example: If my dried sample weighs 71g

71 x 4 = 284
284 x 10000 = 284000
2840000 / 1000 = 2840 kg/ha

Other factors to consider

Following a significant rainfall event, different pasture paddocks will respond differently, depending on the time of year, the composition of the paddock and how it has been managed through the drought period. Due to decreased ground cover, plants that are normally suppressed by the existing pasture may establish. This may include competitive weeds which will deplete the soil of moisture and nutrients required for your reestablishing pastures.

Additionally, some desirable pasture species can be toxic early in their growth phase.

For Example:

Legumes such as clover, medics and Lucerne can cause bloat especially when there is limited grass in the paddock
Rapid growth of some grasses, cereals, forage brassicas and broadleaf weeds such as variegated thistles can cause nitrate poisoning
Sorghum grazed early or grazing returned/volunteer grain sorghum can cause prussic acid poisoning
Rapidly growing phalaris can cause phalaris stagers

For more information on pasture management following drought, refer to the DPI Drought recovery guide or speak to your nearest Local Land Services Ag advisory staff member or district vet.