Broad leaf
Introduction
In the mixed grain/animal production farming belt, silage is not only of value to the animal enterprise as a supplementary feed and a pasture management tool, but can also provide significant benefits to the grain enterprise. These benefits include weed control during the pasture phase, and weed control and nitrogen fixation when annual forage legume break crops are used for silage production.
Weed control
Weed control can be a significant cost in the pastoral and cropping regions. Although the development of herbicide resistance in grass weeds, such as annual rye grass and wild oats, is not a major problem for the grazing industries, it is becoming a serious problem in cropping regions.
Broadleaf weeds can often be expensive to control in pastures if selective herbicides are needed to avoid damage to the legume component.
In the cropping areas of southern Australia, wild radish (Rhaphanus raphanistrum) is a major problem and farmers are looking to control measures being applied during the pasture phase on farms with crop and animal enterprises.
Strategic silage cutting, either alone or in combination with grazing, provides farmers with another weed control option, reducing the requirement for herbicides. Cutting pastures or annual forage legume crops in spring in southern Australia can significantly reduce seed production in annual weeds.
It is generally accepted that most viable seeds present in the cut forage will be sterilised during the ensiling process. However, most weed seeds will survive the hay-making process and can be spread around the farm wherever hay is fed.
Timing of the silage cut in spring is critical to significantly reduce weed seed production. The optimum time of cut will vary with the target weed and should be related to the stage of weed development. A strategic crash grazing of the regrowth may be required if there is any regrowth of the target weed.
Wld radish
Some annual forage legume crops suitable for silage production have the added bonus of competing effectively with weeds and suppressing their establishment and growth through autumn and winter.
For example, peas and vetch sown at high rates, preferably with a low cereal sowing rate, have been found to suppress annual ryegrass in studies at Wagga Wagga, NSW.
Soil acidification.
All producers should be aware of the possible long-term effects of the removal of agricultural products from a farm, whether it be grain, forage, meat, milk or wool, on soil acidity. Acidification rates vary between soil types and production systems, with greatest concern for declining pH being on naturally acid (low pH) soils under high production systems.
Soil tests should be used to monitor soil pH. Lime application may be required to counteract a decline in soil pH. If soil pH is allowed to fall below critical levels, production will suffer.
Table 1.7 shows indicative lime requirements for a number of silage parent crops. Note that acidification rates will be higher when the forage has a high legume component.
The majority of silage is fed back onto the farm (perhaps not on the same paddock), so the question arises as to whether this system is any more exploitative than one which removes the same quantity of forage by grazing.
For example, the acidifying effects of a silage cut may be less if the silage is fed back on that paddock. Longterm studies are required to investigate these issues of nutrient cycling, removal and transfer.
Rye grass
Nutrient cycling, removal and transfer
Large quantities of nutrients are removed when crops and pastures are harvested for silage (see Chapter 4, Table 4.2, and Chapter 5, Table 5.1).
To achieve a sustainable farming system, redistribution of nutrients must be taken into account when silage is fed to animals — the portion that is recycled via excreta and that which is exported off-farm in animals and animal products.
Most nutrients, including phosphorus and potassium, are available to plants through fertiliser inputs or the soil’s natural fertility. Nitrogen fixation by legumes makes nitrogen unique.
The cycling of nitrogen is highlighted in the following exercise, where high-quality legume silage is fed for beef or lamb production on a mixed livestock/crop farm.
In both systems, approximately 70% of the silage nitrogen is excreted by the animals in dung or urine, while the remaining 30% is retained in the animal and is exported off-farm when the animals are sold.
In the grazing situation the nitrogen is returned directly to the paddock, but the nitrogen in silage is transferred to the paddock where the silage is fed. By controlling the site of feeding, producers can decide where the nitrogen is returned. The transferred nitrogen may be used to
boost the fertility of pasture paddocks or those to be cropped. Nutrient redistribution by livestock complicates the issue and should be taken into account.
The simplified version of the nitrogen cycle in Figure 1.8 illustrates the effect of the options outlined on the previous page, using a mixed farming system as the example. Losses of nitrogen from the system, due to volatile losses or leaching down the soil profile, although important, are not included.
The main features of the cycling, transfer and loss of nitrogen from this mixed farming system are:
- The quantity of nitrogen fixed by legume pastures and forage crops (and remaining in soil) is generally considered to be approximately 20 kg of nitrogen for each tonne of total legume forage DM produced (grazed and ensiled).
- The nitrogen content of the legume cut for silage is approximately 3% of the DM (or 30 kg N/t silage DM). Therefore, for legume pastures or forage legume crops yielding silage cuts of 4.5 t and 7.5 t DM/ha, the quantity of nitrogen in the silage for each hectare cut would be about 135 and 225 kg, respectively.
If 30% of the silage nitrogen is exported off-farm in animal product, the nitrogen remaining on-farm, either recycled or transferred, would be approximately 95 and 158 kg nitrogen for each hectare of legume pasture or forage crop cut for silage, respectively.
- Feeding the high-quality silage on a stubble paddock to be cropped next season would not only provide the animals grazing poor-quality stubble with a high-quality, high-nitrogen supplement, but also transfer a significant quantity of nitrogen that could be utilised by the subsequent crop.
