The BOM seasonal Climate Outlook has predicted a wetter than normal period between July and September prompting the implementation of management practices to minimise the risk of pugging damage. Dairy farmers can see the pasture damage from pugging, however the impact to soil health is not as visible.
Pugging, depending on its severity can cause soil structural decline, which indirectly affects the living organisms in the soil and nutrient uptake by plants. Re-sowing a pasture is a cost associated with pugging events, however the hidden cost associated with soil structural decline is not quantified and therefore not foremost in a dairy farmer's mind.
What is happening in a soil that is pugged?
Soil consists of aggregates which are a combination of sand, silt and clay particles along with organic matter, water and air. When aggregates form together in the soil, there are spaces between the particles called macropores. Macropores provide an interconnected network in the soil which influences drainage, aeration and enables plant roots to penetrate the soil. Micropores are smaller pores found amongst the aggregates and store water for use by plants. When a soil is pugged these pore spaces are crushed, becoming smaller or disappearing altogether.
Pugging typically occurs when soil reaches saturation in winter and spring. The wetter the soil is the weaker its strength, increasing the chance of pugging occurring if grazed. Grazing animals can exert high forces (up to 185 kilopascals) onto the soil and cover a greater area of a paddock when compared to a tractor (81 kilopascals), which is why animals can do so much damage.
The above ground effects of pugging is obvious as pasture yield is affected by plant burial in the mud, crushing, bruising and a reduction in dry matter production. This can reduce pasture production by 20 – 80 per cent for four to eight months depending on the severity of the pugging. A reduction in pasture utilisation by 20 – 40 per cent and ryegrass tillering density by 39 – 54 per cent is also experienced. This can result in bare ground, which increases the likelihood of weed infestation and undesirable grasses in the spring and the following year.
The reduction in the number and size of macropores reduces the movement of air and water in the soil. A pugged soil drains slower which increases the potential for the soil to become waterlogged. Waterlogged soils will remain colder for longer, which slows pasture growth coming into spring. The wet soil creates an anaerobic environment which can lead to nitrogen loss through de-nitrification and the leaching of potassium, sulphur and nitrogen through the soil profile.
The anaerobic conditions will also result in a decline of favourable biological activity which is needed to assist in the restoration of soil structure. Reduction in soil pores can restrict root penetration, reducing the ability for the roots to access nutrients and water later in the season. Undesirable gases are also produced which further affects soil organisms.
Grazing management strategies
It can be difficult to completely prevent pugging, however it is possible to minimise the damage with grazing and stock management practices. The following are some grazing management strategies to consider to reduce the incidence of pugging occurring once the soil is saturated.
- If some paddocks are prone to pugging, aim to graze them before it gets too wet.
- Build a pasture cover going into winter with pre-grazing levels between 2,500 – 3,000 kilograms of dry matter per hectare (kg DM/ha). Longer grass helps cushion the soil which can reduce the severity of damage.
- Where feasible try to maintain a high post grazing level of pasture, leaving about 1,500 kg DM/ha (4-6 centimetres) after grazing. This encourages faster pasture regrowth and maintains pasture density.
- If you feed hay in the paddock, roll the bale out prior to putting the cows in the paddock. This will stop the cows following the tractor. Also try to feed along fence lines to minimise trampling.
- Use on-off grazing on your most vulnerable paddocks. This is where cows are taken off the pasture after two to four hours and put on feed pads, laneways, sacrifice paddocks, cow yard, hard stand-off areas and supplementary fed with hay or silage. Research in southwest Victoria has shown that cows can consume about 6 – 7 kg DM/cow or 77 per cent (pre-grazing cover of 2,100 kg DM/ha) to 8.5 – 10.5 kg DM/cow or 88 per cent (pre-grazing cover of 3,100 kg DM/ha) of the 12 hour allocated feedfed in the first two to four hours.
- Avoid allocating the cows a larger grazing area in an attempt to minimise the damage, while this might work in the short term, it has the effect of speeding up the rotation and, if done for too long, will soon result in running out of pasture to graze.
- Consider offering cows their morning and night feeds(2/3 and 1/3 of daily allocation respectively) separately. If offered in one allocation, cows will foul the night feed with poop and mud compared to entering a fresh feed.
- Be mindful when strip grazing that you allocate enough feed. Cows should not back graze paddocks too much if they have been allocated sufficient pasture. If it does occur, get the cows off the paddock.
Research has found that light pugging events have little impact to pasture growth rate and the soil structure will restore through natural processes usually over the summer/autumn period. However, if pugging is re-occurring on the same paddocks each year, they are more likely to become infested with weeds and will need to be re-sown on a regular basis.
These paddocks are more prone to future pugging events due to impeded drainage. If a large proportion of your property is prone to annual pugging events, consideration should be given to constructing a dedicated feeding area to reduce this hidden cost of soil structural decline. Thought might also be given to physically improving paddock drainage, but this is a whole other topic but worth pursuing.