Sweet potato silage as pig feed

By MICHAEL DOM
ALTHOUGH sweet potato is the predominant livestock feed grown and used by village far­mers in PNG, the tubers, vine and leaf cannot be stored for extended periods.
Sweet potato tuber would begin to rot after two weeks, while the vine would decay more rapidly. Therefore, soon after harvest, the nutritional value of the food/feed is lost.
Sweet potato is continuously grown for household food and for livestock, particularly pigs.
This pattern of subsistence agriculture is well known in PNG villages, especially where sweet potato-pig based farming system is common.
Preservation and storage of excess tubers and vines for extended use long after the harvest season affords an added value to garden forage.
NARI is assisting farmers develop sweet potato silage for pig feed, a technology which was officially released to the farming community last May.
Sweet potato silage is a simple feed preservation technology that enables the tuber and vine forage to be stored for up to six months after harvesting. It is developed using sweet potato forage for the garden or sourced from local markets using locally available tools and materials.
The process, known as ensiling, can be applied to any sweet potato. 
The basic requirements for proper ensiling of sweet potato are harvesting forage at the right stage of maturity, chopping tuber and vine into small pieces using hand knives and kitchen graters or a mechanical chopping machine, mixing in salt additive at 0.5% of the total mixture weight, rapid packing of the mixture into airtight bags and ensuring the materials are firmly compacted and sealing the bags within a silo inside a bucket or bin that provides a pest-proof and air-tight storage environment.
It is also useful to understand certain things about sweet potato silage, including:
*Sweet potato forage contains mainly carbohydrates and very little protein;
*Ensiling of sweet potato involves fermentation of the carbohydrates;
*Naturally present micro-organisms are involved in the fermentation process;
*Silage stores much longer than fresh sweet potato material. There are specific fermentation parameters that need to be achieved in order for the silage to store well through the correct use of the adapted ensiling technique; and
*Sweet potato silage is mixed with protein containing feeds such as grower or fish meal and copra meal to provide nutritious rations that are effective in growing crossbred pigs to finish weights within four months.
Silage is the same nutritious starch energy feed when fed fresh.
The difference is its preparation and ability to be stored for a longer period.
Most village and smallscale farmers cook sweet potato or cassava tubers and may offer fresh sweet potato vine as part of the daily pig ration. But, cooking requires fuel, usually wood, and that means the added tasks of gathering and chopping or buying firewood.
In some parts of the country, the need for firewood has become a severe problem to cooking household meals, let alone preparing pig feed.
There are socio-economic advantages to be gained by using ensiling technology in PNG.
The ensiling sweet potato technology can help to relieve the added burden of cultivating feed crop for animals and help farmers to capitalise from lush harvests of forage when processed into silage. Using sweet potato stored as silage enables smallscale farmers to provide more consistent feeding to growing pigs, even during the lean seasons.
Growing sweet potato crops specifically for ensiling allows greater control over production cycles and, therefore, improved ability to plan and allocate garden work times and costs with other activities.
This can be an important advantage in village scenarios, where gardening is a daily chore. In particular, it is vital to free women in PNG farming communities from the drudgery of subsistence agriculture.
On the other hand, more progressive farmers with larger pig herds and readily available land may organise family members or other farm workers to cater to their needs during sweet potato cropping and for silage production. Land and labour availability remain the constraints in getting maximum sweet potato production. But, the ensiling technology enables improved productivity from what is available at farm level. This allows farmers to plan their production to achieve required crop for their own consumption as well as for their livestock.
Labour, material and other operating costs have been estimated for silage technology.
A model budget for a smallscale pig farm, raising four grower pigs by feeding them sweet potato silage, showed that the gross margins and benefit-cost ratios were advantageous at a simplified smallscale operation.
While further socio-economic evaluations of this mode of ensiling sweet potato feed for pig production are required, the overall outlook for smallholder farming systems using sweet potato silage for growing pigs in PNG is positive.
For more information on the livestock feed systems, contact the livestock research and development project leader on telephone (675) 475 1066 or email [email protected]