Crop Residues

Conversion of crop residues

By their content crop residues can compensate for about only half the nutrients that are required for the next cropping. However significant losses occur during decomposition. Losses are even higher if residues are converted by animals into manure and again higher if manure is deposited in a kraal or stored for a longer period of time. Depending on climatic and storage conditions, many nutrients can get lost: N through volatization (NH3) and leaching (NO3); K through leaching; organic matter through decomposition (CO2, CH4). Only P losses are low. Consequently the percentage of plant nutrients and organic matter that remain at the time manure is brought to the fields is much lower than the quantities at the start.

In general the feed quality of crop residues is just sufficient to allow a feed intake that covers maintenance requirements. As a result the export of nutrients through animal products will be very low.

Below are presented some general estimates of the percentage of plant nutrients from crop residues again available for crop cultivation after conversion by livestock and storage at the farm:

N + 25 % (50% of intake excreted in urine nearly all lost, 50 % in faeces half of it lost)

K + 25 % (mainly in urine, ¾ lost through leaching, very small % exported with products)

P + 90 % (in faeces, 10 % transport and storage losses, small % exported with products)

Organic Matter + 25 % (40- 50 %) decomposed by animals, faeces half of it decomposed during storage)

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