The international unit of work and energy is the joule. One joule is the work done by a force of 1 Newton exerted through a distance of 1m. One Newton is the force that will give a mass of 1kg and an acceleration of 1m/sec2.
1kcal = 4.184 kJ or 1 kJ = 0.239 kcal
Previously energy content of feeds was expressed as TDN (total digestible nutrients), whereas ME (metabolisable energy) is the modern unit used.
To convert TDN to MJ of ME, multiply the TDN value by 0.15%.