Statistics for Early weaning of twin-born lambs onto a herb-clover mix containing plantain (Plantago lanceolata), chicory (Cichorium intybus), red clover (Trifolium pratense) and white clover (Trifolium repens) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Palmerston North, New Zealand