A comparative measurement of in vitro digestibility and volatile fatty acids from Nui/white clover and Matua/white clover grass swards : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science in the University of Canterbury [Lincoln College]

Abstract

Pasture yield and In Vitro Digestibilities (IVD) were measured for Matua/white clover and Nui/white clover swards growing in Canterbury. Samples were collected at two weekly intervals over 10 different periods of regrowth from defoliation in May, until September. Each cultivar had two starting dates one week apart. The volatile fatty acids (VFA's) from the fermentations were separated for composition, and the total VFA produced was examined for linkage to IVD. The dry matter yield accumulation of herbage produced a difference of 247 kg/ha for Matua and 1494 kg/ha for Nui. Regrowth after defoliation to soil level showed Nui had recovered to 31.3% grass of the sward composition by week 2, but Matua in comparison had only reached 3.8%. Clover proportion from the Nui sward was higher than from the Matua sward throughout the collection period. The maximum difference was 29.2%, and this occurred 6 weeks after defoliation, when Matua clover% was 2.2% and Nui clover was 31.4%. The mean IVD values arising from different cultivars were Matua 39.5% and Nui 60.9%. The period of growth accumulation caused the mean IVD to drop from an initial 40.5% at week 2, t a minimum of 26.4% at week 6, and then rose continuously to 71.9% at week 20. The starting date also had a significant effect on the IVD. The mean IVD's were 51.8% for start date A, and 48.5% for start date B. The GLC separation of VFA's after in vitro fermentation of samples for 48 hours could detect differences of individual VFA composition, and the total VFA produced. The mean total VFA concentration over the 20 weeks were 212 mg/100ml for Matua, and 321 mg/100ml for Nui. Start date A produced 258 mg/100ml and start date B produced 275 mg/100ml. There was a minimum at 8 weeks from defoliation which rose to 378 mg/100ml at week 20. Interaction occurred between all three factors. The composition of Matua acetic acid was 63.8 mmol% at week 2 and had dropped to 57.7 mmol% by week 18. Nui was 63.4 mmol% and 59.55 respectively. Matua propionic acid was 24.1 mmol% at week 2 and rose to 29.3% at week 18. Nui was 24.8% and 28.0 mmol% respectively. The acetic/propionic acid ration rose to a maximum at week 6, and this was coincident with the lowest yield and lowest IVD. The start dates had different effects on the Matua and Nui cultivars. The total VFA's produced the following correlation coefficients to IVD : r² = 0.98 for Matua and r² = 0.87 for Nui. The change of VFA composition die to IVD was also evident.