A comparison of gas production during incubation with rumen contents in vitro and nylon bag degradability as predictors of the apparent digestibility in vivo and the voluntary intake of hays

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Abstract
Ten hays harvested at three stages (early bloom MB, mid bloom MB or in seed) made from lucerne (Medicago sativaj, sweet clover (Melilotus segetalis), Persian clover (Trifolium resupinatum) and pre-bloom (PB) Italian ryegrass (Lolium multiflorum var.), were offered ad libitum to four Merino male sheep and daily intake (g dry matter (DM) per kg M0·75) and DM apparent digestibility (DMD) were measured. In sacco DM degradation (g per 100 g DM), gas production (ml per 200 mg DM), in vitro digestibility and fibre composition (g/kg DM) of the hays were also studied. Gas production or DM degradation were calculated at 6, 12, 24, 48, 72 or 96 h and their kinetics were described using the equation p = a + b(1 - e-ct). Intake and in vivo DMD of the hays were variable (P < 0·01). Lucerne EB and Persian clover (all stages) had the highest nutritive value, whereas sweet clover (all stages) had the lowest. Apart from neutral-detergent fibre, which was only related to intake (r = -0·68; P < 0·05), chemical components and in vitro digestibility were poorly (P > 0·05) related to animal performance. Between 12 and 96 h incubation, intake and in vivo DMD were better related to DM degradation (r = 0·79 to 0·83; r = 0·61 to 0·77) than to gas production (r = 0·73 to 0·80; r = 0·58 to 0·78). Prediction of intake and in vivo DMD from the (a + b) values did not provide a great advantage over using some of the static values of gas production or DM degradation.Multiple regression using separated kinetics of degradation resulted in highest accuracy for predicting intake and apparent digestibility from gas production (R2 = 0·63; R2 = 0·78) and nylon bag degradability (R2 = 0·77; R2 = 0·89). It was concluded that the gas test has good potentiality as it was capable of predicting not only apparent digestibility, but also intake to a level close to that of the nylon bag technique.

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