THE EFFECT OF STAGE OF HARVEST ON THE PROCESSES OF DIGESTION IN CATTLE FED WINTER OATS INDOORS .1. DIGESTION OF ORGANIC-MATTER, NEUTRAL DETERGENT FIBER AND WATER-SOLUBLE CARBOHYDRATES

The objective of this trial was to study the digestion process of organic matter (OM), neutral detergent fibre (NDF) and water-soluble carbohydrates (WSC) of winter oats (Avena sativa L.) of different stages of harvest fed indoors. Treatments and dates of harvest were: A (25 May, autumn), EW (25 June, early winter), W (9 August, winter), ES (22 September, early spring), LS (20 October, late spring). Two Holstein heifers weighing 567.6 +/- 17.8 kg were used, each fitted with cannulas in the rumen, proximal duodenum and distal ileum. Chromic oxide and Co-EDTA were used as external markers. A completely randomized design was arranged considering dates of harvest as treatment effect. OM intakes (g kg-1 live weight (LW)) were higher (P<0.03) in A (16.5) than in the other treatments. Apparent OM digestibilities (g g-1) ranged (P<0.002) from 0.63 (A) to 0.76 (LS). The fractions of digestible OM intake (DOMI) apparently digested (OMADR) and truly digested (OMTDR) in the rumen were not influenced (P>0.05) by treatments (mean 0.66 g g-1 and 0.87 g g-1 DOMI for OMADR and OMTDR, respectively). A high correlation was found between DOMI and OMTDR (OMTDR (g)=-287.2+0.925DOMI (g); r=0.89, P<0.001). DOMI digested in the small intestine was not different (P>0.05) between dates (mean 0.27 g g-1). DOMI digested in the large intestine (g g-1) was lower (P<0.04) in ES and LS (mean 0.06) than for other dates. The NDF intakes (g kg-1 LW) ranged between 6.6 and 9.5, being higher (P< 0.002) in A. Apparent OM digestibility was lower (P<0.01) in LS (0.51 g g-1). The digestible NDF intake digested in the rumen and small and large intestines was not different between dates. The WSC intake (WSCI) and apparent digestibility were higher (p< 1 X 10(-7)) in ES (4.13 g kg-1 LW, 0.99 g g-1) than in A (0.86 and 0.87, respectively). Nearly 98% of the digestible WSCI was apparently digested in the rumen. A correlation between pH and digestible WSC intake (DWSCI) was found (pH = 6.51 - 2.16 x 10(-4) DWSCI, r = 0.65, P < 0.04). The acetic: propionic ratio was lower (P < 7 X 10(-4)) in ES (3.2) and highest in A (4.42). In fresh winter oats, OMTDR was high and the differences between dates were important only for WSC content and these could have affected pH and the acetic: propionic ratio.