Abstract :
It was hypothesized that differences in starch degradability account for observed differences in rumen vaccenic acid (t11-18:1)
and milk rumenic acid (RA) concentrations. To test this hypothesis, starch degradability was varied through grain source and by
processing. Eight Holstein cows in mid-lactation were assigned to two 434 Latin squares with four 21-day periods and four diets:
dry rolled barley, ground barley, dry rolled corn and ground corn. Diets contained similar starch content and were supplemented
with whole sunflower seed to provide similar total polyunsaturated fatty acid (PUFA) (18:2n-6118:3n-3) contents. Forage/
concentrate ratios of all diets were 42 : 58. Rumen, plasma and milk samples were collected in the third week of each period.
In situ degradation rates (%/h) for rolled corn, ground corn, rolled barley and ground barley were 5.4, 8.9, 17.0 and 19.4,
respectively, for dry matter (DM) and 6.3, 10.8, 25.3 and 43.8, respectively, for starch. DM intakes were greater for corn-based
diets (CBD) than for barley-based diets (BBD) with no difference between rolled and ground diets. Daily minimum rumen pH was
less (5.2 v. 5.5) and pH duration ,5.8 (h/d) was greater (7.4 v. 4.3) for BBD than for CBD. Milk fat content and yield were less for
BBD than for CBD with greater values observed for rolling compared with grinding. Variability in milk fat yield was strongly related
(R250.55; P,0.01) to total starch intake (45%) and milk c9t11-CLA (10%) and none of the t-18:1 isomers or CLA isomers that
are typically associated with milk fat depression entered the model. The concentrations (%) of t10-18:1 and t11-18:1 were greater
for BBD than for CBD in rumen contents (t10-18:1, 3.5 v. 1.3; t11-18:1, 3.2 v. 1.9), plasma (t10-18:1, 1.2 v. 0.2; t11-18:1, 0.97 v.
0.58) and milk (t10-18:1, 3.8 v. 1.0; t11-18:1, 2.6 v. 1.7) despite greater total PUFA intakes for CBD. Milk RA concentration was
greater for BBD than for CBD (1.46 v. 0.89) but was not influenced by the method of grain processing. This study clearly
demonstrated that the milk content and profile of t-18:1 and CLA isomers were more strongly influenced by the source of grain
starch (barley . corn) than by the method of grain processing indicating that factors inherent in the source of starch were
responsible for the observed differences and these factors could not be modified by the processing methods used in this study.
Keywords :
grain processing , Conjugated linoleic acid , corn , barley , vaccenic acid
