Results and Discussion
Experiment 1
Data of fatty acid flows, biohydrogenation, and bacteria number are summarized in Table 1.
| Treatment1 | |||||
| Control | HSDR | HLA | LPH | SE | |
| Total FA, mg/day | 3356a | 2807ab | 3318a | 2394b | 196 |
| C16:0, mg/day | 544 | 424 | 332 | 414 | 66 |
| C18:0, mg/day | 1157b | 1268ab | 1628a | 401c | 119 |
| Trans-C18:1, mg/day | 387a | 144b | 152b | 352.0a | 33 |
| Cis-C18:1, mg/day | 329b | 241b | 257b | 407a | 45 |
| C18:2, mg/day | 142b | 111b | 156b | 282a | 36 |
| C18:3, mg/day | 14.8 | 12.9 | 12.0 | 9.5 | 1.4 |
| CLA, mg/day | 9.4c | 10.9bc | 14.5a | 12.5ab | 0.8 |
| Biohydrogenation, % | 61.1b | 72.8ab | 81.1a | 30.4c | 3.8 |
| Log bacteria number | |||||
| Total | 9.85 b | 9.22c | 10.39 a | 9.27c | 0.09 |
| Cellulolytic | 8.53 b | 8.09c | 9.63 a | 8.14c | 0.10 |
| 1 Control = pH 6.5, 1% linoleic acid
(LA), and 4%/hour solid dilution rate (SDR); HSDR = high SDR
(8%/hour), pH 6.5, and 1% LA; HLA = high LA (3%), pH 6.5, and 4%
SDR; and LPH = low pH (5.8), 1% LA, and 4% SDR.
abc Means in the same row with different letters differ (P < 0.05). |
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Effects of pH. The LPH resulted in a relatively higher flow of cis-9, trans-11-18:2, accompanied with higher flows of trans18:1, cis-18:1, and 18:2, lower flow of 18:0, lower BH, and lower total and cellulolytic bacteria number compared to control. These data indicate that reduced ruminal pH may increase the flow of CLA to duodenum in a cow by inhibiting ruminal BH. Although it is known that factors altering rumen fermentation and the microbial population are undoubtedly keys to controlling the regulation of CLA synthesis (McGuire et al., 1997), very few studies have associated CLA production with pH. Some studies investigated the effect of forage to concentrate ratio, but no detailed information on ruminal pH was given and the results were not consistent (Chouinard et al., 1998; Jiang et al., 1996).
Effects of SDR. We had expected that a shorter solid retention time would lead to an incomplete BH, and thus HSDR should result in a higher content of CLA in the outflow than control. The actual results revealed that HSDR only tended to increase CLA content in the outflow (P = 0.10) and in total FA (P = 0.06) (data not shown). The total flow of CLA, however, was not significantly increased by HSDR. This may have occurred because of two reasons. First, the design of the fermentor can not ensure a very accurate SDR because the filtrate may also contain some small feed particles, which might compromise the difference between fermentors with high and low SDR. Second, all fermentors had the same DM intakes. The HSDR treatment could not result in a much higher DM outflow than others could and thus this reduced the possibility of a higher CLA flow. We speculate that, in a cow, a higher solid passage rate would usually result in a higher DM intake, which in turn, may cause a higher duodenal flow of DM, as well as CLA.
Effects of level of LA. With more LA available as a substrate for CLA production, HLA resulted in the highest flow of CLA among treatments. Interestingly, HLA also resulted in the highest BH and bacteria number. These data indicate that continuous infusion of LA into the fermentors may have prevented any adverse effect on bacteria. This study further confirms the information from some previous studies that feeding oil or oilseeds for providing LA to cows can be an effective way to increase CLA content of milk fat (Dhiman, 2000; Palmquist, 2000).