Results and Discussion
Experiment 2
Dry matter intake, milk yield, and milk components. Cows fed LDMI had significantly lower DM intake but higher MUN than the cows on other three treatments (Table 2). However, milk yield, 4% FCM yield, milk fat and protein percentages, and daily milk fat and protein yields were similar among treatments. This happened because CP and energy concentrations for LDMI were designed to be higher than the diets for other treatments so that the daily CP and energy intakes would not be lower for cows with restricted intake. The MUN was higher for this treatment possibly because the reduced DM intake resulted in a longer retention time, consequently leading to a more extensive ruminal digestion of protein. It is well known that feeding high levels of unsaturated fat of marine origin will generally compromise animal performance and cause milk fat depression (Cant et al., 1997; Pennington and Davis, 1995). In this study, while other indexes are difficult to evaluate, obvious milk fat depression was observed. Milk fat percentage was 2.11, 2.18, 2.26, and 2.88 for control, BUFF, LDMI, and SBO, respectively, much lower than the average of the University's herd located in Columbus (3.67%).
| Treatments1 | |||||
| Control | BUFF | LDMI | SBO | SE | |
| DM intake, lb/day | 40.7a | 38.3a | 33.0b | 38.9a | 1.3 |
| Milk yield, lb/day | 70.6 | 67.1 | 70.8 | 70.4 | 4.6 |
| 4% FCM2, lb/day | 49.9 | 48.4 | 51.5 | 57.9 | 5.1 |
| Milk fat, % | 2.11 | 2.18 | 2.26 | 2.88 | 0.21 |
| Milk fat, g/day | 659 | 653 | 702 | 898 | 105 |
| Milk protein, % | 3.12 | 3.15 | 3.11 | 3.14 | 0.04 |
| Milk protein, g/day | 1005 | 961 | 995 | 998 | 53 |
| MUN3, mg/dl | 13.7 b | 13.2 b | 19.7 a | 13.2 b | 0.6 |
| SCC4, (cells/ml) x 1000 | 68.1 | 61.9 | 194.9 | 155.4 | 70.6 |
|
1BUFF = buffer addition, LDMI = low DM intake (80% of
ad libitum intake), and SBO = soybean oil (2% of DM ). Control,
BUFF, and LDMI contained 2.0, 2.0, and 2.5% fish oil,
respectively.
2FCM = fat-corrected milk. 3MUN = milk urea nitrogen. 4SCC = somatic cell count. ab Means in the same row with different letters differ (P < 0.05). |
|||||
Duodenal FA flow and milk FA composition. Generally, control and BUFF were similar in duodenal flow of total FA and all individual FA (Table 3). The LDMI resulted in lower duodenal flow of C16:1 and trans-C18:1 than control. Feeding SBO instead of fish oil resulted in the highest flow of stearic acid (C18:0) but the lowest flows of C14:0, C16:0, C16:1, and trans-C18:1. The LDMI treatment resulted in similar CLA flows compared to control. The SBO treatment resulted in the lowest duodenal flow of CLA. Kepler et al. (1966) and Noble et al. (1974) found that accumulation of VA in the rumen seems to occur most consistently when the concentration of free 18:2 is high in ruminal contents. However, in Experiment 1 of this study, continuous addition of 3% LA to a continuous culture system decreased rather than increased LA and trans-C18:1 flows as compared to control. Similarly, in Experiment 2, most of the linoleic and linolenic acids were hydrogenated into stearic acid and not much trans-C18:1 and CLA accumulated in the rumen when the cows were fed soybean oil. This also was supported by the BH data (Table 3). The calculated BH was much higher for SBO than the other three treatments, especially because of the high concentration of C18:2 in SBO compared to fish oil.
Table 3. Effect of buffer addition, DM intake, and fat source on duodenal flow of fatty acids (FA) in Experiment 2.
| Treatments1 | |||||
| Control | BUFF | LDMI | SBO | SE | |
| Total FA, g/day | 627 | 557 | 462 | 508 | 43 |
| C14:0, g/day | 19.6a | 16.7a | 16.8a | 6.60b | 1.96 |
| C16:0, g/day | 164a | 144a | 122ab | 84.9b | 12.5 |
| C16:1, g/day | 13.6a | 14.3a | 10.3b | 1.35c | 0.79 |
| C18:0, g/day | 89b | 78b | 101b | 263a | 18 |
| Trans-C18:1, g/day | 148a | 142ab | 76bc | 28c | 20 |
| Cis-C18:1, g/day | 0.33 | 0.38 | 0.25 | 0.16 | 0.08 |
| C18:2 n6, g/day | 38.7 | 31.4 | 28.8 | 37.6 | 3.84 |
| C18:3 n3, g/day | 3.45 | 2.85 | 3.90 | 1.43 | 1.15 |
| CLA2 , mg/day | 1060ab | 1482a | 829b | 304c | 143 |
| BH3 ,% | 49.0b | 51.1b | 57.1b | 81.7a | 3.5 |
|
1BUFF = buffer addition, LDMI = low DM intake (80% of
ad libitum intake), and SBO = soybean oil (2% of DM ). Control,
BUFF, and LDMI contained 2.0, 2.0, and 2.5% fish oil,
respectively.
2CLA = conjugated linoleic acid. 3BH = biohydrogenation. abc Means in the same row with different letters differ (P < 0.05) |
|||||
There were no treatment effects (P > 0.10) on milk concentrations of FA with chain length of twelve or shorter (Table 4), indicating a similar extent of de novo syntheses of these FA among treatments. Feeding SBO resulted in higher C18:0, trans-C18:1, cis-C18:1, and C18:2 concentrations in milk fat, but there were no differences in these FA among the three treatments feeding fish oil. This may have occurred because of the different FA profile between fish oil and soybean oil and their different impacts on ruminal BH. The CLA concentrations in milk fat were 24.5, 17.9, 18.5, and 10.1 mg/g of FA for control, BUFF, LDMI, and SBO, respectively (Table 4). The SBO treatment resulted in the lowest CLA concentration in milk fat. There were no differences in milk CLA concentrations among control, BUFF, and LDMI.
| Treatments1 | |||||
| Control | BUFF | LDMI | SBO | SE | |
| ------- mg/g of FA ------- | |||||
C4:0 |
5.61 | 5.18 | 6.16 | 6.38 | 0.76 |
| C6:0 | 10.4 | 8.7 | 14.0 | 12.8 | 1.7 |
| C8:0 | 11.5 | 10.7 | 13.4 | 12.1 | 1.3 |
| C10:0 | 28.7 | 26.3 | 32.6 | 29.0 | 3.0 |
| C12:0 | 37.8 | 36.6 | 40.4 | 35.9 | 3.2 |
| C14:0 | 129 | 124 | 129 | 115 | 4 |
| C16:0 | 354 | 326 | 335 | 290 | 14 |
| C16:1 | 41.9 | 44.6 | 33.8 | 19.6 | 6.1 |
| C18:0 | 38.6b | 43.4b | 53.4b | 107.0a | 12.2 |
| Trans-C18:1 | 95.6a | 99.5a | 70.7ab | 35.8b | 15.5 |
| Cis-C18:1 | 136b | 152b | 163b | 242a | 16.3 |
| C18:2 n6 | 28.8b | 30.6b | 27.8b | 39.6a | 2.4 |
| C18:3 n3 | 3.72 | 3.91 | 3.64 | 4.36 | 0.42 |
| CLA2 | 24.5a | 17.9a | 18.5a | 10.1b | 2.4 |
|
1 BUFF = buffer addition, LDMI = low DM intake (80%
of ad libitum intake), and SBO = soybean oil (2% of DM ).
Control, BUFF, and LDMI contained 2.0, 2.0, and 2.5% fish oil,
respectively.
2 CLA = conjugated linoleic acid. abc Means in the same row with different letters differ (P < 0.05). |
|||||
Large variations in milk CLA concentrations were found among individual cows with the lowest concentration at 3.2 and the highest concentration at 31.6 mg/g of FA. Substantial variation in CLA concentration of milk fat was also observed by Jiang et al. (1996) and Kelly et al. (1998).
In this study, duodenal flow of CLA and CLA concentration in milk fat were lower for SBO than control indicating that fish oil is more effective than soybean oil in increasing CLA production in dairy cows. It is well recognized that CLA can be endogenously synthesized from VA in the mammary gland by the activity of D9-desaturase (Griinari et al., 2000). However, it is not quite clear to the extent that the endogenous synthesis contributes to the total CLA in milk. We estimated the daily milk CLA secreted by using CLA concentration in milk FA times fat yield times 0.9, assuming that FA account for 90% of the milk fat. Thus, milk CLA yields were 15.5, 9.8, 6.9, and 6.7 g/day for control, BUFF, LDMI, and SBO respectively. Compared to the duodenal flows of CLA (1.1, 1.5, 0.8, and 0.3 g/day, respectively), it is obvious that most of the CLA in milk was endogenously synthesized from VA.