Milk analysis is receiving increased attention. Milk contains conjugated octadecadienoic acids (18:2) purported to be anticarcinogenic, low levels of essential fatty acids, and trans fatty acids that increase when essential fatty acids are increased in dairy rations. Milk and rumen fatty acid methyl esters (FAME) were prepared using several acid- (HCl, BF3, acetyl chloride, H2SO4) or base-catalysts (NaOCH3, tetramethylguanidine, diazomethane), or combinations thereof. All acid-catalyzed procedures resulted in decreased cis/trans (delta 9c,11t-18:2) and increased trans/trans (delta 9t,11t-18:2) conjugated dienes and the production of allylic methoxy artifacts. The methoxy artifacts were identified by gas-liquid chromatography (Gl.C)-mass spectroscopy. The base-catalyzed procedures gave no isomerization of conjugated dienes and no methoxy artifacts, but they did not transesterify N-acyl lipids such as sphingomyelin, and NaOCH3 did not methylate free fatty acids. In addition, reaction with tetramethylguanidine coextracted material with hexane that interfered with the determination of the short-chain FAME by GLC. Acid-catalyzed methylation resulted in the loss of about 12% total conjugated dienes, 42% recovery of the delta 9c,11t-18:2 isomer, a fourfold increase in delta 9t,11t-18:2, and the formation of methoxy artifacts, compared with the base-catalyzed reactions. Total milk FAME showed significant infrared (IR) absorption due to conjugated dienes at 985 and 948 cm-1. The IR determination of total trans content of milk FAME was not fully satisfactory because the 966 cm-1 trans band overlapped with the conjugated diene bands. IR accuracy was limited by the fact that the absorptivity of methyl elaidate, used as calibration standard, was different from those of the other minor trans fatty acids (e.g., dienes) found in milk. In addition, acid-catalyzed reactions produced interfering material that absorbed extensively in the trans IR region. No single method or combination of methods could adequately prepare FAME from all lipid classes in milk or rumen lipids, and not affect the conjugated dienes. The best compromise for milk fatty acids was obtained with NaOCH3 followed by HCl or BF3, or diazomethane followed by NaOCH3, being aware that sphingomyelins are ignored. For rumen samples, the best method was diazomethane followed by NaOCH3.

We measured effects of continuous vs twice-daily feeding, the addition of unsaturated fat to the diet, and monensin on milk production, milk composition, feed intake, and CO2-methane production in four experiments in a herd of 88 to 109 milking Holsteins. Methane and CO2 production increased with twice-daily feeding, but the CO2:CH4 ratio remained unchanged. Soybean oil did not affect the milkfat percentages, but fatty acid composition was changed. All saturated fatty acids up to and including 16:0 decreased (P < .01), whereas 18:0 and trans 18:1 increased (P < .001). The 18:2 conjugated dienes also increased (P < .01) when the cows were fed soybean oil. Monensin addition to the diet at 24 ppm decreased methane production (P < .01); the CO2:CH4 ratios reached 15, milk production increased (P < .01), and milkfat percentage and total milkfat output decreased (P < .01), as did feed consumption, compared with cows fed diets without monensin (P < .05). Milk fatty acid composition showed evidence of depressed ruminal biohydrogenation: saturated fatty acids (P < .05) decreased and 18:1 increased (P < .001); most of the increase was seen in the trans 18:1 isomer. As with soybean oil feeding, addition of monensin also increased (P < .05) the concentration of conjugated dienes. The monensin feeding trial was repeated 161 d later with 88 cows, of which 67 received monensin in the diet in the first trial and 21 cows were newly freshened and had never received monensin. Methane production again decreased (P < .05), but this time the CO2:CH4 ratio did not change and all other monensin-related effects were absent. The ruminal microflora in the cows that had previously received monensin seemed to have undergone some adaptive changes and no longer responded as before.

Methane and CO2 emissions from a herd of 118 lactating cows were measured directly by continuous monitoring with an infrared gas analyzer from 24 gas sampling locations. A total of 112 d of gas output was recorded between June 1993 and November 1993. Recordings were integrated at .5-h intervals, so that there were 48 data points for each 24-h period. The mean 24-h CH4 emission per cow was 587 +/- 61.3 L; the range was 436 to 721 L. The mean 24-h CO2 emission per cow was 6137 +/- 505 L, and the range was 5032 to 7427 L. These values were not corrected for gas emissions from stored manure, which contributed 5.8 and 6.1%, respectively, to CH4 and CO2 output under conditions of this experiment.

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Four ionophores differing in cation selectivity were compared for their effect on microbial fermentation and biohydrogenation by ruminal bacteria in continuous culture. Monensin and nigericin are monovalent antiporters with selective binding affinities for Na+ and K+, respectively. Tetronasin is a divalent antiporter that binds preferentially with Ca2+ or Mg2+. Valinomycin is a monovalent uniporter and does not exchange K+ for H+. Steady-state concentrations of 2 micrograms/ml of monensin, nigericin, tetronasin, or valinomycin were maintained by constant infusion into fermenters. Molar percentages of acetate were lower, and those of propionate were higher, in the presence of monensin, nigericin, and tetronasin; all three ionophores also decreased CH4 production. Concentrations of valinomycin as high as 8 micrograms/ml had no effect on volatile fatty acids or CH4 production. Monensin, nigericin, and tetronasin inhibited the rate of biohydrogenation of linoleic acid. Continuous infusion of C18:2n-6 at a steady-state concentration of 314 micrograms/ml into fermenters receiving monensin, nigericin, or tetronasin resulted in lower amounts of stearic acid and higher amounts of oleic acid. Ionophores increased total C18:2 conjugated acids mainly because of an increase in the cis-9, trans-11-C18:2 isomer. If reflected in milk fat, ionophore-induced changes in ruminal lipids could enhance the nutritional qualities of milk.