Absorption of acetic, propionic and butyric acids is known to occur from the rumen of sheep but some confusion concerning relative rates of absorption of these acids has arisen, for although it is agreed that free acids are absorbed more rapidly than their sodium salts when aqueous solutions are in the rumen (Danielli, Hitchcock, Marshall & Phillipson, 1945;Gray, 1947) it has been claimed recently that no absorption at all occurs from the rumen when the pH is over 7 (Gray, 1948).Under normal feeding conditions the pH of the contents of the rumen rarely falls below 5'5; for this reason the quantity of free acid present is small compared to the quantity of anion. Absorption, therefore, is likely to be insignificant if the membrane lining the rumen is permeable only to free acid. Further investigations, which are reported in this paper, were undertaken to compare the mixture of acids present in the rumen with the mixture present in the blood leaving the rumen, for this should show whether the mixture of acids present in the rumen is indicative of the mixture passing to the liver from this organ.The majority of these comparisons were made while two of us (R. A. M. and A. T. P.) were working in the Unit of Animal Physiology, Cambridge, whilst confirmation of these results using a different method of analysis was obtained at the Rowett Research Institute. METHODSExperimental. Normal ewes were taken straight from pasture to the laboratory. They were anaesthetized by intravenous injection of nembutal, the abdomen was opened on the right side and the posterior vein of the rumen was located as previously described (Barcroft, McAnally & Phillipson, 1944). In addition, the right carotid artery in the neck was exposed. The experimental procedure was usually as follows.After exposing the posterior vein of the rumen 5 ml. of blood were withdrawn from the carotid

An electronic probe with parallel stainless steel electrodes was designed to measure changes in electrical resistance in the anterior vagina associated with increased cervical mucus secretion at estrus. Three groups of Holsteins were probed, and a sample of residual milk after milking was taken for progesterone assay every 2nd day for 28 to 40 days. Group I consisted of 29 cycling virgin heifers, Group II consisted of 24 lactating cows in the Cornell herd, and Group III consisted of 62 lactating cows in three commercial herds. Lactating cows were 26 or more days postpartum when sampling began. In Group I, 77% of the measurements at the ventral surface of the anterior vagina were lowest on the day of estrus and correlated well with days KaMaR Heatmount Detectors were triggered or chalk was erased. In Groups II and III, average electrical resistance also was minimal at estrus and was correlated .92 to .99 with average milk progesterone during the 4 days preceding and including estrus. The electronic probe appears to enable one to detect which cows are cycling normally and to aid in determining when to expect estrus.

A new vaginal probe with two pairs of parallel electrodes 90 degrees apart was designed with a switching arrangement so that electrical resistance (ohms) of the dorsal and the ventral areas of the anterior vagina could be monitored separately during the estrous cycle. Three inseminators probed cows during February--April, 1977, in nine herds in stanchion barn housing. Cows not seen in estrus by 40 to 60 dyas after calving were probed every 3rd day. At each dairyman's option normal cows with a low porbe reading were inseminated. Controls consisted of cows seen in estrus by the dairyman and reported for insemination in the routine manner. The diagnosed pregnancy rate for 86 first service controls was 49% versus 52% for 58 cows not seen in estrus, but identified by low probe readings. Cows varied in electrical resistance at breeding, but the fact that pregnancy rates were equivalent in the two groups is interpreted to mean that the probe was as effective in identifying cows suitable for insemination as was visual observation of estrus by dairymen.

The following conclusions were reached by placing a solution of sodium acetate, propionate and buty-rate in the rumen of sheep and studying the rate at which these substances are absorbed: (1) When the rumen contents are at pH 7.5, fatty acid anion only is absorbed, i.e. no free fatty acid is absorbed, and the anion is accompanied by a roughly equivalent amount of sodium. The anion is absorbed by passage through water-filled pores, the diameter of which is large compared with retric acid. These pores probably lie in the intercellular cement of the rumen epithelium. (2) When the rumen contents are at pH 5.8, free acid is lost from the rumen in large amounts in addition to the loss of fatty acid anion. Part of this free acid is absorbed by passage through water-filled pores, but the greater part of it passes through the lipoid membranes of the epithelial cells. (3) In consequence of the difference in the two mechanisms, at alkaline pH the substances are lost in the order acetate > propionate > butyrate, whereas at acid pH the order is butyrate > propionate > acetate. (4) All the substances studied (fatty acid anions, sodium and water) move from regions of higher to regions of lower concentration. (5) The total fatty acid absorbed from the rumen of a sheep on pasture grass may readily be twice that absorbed from the rumen by the same sheep on hay. (6) The permeability of the rumen epithelium is such that the pH of the ingesta tends to move towards neutrality, independently of the neutralizing action of the saliva.

A mixture of volatile organic acids, consisting for the most part of the fatty acids, acetic, propionic and butyric, is a characteristic product offermentation within the alimentary tract. It was shown previously that volatile acids are absorbed directly from the organ in which they are produced (Barcroft, McAnally & Phillipson, 1944), and this fact, together with a consideration of the quantities present in the digesta—a subject with which the present paper is concerned—suggest that they form a valuable part of the calorific material utilized by the animal.