Responses to post-ruminal infusion of casein and glucose in lactating goats.

Abstract

RESPONSES TO POST-RUMINAL INFUSION OF CASEIN AND GLUCOSE IN LACTATING GOATS S.S.E. RANAWANA* and R.C. KELLAWAY* Summary Lactating goats fed a basal ration containing 11% crude protein, were infused intra-abomasally with 45g/d of either casein or glucose, in an attempt to determine whether milk production responsesto casein infusions could be attributed to gluconeogenesis from the casein. Infusions of casein increased the production of milk and milk constituents whereas the-production during glucose infusions was similar to that on the basal ration. Milk fat content was depressed by the glucose infusion. The output of nitrogen in milk and urine was increased during the casein infusion. Consideration of the data from this and other similar experiments leads us to conclude that the production response to casein infusion post ruminally, cannot be attributed to an increased availability of glucose precursors. I. INTRODUCTION Feeding standards for lactating ruminants are based on apparently digestible crude protein (DCP). The intake of DCP, however, often bears little relationship to the .actual amino acid supply (Satter and Roffler 1974). A number of workers have reported that postruminal infus+ ion of casein leads to increased milk production in cows fed rations containing levels of crude protein considered adequate by accepted feeding standards (Clark, Spires and Derrig 1973; Vik-Mo, Emery and Huber 1974). Recently, we obtained similar results with lactating goats in which successive increments in milk production were recorded following intra-abomasal infusions of increasing levels of casein (Ranawana and Kellaway 1975). It is well recognised that glucose is a key nutrient in milk synthesis (Linzell and Peaker 1971) and that amino acids are major precursors of glucose in ruminants (Wolff and Bergman 1972). The present study was carried out to examine the extent to which increases in milk yields following casein infusions could be attributed to increased gluconeogenesis in response to the extra supply of absorbed amino acids. . II. MATERIALS AND METHODS Three, multiparous Saanen does, in mid-lactation, of similar live weight and milk production, were used for the experiment, Details of surgical and management procedures have been described in an earlier paper (Ranawana and Kellaway 1975). The basal ration contained 62% rolled barley, 20% lucerne chaff, 10% oat straw, 7% molasses and 1% bone meal. On analysis it was found to contain 87.9% dry matter, 11.2% crude protein, 17.9% acid detergent fiber and 4.8% lignin. The daily allowance was divided into eight equal portions and offered every three h, using an automatic feeder. * Department of Animal Husbandry, University of Sydney, Camden, N.S.W., 2570. 489 The three goats were assigned at random to three treatments in ,a 3 x 3 Latin square such that each goat was subjected to each treatment for a period of 14 d. Within each period, the goats were allowed seven d to adjust after which measurements were made for five d to estimate milk yield and nitrogen balance. The treatments consisted of a control (no infusion) and infusions into the abomasum of 45 g/d of either glucose or casein. The glucose infusate contained 50 g/l of glucose, 62.5 mg/l dihydra streptomycin sulphate and 62,500 units/l penicillin G. The composition of the casein infusate and details of the infusion procedures have been described previously (Ranawana and Kellaway 1975). During the appropriate period, the goats received as a constant infusion 900 ml/d of infusate which was equivalent to 45 g of either glucose or casein. Details of the collection, sampling, storage and analysis of milk, feed, faeces, urine and the infusate.have been described previously (Ranawana and Kellaway 1975). The data were submitted to a conventional Latin square analysis and the treatment means were subjected to Duncan's new Multiple Range Test to assess significant differences (Steel and Torrie 1960). III. RESULTS One goat refused 60 g/d of the basal ration when infused with casein and 330 g/d during the glucose infusion. As these seemed to be her maximum intakes and she appeared to be in good health, the data are included in the results. The effects of-infusions on the production of milk and milk constituents are shown in the table. 490 Relative to the control, milk production was significantly (P(O.01) increased during the infusion of casein but not glucose. Fat content of milk was depressed.by the glucose infusion (P(O.05) whereas the slight depression during the casein infusion was not significant (P(O.05). The production of other milk constituents paralleled the increase in total milk production. , The nitrogen balance data are presented in the table. All the infused casein apparently was absorbed since there was no increase in faecal nitrogen output, resulting in a significant (X0.05) increase,in nitrogen digested and absorbed. Although milk nitrogen was increased (P(O.05) by the casein infusion, most of the excess nitrogen absorbed from the casein was excreted in the urine. The goats were in positive nitrogen balance on the basal ration and the quantities of nitrogen retained were unaffected by either infusion. 'Glucose infusions had no effect on any of the nitrogen balance measurements. IV. DISCUSSION Annison and Lime11 (1964) showed that 60 to 85% of the glucose entering the circulation of the high yielding goat is utilised by the mammary gland. Since little glucose as such is absorbed by the ruminant intestine (Leng 1970) the glucose requirement of the lactating mammary gland is met largely by gluconeogenesis from various dietary precursors. Although propionate is the chief substrate utilised; Wolff and Bergman (1972) showed in- sheep fed a lucerne ration containing 20% crude protein, that 11 to 30% of the'glucose entering the circulation originated from amino acids. The 45 g of glucose infused in the present experiment would have supplied approximately 11% of the daily glucose turnover (Annison and Lime11 1964). Although intravenous (Lime11 1967) or intraarterial (Mepham and 1 Lime11 1974) infusion of glucose increases milk yield in goats, evidence that post-ruminal infusion of glucose stimulates milk yield in lactating ruminants, is equivocal. Thus, Clark, Spires and Derrig (1973) reported that post-ruminal infusion of casein but not glucose stimulated milk production in cows. In contrast, Vik-Mo, Emery and Huber (1974) found in one of several experiments where glucose and casein were infused postruminally, that milk yield was stimulated equally by casein and glucose. They suggested that the response to glucose infusion was due to a sparing action on amino acids. However, it seems doubtful that this would occur since the amino acids likely to be spared (alanine, aspartate, glutamine, glycine and serine - Wolff and Bergman 1972) appear to be relatively unimportant for milk protein synthesis. It is, clear from the results of the present study that the intraabomasal infusion of glucose was without effect on the level of milk production. This finding leads us to suggest that where a production response to infusion of glucose has been obtained, the mechanism involved differs from that by which casein infusion stimulates milk yield. Consistent with this suggestion is the report of Tyrell et al. (1972) that when casein was infused into th;! abomasum of lactating cows, 48% of the total energy infused appeared in milk. In contrast, the corresponding figure for glucose infusion was 16%. The depression in fat content of milk in the present study is consistent with the results of several previous studies (Fisher and Elliot 1966.; ,Clark, Spires and Derrig 1973; Vik-Mo, Emery and Huber lg@), The depression of milk fat levels is apparently caused by the lowering of 491 levels of blood-borne precursors of milk fat and, thus, a reduced mammary extraction of these metabolites (Annison, Bickerstaffe and Linzell 1974). Nitrogen balance data were presented to demonstrate the nitrogen. status of the goats in the present study.. The results obtained are similar to those of Clark, Spires and Derrig (1973) who reported that infusion of casein in cows increased the output of nitrogen in milk and urine. However, utilisation of absorbed nitrogen from casein in the ' present study, was poor when compared to the results of an earlier experiment (Ranawana and Kellaway 1975). This could be due to the fact that i&mid-lactation utilisation of nutrients is less efficient than in early lactation. In conclusion, it appears that the response in milk production to post-ruminal infusion of,casein is not dueto the provision of glucogenic amino acids. The possible mechanisms by which the response is elicited have been discussed by Clark (1974) who pointed out that the facto:s involved have not been resolved. A fuller understanding of the nature of the response would lead to optimum use of scarce-animal feedstuffs . for milk production. ACKNOWLEDGMENTS The study was supported by the Australian Dairying Research Committee. 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