Nitrogen and amino acid metabolism in dairy cows

anglais. For the process of milk production, the dairy cow requires nutrients of which energy supplying nutrients and protein or amino acid supplying nutrients are the most important. Amino acid supplying nutrients have to be absorbed from the small intestine and the research reported in this thesis mainly concentrates on the study of methods by which the intestinal supply of amino acids in dairy cows can be manipulated. Information of this sort can only be obtained if one can get access to the small intestine of normally functioning animals. For this purpose the small intestine of mature dairy cows was cannulated in such a way that all the digesta flowing through the small intestine had to pass an "artificial intestine" just on the outside of the animal. This "artificial intestine" consisted of two pieces of PVC tubing inserted through the body wall into the two blind sacs of the dissected and sewed up small intestine and a piece of soft tubing connecting both PVC cannulae. By removing the connecting tube one gets access to the interior of the small intestine and the intestinal flow of digesta can be measured and sampled.The intestinal flow of protein is governed by the microbial degradation of dietary protein in the forestomachs of the ruminant animal on the one hand and by the microbial protein synthesis in the same forestomachs on the other. Both processes occur simultaneously. A detailed discussion of the process of microbial degradation of dietary protein in ruminants is given in chapter 1. It appears that dietary protein is degraded to a varying degree, depending on nature and solubility of the dietary protein, rate of passage through the forestomachs and level of feed intake. A further quantification of the differences in degradation and factors which influence these differences is hampered by the lack of accuracy or reliability of the measuring techniques. The process of microbial protein synthesis and measuring techniques was studied and discussed in chapter 2. Measuring the contribution of microbial protein to the intestinal protein supply is usually based on the use of marking substances which are present in the microbes, but not in the feed. For this purpose nucleic acids (RNA in particular), 2.6-diaminopimelic acid (DAPA) and various radio-isotopes such as 15 N, 32 P or 35 S can be used. The efficiency of microbial protein synthesis (microbial N/100 g of carbohydrates apparently fermented in the forestomachs) was compared using 3 different methods. The methods were the use of the microbial markers RNA and DAPA and a regression method. All methods showed considerable variation and the results of the different methods differed considerably from each other.Despite an observed close relationship between energy intake and duodenal protein flow it seems reasonable to assume that apart from energy intake some other factors may have an influence on the intestinal protein flow. Some of these possible factors were investigated and the results of these investigations are reported in the following chapters.Comparing two protein sources of a widely different nature viz. maizegluten feed and soyabean meal showed that after feeding maizegluten feed substantially more protein entered the small intestine of a dairy cow than after feeding soyabean meal. The difference was explained by a difference in extent of degradation in the forestomachs between both protein sources. The value of this experiment is somewhat limited because only one animal was involved in it. However some provisional studies on the degradation of different protein sources in the rumen by using the so-called dacron bag technique seems to confirm the findings presented in chapter 3 as can be concluded from the results presented in chapter 11 (table 3).A further question is if the degradation of the dietary protein in the forestomachs of ruminants can be reduced by treatments like heating or the application of chemicals. This aspect was also studied and the results in chapter 5 do suggest that heat treatment (achieved by artificial drying) as well as treatment with certain chemicals (formaldehyde in particular) of roughages can improve the intestinal protein flow. Sow indications were found however that treatment with formaldehyde may slightly decrease the efficiency of absorption of amino acids from the small intestine, but the increase in duodenal protein flow appeared higher than the decrease in absorbed protein. Heat treatment and formaldehyde treatment can easily lead to overprotection. Under such conditions the decrease in absorption may exceed the increase in intestinal flow and the total result is negative. Such treatments must therefore be applied with great care.Because of differences in their nature, protein degradation may differ between long roughage and ground concentrates. Diets were therefore fed in which the roughage to concentrates ratio varied widely. A tendency was found that a larger proportion of the apparently digestible organic matter was digested in the stomachs with a larger proportion of long roughage in the diet.Despite the wide variation in the roughage to concentrates ratio no detectable changes took place in the degradation of total feed protein or in the synthesis of microbial protein. Approximately 70% of the ingested feed protein became degraded in the stomachs whereas for each kg of carbohydrates fermented some 32 g of microbial N was produced.Factors influencing duodenal protein flow discussed sofar were largely associated with differences in rate of degradation of protein in the forestomachs. It was realised that decreasing the time during which the feed proteins were subjected to microbial degradation might also reduce the extent of degradation. Reducing the time that feed particles spend in the rumen can be achieved by increasing the level of feed intake. The effect of the level of feed intake on N entering the small intestine was therefore studied and the results are reported in chapter 7. Increasing the level of feed intake appeared to increase the duodenal flow of non-ammonia-nitrogen (NAN), not only the absolute quantity, but also the quantity per kg feed ingested, which could entirely be explained by a reduction in extent of protein degradation in the forestomachs.From the preceding chapters it became evident that there are ways to reduce the extent of degradation of dietary protein in the forestomachs of ruminants. The question arises what measures can be taken to improve microbial protein synthesis or to prevent that microbial protein synthesis becomes reduced. Frequent feeding, particularly of concentrate rich diets, is thought to have a stabilising effect on rumen fermentation and this might be advantageous for microbial growth and hence for the microbial protein production. The effect on duodenal N flow of feeding the concentrate part of the diet in smaller portions but more frequently per day was further investigated. The results are shown in chapter 8. Feeding the concentrate part of a low N diet twice daily resulted in a relatively low N flow which could be increased to more normal values if the concentrates were administered in 6 portions per day. Repeating the experiment with diets containing more normal protein levels showed only a small positive effect of more frequent feeding on the duodenal N flow. Evidence was obtained from regression calculations that frequent feeding decreased the proportion of the dietary protein escaping degradation in the forestomachs but that this decrease was more than compensated for by an increased production of microbial protein.In the last three chapters attempts are being made to evaluate the practical significance of the findings reported or discussed in previous chapters. Chapter 9 shows that it seems more promising to study and develop methods by which the total duodenal protein flow is enhanced rather than to concentrate on methods which try to improve either the intestinal flow of undegraded feed protein or the intestinal flow of microbial protein. The relationship between energy intake and intestinal protein flow is emphasized again. Evidence is obtained that the maximum flow of intestinal protein if related to energy intake is likely to be in the order of 3 g of NAN/MJ ME ingested.The ultimate aim of improving the intestinal supply of protein is to provide the dairy cow with more amino acids in the blood so that more milk can be produced, provided the genetic capacity potential and physiological state of the animal will allow such an improvement. After entering the blood the amino acids can be used for various processes. In chapter 10 an attempt is made to get some information, mainly based on data from literature, on the amino acid requirements for maintenance, gluconeogenesis, protein retention in the body and protein output in milk. Because of a lack of precise quantitative data the result remains rather descriptive. Evidence is presented however that amino acids do not play an important role as precursors for gluconeogenesis in dairy cows, not even at high levels of milk production. Evidence is also presented that if milk output is limited because of an inadequate supply of amino acids, this is often because of the total supply of amino acids is limiting. Under some conditions one single amino acid may be limiting and then methionine appears more often limiting than one of the other essential amino acids.The final chapter 11 deals with a general discussion of the feeding principles of the high yielding dairy cow. Following the concepts developed in the previous chapters some guidelines for the protein feeding of high yielding dairy cows are given. It is stressed that maintaining optimal conditions for fermentation, microbial activity, microbial growth and protein production in the forestomachs are the best guarantee for an adequate intestinal protein supply. In this respect the significance of protein/energy interactions is emphasized. Only if an optimal rumen fermentation cannot be maintained there seems scope for the feeding of protein with a high resistance against degradation in the rumen, provided the intestinal digestibility of this protein is high and it has an amino acid composition resembling the composition which is required by the animal.

néerlandais; flamand. Het zoeken naar methoden waardoor het aanbod van aminozuren in de dunne darm van de koe beinvloed kan worden staat centraal in deze dissertatie. Deze toevoer van aminozuren wordt bepaald enerzijds door de mate van microbiele afbraak van opgenomen voereiwit en anderzijds de synthese van microbieel eiwit. Hoewel er een verband aangetoond kan worden tussen de opname van energie en de toevoer van aminozuren naar de dunne darm, lijkt het ook aannemelijk dat er nog andere factoren zijn die op de genoemde toevoer van invloed zijn. Een aantal van deze factoren werden nader bestudeerd: (1) het soort voereiwit, (2) een eventuele voorbehandeling van het voer, (3) de verhouding tussen ruwvoer en krachtvoer in het dieet, (4) een verhoging of verlaging van het niveau van voeropname en (5) het toedienen van krachtvoer in meerdere porties per dag. Tot slot wordt nog ingegaan op het gebruik van de aminozuren in het dieet door de koe en worden richtlijnen gegeven voor de eiwitvoeding van hoogproduktieve melkkoeien. Gezorgd moet worden voor een optimale fermentatie in de voormagen van de koe