Pour experiments were carried out with Friesian cows in different stages of lactation to determine the influence of the crude-protein content of the diet on the digestibility and utilization of dietary constituents and on the cow's performance and milk composition.The rations were composed so that they met the energy requirements of the cows, but crude-protein content was widely variable.Our findings on the digestibility of the dietary constituents were in agreement with the data given in the literature.The utilization of dietary N for milk secretion was influenced by the level of N intake only when the crude-protein content of the diet was lower than a limiting value, which varied according to stage of lactation. When this value was not attained the daily milk production was affected, but not the protein content of the milk.The optimal crude-protein content of the diet was 15-16% (dry-matter basis) when milk production exceeded 20kg/d, 12-13% for a production of 15-17 kg/d and 11-12% for cows giving less than 10 kg of milk daily.
In different experiments, the nutritive value of maize silage associated with urea was compared with that of the same forage without any N supplement. The ammonia concentration in the rumen liquor was measured with two dry cows fitted with a permanent rumen cannula. Urea was rapidly hydrolysed in the rumen and gave rise to high ammonia concentrations. The control diet did not supply enough fermentable nitrogen to the ruminal flora. The duodenal N flow was measured with two dry cows fitted with a re-entrant cannula at 10 cm from the pylorus. When increasing amounts of urea were fed, the duodenal N flow initially increased to a maximum, but it leveled off with higher amounts of urea in the diet. With 13 cows in the second part of lactation, the feeding of urea increased the nitrogen balance of the animals and the metabolizable energy density of the rations. However, the milk production and the milk protein secretion were only slightly enhanced.
The dietary and individual animal factors influencing the utilization of digestible nitrogen and of metabolizable energy for milk production were studied and change in. the optimal nitrogen:energy ratio during lactation was investigated using 35 different experimental diets and the individual results obtained from 119 Friesian cows whose daily milk production varied from 4 to 30 kg.The amount of N secreted in the milk and the daily milk production depended above all on the metabolizable energy intake and on the stage of lactation. They were related to the dietary N: energy ratio.The percentages of digestible N and of metabolizable energy utilized for milk production were also related to the dietary digestible N: metabolizable energy ratio of the diet.The optimal digestible N:metabolizable energy ratio ranged from about 2-2 g/MJ during the first 3 months of lactation to about 1-7 g/MJ in the sixth and seventh months and 1-3 g/MJ from the tenth month. When expressed in g N intake per MJ energy intake, this optimal ratio ranged from 1-6 after calving to 1-1 at the end of lactation.In the ruminant, close relationships exist between N metabolism and that of other dietary constituents.In a previous paper (Paquay, De Baere & Lousse, 1972a), the factors which influence the digestibility of N in the cow were studied. The results obtained for dry cows and lactating cows were studied together, since it was observed that the production of milk did not modify the digestibility of ration constituents. In agreement with other authors (Elliott & Topps, 1963a;Van Niekerk, Smith & Oosthuysen, 1967), it was found that N digestibility is strongly related to the crude-protein content of the diet, and the amounts of digestible N are determined by the N intake level and also by the dry-matter intake level.On the contrary, when studying the utilization of digestible N, it is necessary to consider separately the values obtained for dry cows and lactating cows, since the production of milk largely modifies the fate of dietary N and the interactions between the dietary constituents. The influences of dietary and individual animal factors on N utilization in the dry cow were studied previously (Paquay, De Baere & Lousse, 19726).
Zusammenfassung
Untersuchung über die Magnesiumabsorption bei Kühen
In einer Versuchsreihe wurde der Einfluß verschiedener Fütterungsfaktoren auf die Magnesiumabsorption aus den Mägen und Darm von Kühen gemessen.
Der größte Teil der Magnesiumabsorption findet in den Mägen statt. Im Dünndarm wurde eine leichte Mg‐Sekretion und im Dickdarm eine leichte Mg‐Absorption nachgewiesen.
Eine kontinuierliche Panseninfusion von steigenden Wassermengen erhöhte die Flußrate am Duodenum. Dadurch wurde die Magnesiumabsorption reduziert.
Die Absorption des Magnesium aus den Mägen war negativ gekoppelt mit dem Rohproteingehalt der Rationen. Die Art der Stickstoffsupplemente hatte keinen Einfluß.
Diese Feststellungen werden diskutiert im Zusammenhang mit dem Auftreten der Grastetanie bei Milchkühen.
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