I . Six experiments were undertaken with mature, dry, non-pregnant cows to determine the capacity to lose and recover nitrogen and to study the significance of the labile and total protein reserves.2. It was concluded that, without altering its ability to reach N equilibrium, the mature cow is able to store and lose large amounts of body proteins (certainly more than 15 kg), when its N and energy intakes are greatly varied. The repletion or depletion of protein reserves can occur over a long period of time (sometimes more than 5 months). Total protein reserves include a very labile part. This part is more rapidly lost during fasting than during feeding on low-protein diets; it can be used to meet temporarily the energy requirements of the animal.3. N balance and body-weight gains were generally very well correlated.More than a century ago, Voit (1866) concluded that the body of the well-nourished animal contains a store of labile proteins. The magnitude and significance of this store are still not clearly established. According to many authors (see Munro, 1964), the amount of labile protein deposited in the body does not exceed 5% of the total body protein and this labile protein disappears within a few days when the subject is fasted, or fed on a protein-free diet. But Allison (1964)) Allison, Wannemacher, Banks & Wunner (1964) and Allison & Wannemacher (1965) concluded that there was a larger protein reserve which can contribute to the maintenance of essential tissue structure. The protein reserve includes a very labile part.Most of the previous experiments were undertaken with simple-stomached animals. The present paper describes the results of six experiments intended to determine the capacity of the mature, dry, non-pregnant cow to lose and regain nitrogen and to study the significance of labile and total protein reserves. E X P E R I M E N T A LFriesian cows were used and were kept in metabolism stalls allowing the separation of urine from faeces. One of the stalls was so arranged that the cow's weight was automatically recorded every hour.Expt I. After being undernourished for z months, a 7-year-old cow received 4 kg pea-straw hay and 4 kg ground barley each day for a period of 5 months. According to Kellner & Becker (1962), this ration provided 130% of the energy maintenance requirements of the cow. During another 5-month period the ration was reduced by half ( z kg pea-straw hay and 2 kg barley). Urine and faeces were collected during Expt 2. Two groups of four and three cows, 6-10 years old and of almost the same body-weight, were so nouiished during 3 months that the cows in one group became nearly I 30 kg heavier than those in the other. Later, all the cows were given the same ration (3 kg barley straw, 2 kg ground barley, 2 kg concentrated foods each day) during a 2-month period. Urine and faeces were collected three times during 6 d periods.Expt 3. Three cows, 7-9 years old, received a winter ration of 3 kg hay and 3 kg ground barley for a month. Afterwards, grass, freshly cut each day, was given a...

SUMMARYStatistical tests were made on the data obtained from 100 non-pregnant lactating cows, whose milk production ranged from 4 to 22 kg/day. The cows were kept in metabolism stalls and fed 37 different rations.The authors have studied the correlation between total water intake, faecal and urinary water losses, absorbed and available water and the numerous other factors which were analysed in each of the 37 rations.Total water intake is closely related to the dry-matter intake and the dry-matter content of the diet; it is related also to the composition of the ingested dry matter but is unrelated to the daily milk production.Faecal water losses are closely related to the dry-matter intake and the dry-matter content of the diet; increase of pentosans and crude-fibre intake enhances the losses.Urinary water excretion is related to the amount of absorbed water and to the dry-matter content of the diet; a highly significant correlation exists also between urinary water and urinary nitrogen and potassium. Daily milk production and urinary water are negatively correlated.Water secretion in the milk depends on the dry-matter intake and on its composition; but there is no significant correlation between total water intake and milk water.

SUMMARYStatistical analyses were carried out on the data obtained under strictly controlled conditions in metabolism stalls with 41 different rations fed to 127 adult non-pregnant dry cows, and with 14 other different rations fed to 35 adult non-pregnant lactating cows that had calved 2–6 months earlier and whose daily milk production ranged from 11 to 20 kg. The authors have calculated and studied the correlations between faecal and urinary calcium losses, calcium excretion in the milk, digestible calcium, and calcium balance, and the 75 other nutritive factors which were analysed for each of the 55 above-mentioned experimental diets. The results showed that calcium metabolism is regulated in the digestive tract, and the fact that the lactation calcium requirements increase at the same time as the amounts of digestible calcium and the calcium balance corroborates this statement. But except for this endogenous factor of resorption, the fate of the dietary calcium does not depend on the ingested amounts but on the nature itself of the components of the diets and their proportions in the diets. Cereals, and especially barley, have from this point of view a particular beneficial effect. These factors act on the absorption of calcium and thus directly on the balance, since the daily urinary excretion of calcium is generally small and independent. It has also been demonstrated that the balance is not inevitably negative in cows with a daily milk production of 11–20kg, and that a careful composition of the rations should enable one to equilibrate it.The other nutritive factors do not work on calcium utilization, but a close parallel exists between digestibility, urinary excretion and balance of both calcium and magnesium.

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.

Statistical analyses were carried out on the data obtained under strictly controlled conditions in metabolism stalls with 41 different rations fed to 127 adult non-pregnant dry cows, and with 14 other different rations fed to 35 adult non-pregnant lactating cows that had calved 2-6 months earlier and whose daily milk production ranged from 11 to 20 kg.The authors have calculated and studied the correlations between faecal and urinary sodium losses, sodium excretion in the milk, digestible sodium and sodium balance and the 75 other nutritive factors which were analysed for each of the 55 above mentioned experimental diets.The most important factor in the fate of sodium is the amount of dietary sodium. Sodium digestibility is neither total nor constant: faecal losses and digestible amounts are enhanced when sodium intake is increased and faecal losses are also positively correlated with dry matter and nitrogen intakes. An increase in digestible sodium benefits both urinary excretion and balance, the latter being reduced by an increase in dry matter and nitrogen intake.Sodium balance is always negative when sodium content of the diet is lower than 0-1 % in the dry cows and 0-2 % in the lactating cows. In our experimental conditions, the sodium requirements for milk production do not influence sodium digestibility, but are met above all to the detriment of urinary losses.Statistical research on the extensive data ob-rations were used and that for each of them we had tained in metabolism stalls with adult dry and 80 data on intake, digestion and utilization of ration lactating non-pregnant cows fed 55 different ex-constituents. The trials were conducted in metabol