The effect of starch on forage fiber digestion kinetics in vitro

Purified corn and wheat starch were added to alfalfa, Coastal bermudagrass, fescue, and orchardgrass hays at 0, 40, 60, and 80% of the total as-fed substrate, and fiber digestion kinetics were determined in vitro. Kinetics were estimated by the model R = Doe-k(t-L) + U where R is residue remaining at time t, Do is digestible fraction, k is digestion rate constant, L is discrete lag time, and U is indigestible fraction. Parameters of the model were estimated by logarithmic transformation and a direct nonlinear least squares procedure. Corn and wheat starch did not differ in their effect upon lag time of fiber digestion, digestion rate, or potential extent of digestion. Alfalfa had a shorter lag time of fiber digestion (.86 h) than Coastal bermudagrass (3.05 h), but not than orchardgrass or fescue (1.66 and 2.42 h). Orchardgrass differed in fiber digestion rate (.0542h-1) from Coastal bermudagrass (.0698h-1) but not from alfalfa or fescue (.0670 and .0658h-1). The potential extent of fiber digestion was similar for fescue (75.8%) and orchardgrass (76.0%). The potential extent of fibre digestion for alfalfa (50.9%) differed from Coastal bermudagrass (64.3%), and both of these forages differed from fescue or orchardgrass. Addition of starch resulted in a linear increase in lag time of fiber digestion, but digestion rate was not affected. Potential extent of digestion was decreased when starch was added.

Some effects of starch on protein utilization by sheep

1. One feeding experiment was carried out with seven adult rams (49–65 kg) and a second with eight young rams (39–51 kg) to compare the influence of starch supplementation on the utilization of protein; soya-bean meal was the main protein source in all diets. Cottonseed hulls and Rhodes hay served as energy sources in the basal diets. The energy content of the experimental groups (four adult or four young rams) was increased to 150% of that of the control groups by addition of crushed maize while the intake of digestible protein was equalized by adding maize gluten to control diets.

2. Adult rams retained 20% more nitrogen and young rams 20% less nitrogen on the highenergy than on the control diet. The effect in young rams was not significant.

3. Addition of maize to the diets was followed by a decrease in protein digestibility, but no parallel trend was found between protein digestibility and efficiency of nitrogen retention.

4. In both experiments rumen ammonia and blood urea values were higher in sheep given the control diet than in those receiving the diet supplemented with maize.

5. The dehydrogenase activity of rumen contents from young rams was only slightly raised by the maize supplement compared with the much larger increase that occurred in adult rams.

6. The concentrations of total volatile fatty acids in the rumen contents of adult and young rams were not markedly influenced by the addition of maize to their diets but a slight increase in the concentrations of propionic and butyric acids and a decrease of the ratio of acetic to propionic acid occurred.

A study of the protein requirements of the mature breeding ewe. 2. Protein utilization in the pregnant ewe

1. An experiment was carried out in which protein utilization in the pregnant ewe was studied using the nitrogen balance technique.

2. Eight diets supplying four different intakes of crude protein and two different intakes of energy were each offered to eight individually penned ewes.

3. The mean crude protein intakes per day were 7·2, 5·5, 4·1 and 3·0 g/kg W0·73 (where W = body-weight) and the metabolizable energy intakes 134 and 113 kcal/kg W0·73.

4. N balances were carried out at 10–12, 14–16 and 18–20 weeks of gestation on five ewes from each treatment.

5. The apparent digestibility of both dry matter and crude protein decreased with decreasing protein intake. With the high energy intake, the apparent dry-matter digestibility was increased and the apparent digestibility of crude protein decreased. Stage of gestation had no significant effect on the apparent digestibility of either of these constituents.

6. N retention was not affected by the number of foetuses carried. With the higher energy intake and the higher protein intakes, the absolute retention of N was significantly increased at all stages of gestation. N retention increased with advancing pregnancy; the retentions at 10–12, 14–16 and 18–20 weeks of gestation being 0·086, 0·114 and 0·163 g/kg W0·73 per day respectively.

7. The efficiency of utilization of apparently digested N was calculated from the regression of retained N as a percentage of apparently digested N against apparently digested N.

8. The daily intakes of apparently digested N required for maximum efficiency were 0·551 and 0·620 g/kg W0·73 on the high and low energy intakes respectively. The daily intake for maximum efficiency decreased with advancing pregnancy, the values being 0·623, 0·587 and 0·567 g/kg W0·73 for the 10–12, 14–16 and 18–20 weeks of gestation respectively.

9. The levels of N retained at maximum efficiency were 0·235 and 0·202 g/kg W0·73 per day for the high and low energy intakes respectively. The levels of N retained increased during pregnancy from 0·170 g/kg W0·73 per day at 10–12 weeks to 0·286 g/kg W0·73 at 18–20 weeks. The requirements for zero N balance were 0·072 and 0·153 g apparently digested N/kg W0·73 per day for the high and low energy intakes respectively. The requirement for zero N balance decreased from 0·176 g/kg W0·73 per day at 10–12 weeks to 0·071 g/kg W0·73 at 18–20 weeks.

10. The results are discussed in relation to other research findings and current recommendations.

Binding of magnesium and calcium in the contents of the small intestine of the calf

1. In ileal contents from differently fed ruminating calves, examined under conditions approximating to those obtaining in vivo, 34–74% of the magnesium and 63–93% of the calcium were non-ultrafilterable. The binding was shown to be due to at least two processes, one depending on the presence of phosphate, the other not. 2. Considerable non-phosphate binding occurred in samples adjusted in vitro to pH 5.5 and above. The binding material was probably not a single substance but the bound forms of both Mg and Ca were, at least partly, in equilibrium with the soluble forms, and some competition between the two metals occurred. Thus in any one sample the extent of binding for either metal was influenced by the concentration of both. With normal concentrations it was estimated that, in the ileum, about one-third of the Mg and half of the Ca was bound in this way, irrespective of whether pasture or one of a variety of stall diets was given. 3. Samples adjusted to about pH 6.5 and above (in vivo ileum pH was about 7.4–7.9) showed further precipitation of Ca and Mg to an extent which partly depended on the concentration of inorganic phosphate. Ca precipitation appeared to be mainly controlled by the concentration of Ca and inorganic phosphate but for Mg the precipitation depended also on the presence of factors other than Mg and inorganic phosphate. One such factor found to be present was the ammonium ion, but its practical importance is uncertain. 4. Ileal contents from milk-fed calves showed considerable non-phosphate binding of Ca but not of Mg.

THE INFLUENCE OF PROTEIN AND ENERGY CONTENT OF THE RATION ON LAMB AND WOOL PRODUCTION OF RANGE EWES

Two feeding experiments were carried out with range ewes to determine the influence of the protein and energy content of the ration on lamb and wool production. When the energy content of the ration was increased approximately 10 per cent during early pregnancy [digestible crude protein (D.C.P.) intake was approximately 0.14 pound and digestible energy intake was approximately 3.2 therms daily on the basal ration], body weight gains and wool production of the ewes were increased (P < 0.05 and P < 0.01, respectively), but birth weights of the lambs in one experiment and 6-week weights of the lambs in both experiments were not affected significantly. However, when the energy content of the ration was increased during late pregnancy and early lactation (D.C.P. intake between 0.12 and 0.24 pound and digestible energy intake approximately 3.7 therms in Experiment 1, and 2.7 therms in Experiment 2, daily on the basal ration), body weight gains of the ewes were not affected, wool production was depressed (not significant in all cases), birth and 6-week weights of single lambs and birth weights of twin lambs in Experiment 1 were not affected. In Experiment 2, increasing the energy content of the ration reduced (P < 0.05) the birth weights of twin lambs.When the protein content of the rations was increased (from 0.16 to 0.20 to 0.24, and from 0.12 to 0.14 to 0.16 pound D.C.P. intake daily in Experiments 1 and 2, respectively), there was no increase in body weights of the ewes or birth weights of single lambs, but there was an increase (P < 0.05) in the birth weights of twin lambs and an increase (P < 0.01) in wool production of the ewes. Increasing the protein content of the rations increased the 6-week weights of single and twin lambs in Experiments 1 and 2 (P < 0.05). The results of these experiments suggest that the minimum average protein requirement for pregnant and lactating ewes was approximately 0.10 and 0.11 pound D.C.P. per pound of T.D.N. intake for maximum lamb and wool production, respectively.A nitrogen-balance and wool-growth study using non-pregnant, mature ewes showed that increasing the energy intake at low levels of protein intake caused a reduction in the apparent but not the "true" digestibility of the protein, no change in the biological value of the protein, and a reduction in weight of clean wool produced. At a higher level of protein intake, an increase in energy content of the ration resulted in no change in the apparent or true digestibility of protein but in an increase in wool production.