الفهرس 
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Abstract
Two experiments were conducted in the present study in order to investigate the effect of different dietary energy sources (dry-fat or yellow corn supplementation) on productive performance (feed intake, milk yield and composition) of dairy cows; the first experiment was a preliminary experiment with three Ossimi rams (3x3 Latin square design) in order to clarify the effect of the dietary treatment on digestion kinetics. The effect on nutrient digestibility and feeding value as well as rumen fermentation was determined. Blood plasma constituents were measured. The dietary cation-anion balance was also determined. Eighteen dairy Holstein cows were chosen and allocated into a completely randomized block design. Cows were weighed every two weeks during the experimental period, also BCS was measured biweekly during the experimental period, which lasted for 12 weeks. This was done before the morning feeding and drinking. The control ration was that of concentrate feed mixture (CFM) without supplements; the second and third CFM were supplemented daily with either 544g dry fat (2.65% on DM basis; RF) or 1000g corn grain (4.88% on DM basis, RC),respectively. The ratio of concentrate to roughage was nearly 50:50. All cows were fed based on their body weight, milk yield and milk composition. Three digestibility trials were carried out during the three stages of lactation.
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Results revealed that:
1- Dietary energy source increased (P<0.01) intakes of DM, OM, EE and NFE by Ossimi sheep. Water intake followed the same pattern.
2- Addition of fat or corn increased (P<0.01) the digestibility of DM, CP, NFE and EE.
3- Nitrogen balance with sheep were higher (P<0.01) in RF (6.74g) and RC (6.19g) than the control group (4.76g).
4- Lower ruminal pH was found at all times post feeding being for the control and RC ration than RF. The lower pH was due to the higher fermentation ability of corn grain. VFA values took the opposite trend of the pH.
5- Plasma insulin in sheep was highest with the rams fed the corn-supplemented ration than the other rations at all times. Plasma glucose followed the same pattern.
6- Fat-treated ration caused an increase in plasma lipids (tri-glycerides total lipids and cholesterol).
7- Plasma Ca concentration was highest in rams fed the fat-supplemented ration. No other differences were found regarding plasma minerals.
8- Cows on the fat-supplemented ration (RF) showed more (P<0.01) DM intake (21.1 kg/d). No differences were found between R0 (control) and
RC (corn grain supplemented rations) being on the average 19.40 and
19.66 kg/d, respectively.
9- Body weight of all cows in the experimental groups R0 and RF
decreased to reach the lowest values at the 6th week; they started to regain
their loss in body weight; body weight of cows on RC continued to
decrease.
10- Body condition score began to decrease for the RF and reached its least
value at the 6th week (2.5) then it began to gain back till it reached 3.25
at the end of the experiment. The other two groups (R0 and RC) were
semi stable.
11-During the three lactation periods, dietary fat RF increased (P<0.05) the
digestibility of EE comparing to R0 and RC. Digestibility of the other
nutrients was not affected by the treatment.
12- Fat-supplemented ration of dairy cows non-significantly improved the
TDN value by about 3% at the three stages of lactation. Dietary energy
sources did not affect digestible CP.
13- In the 3rdand 7thweek of lactation there was a significant increase in
milk yield for the two treated-groups (RF and RC). Fat corrected milk
followed the same trend.
14-Milk composition did not differ due to dietary energy source.
15- Total milk yield increased (P<0.05) for cows fed RF and significant decrease for R0 while the group RC was intermediate.
16-As for fat yield, there was a significant increase for RF while there was a significant decrease in the other two groups of R0 and RC.
17- Plasma constituents in dairy cows followed the same pattern of sheep.
18- It was clear that DCAD was positive in all rations. Value of DCAD was +28, +19 and +34 for rations R0, RF and RC, respectively. The lower DCAD was associated with the higher performance of lactating cows.
from the results obtained in the present study, it could be concluded that adding more energy especially fat (Ca-soup) is recommended as a feed supplement for dairy cows in order to improve digestibility, feeding value, milk production and milk composition without any adverse effect on the productive performance or the animal health. It is obvious that dietary fat and corn grain supplementation improved nitrogen balance through the improvement of N digestibility. The lowest VFA values reported for the dry fat-treated ration may have been due to a lower microbial activity in the rumen of sheep fed this ration than those received the control or corn-supplemented rations, perhaps because it contained less fermentable carbohydrate.