Effect of protein concentration in the diet on blood composition and productivity of lactating Holstein cows under thermal stress

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Abstract
SUMMARY: An experiment was designed to assess the effect of thermal stress and protein value of the diet on the productivity and some of the blood organic constituents of lactating Holstein cows. Two groups of eight animals each were used in 2 × 2 Latin-square designs with two experimental diets. The percentage of crude protein in the diets was 14.3 and 20.8 %.Rectal temperature and respiration rate were recorded twice weekly and jugular blood samples were collected at weekly intervals for analysis. Daily feed intake, refusals and milk production per cow were recorded. Representative samples of morning and afternoon milk were taken three times a week for chemical analysis.Data on respiration rate and rectal temperature suggested that both groups of animals were under thermal stress. However, significantly lower values were found by increasing the protein level of the diet. The higher level of protein significantly increased total milk production, 4 % fat-corrected milk (4 % FCM), total solids, solidsnot-fat, protein percentage and yield, casein and non-protein-nitrogen in milk. The increase in milk production and 4 % FCM was found to be associated with the increase in total feed and total-digestible-nutrients intake but not with crude-protein intake. Protein levels in the diet had no effect on fat percentage and yield nor on whey protein.The correlation coefficients between respiration rate and rectal temperature with some of the blood organic constituents suggested an inverse relation between thermal stress and the blood organic constituents. The higher level of protein in the diet significantly increased haemoglobin levels but had no effect on oxyhaemoglobin, haemoglobin:oxyhaemoglobin ratio and haematocrit value. Total serum protein was not affected by thermal stress or by the protein value of the diet. Increasing protein levels in the diet increased albumin but decreased the globulin fraction of serum protein. Plasma glucose and non-protein nitrogen were increased by increasing the protein in the diet.