Relationship between production performance and responses of blood chemistry to different physiological states in dairy cattle

Abstract

The aim aiof the research presented in this thesis was to study t he changes "that occur' in blood chemistry as a result of some common physiological f act ors (age, late pregnancy , early lactation, starvation, and s tress due to ACTH injection) and the relationship of such changes with production indices (fertility, milk production and growth rate) in dairy cattle. The potential use of erythrocyte magnesium (EI1g) as an indicator of body Mg status was studied by the experimental induction of hypomagnesaemia in young calves. A magnesium load test which could be used to determine body Mg status in young calves was also developed. Since current methods of metabolic profile testing use mainly post-partum observations, a method that utilised and/or combined changes in blood components 8 weeks before and 8 weeks after parturition could be more informative. This approach could aid in early detection of any blood Chemistry abnormalities which could affect production performance. Blood Chemistry changes in late pregnancy and early lactation were studied in two herds. The blood protein components (PCV, HL, RCC, TPP, albumin and globulin) and plasma Mg concentration decreased (P<0.05) while plasma Na and WCC's increased (P>O.O5)towards calving. PCV, HB,MCH, erythrocyt e Na (ENa) and plasma creatinine concentrations decreased (P<0.05)while TPP, globulin, BUN, plasma, ENa and e rythrocyt e K(EK) concentrations increased(P<O.O5) after calving. The changes in concentrations of the various parameters before and after calv ing were not significantly between herds or age groups with the exception of HB and PCV levels which showe d greater decreases In cows over 4 years than in those under 4 years. Multiple linear regression analyses based on slopes and means of both pre-partum and post-partum periods accounted for 63% to 78.8% of the variation in services per conception, 44% to 65% of that in days open and 53% to 82% of the variation in milk production rank in both herds. Changes in blood chemistry of heifers aged approximately 11 months occurred during a 40 hr starvation period with significant (P<O.OS) between animal variation (except for plasma inorganic phosphorus, Na and K). Changes in plasma Mg, EMg and EK (decrease) and in BUN (increase) concentrations were correlate (P<O.O5) with growth rate. A multiple linear regression equation with changes in plasma Mg, EMg, PCV and albumin as independent variables and growth rate as dependent variable yielded a multiple coefficient of determination of 0.79. Blood chemistry changes occurred between 0 and 4 or 6 hours after an injection of 100 iu ACTH in heifers aged approximately 14 months and showed repeatable significant (P<O.OS) between animal variations. A multiple linear regression equation with changes in plasma glucose, WCC, BUN, absolute e eosinophil count and Plasma Na concentrations as independent variables and growth rate as dependent variable yielded multiple coefficients of determination of 0.71 and 0.78 in repeat experiments. Blood samples were collected monthly from 10 dairy heifers between 2 and 13 months of age . Regression equations based on the concentrations of plasma albumen and Ca at 7 months, plasma globulin at 13 months, overall means (of 12 monthly estimations) of HB, PCV and plasma albumin and changes in Ca and PK between 7 and 13 months of age accounted for 43% to 89% of the variation in growth rate. Similarly blood samples were collected monthly from 10 dairy heifers between 12 and 24 months of age, and regression analyses based on the concentrations of TPP and plasma globulin at 12 and 18 months of age accounted for 75% to 84% of the variation in growth rate. Similarly multiple regression equations based on the concentrations of TPP and plasma globulin at 12 months of age and plasma globulin and plasma Mg at 24 months of age and the overall mean concentrations... for 64 to 72% of the variation in milk production rank at first lactation. Hypomagnesaemia was induced in 4 experimental calves 1-2weeks old by feeding 2 litres of skimmed milk (containing 19 of urea and 19 of KC1) twice a day and with ad libitum access to barley straw containing 5% urea, 10% soyabean oil and 5% KCl. The control calves received the same diet but were supplemented with 19 magnesium oxide daily. EMg concentrations decreased with age in both groups and also decreased following the onset of hypomagnesaemia in the experimental calves. The hypomagnesaemia was accompanied by significant{P<0.05) decreases in thyroid gland activity (FTI), plasma Ca,inorganic phosphorus, alkaline phosphatase concentrations and RCCls. Intramuscular injection of 40mg/kg b.wt of MgS04 solution increased both plasma Mg (P<0.05) and EMg concentrations after 4 hours in both hypomagnesaemic and normomagnesaemic calves but the changes in EMg were small and not significant in either group. The amount of Mg excreted in urine 4 and 24 hours after the M9S04 load was higher (P<O.Ol) in the control than in the hypomagnesaelnic calves and could be used as a method of determining Mg status in young calves