Effect of elevating serum lipids on luteinizing hormone response to gonadotrophin releasing hormone challenge in energy-deficient anestrous heifers.

A study was conducted to evaluate the effect of feeding a bypass fat on luteinizing hormone (LH) response to gonadotrophin releasing hormone (GnRH) in noncyclic Holstein heifers. Twelve cyclic Holstein heifers were fed a complete diet at 40% net energy for maintenance (NE(m)) until cessation of ovarian activity. Based on weights and condition scores, heifers were assigned to either a control or treatment diet containing 0.45 kg bypass fat and fed at an energy level of 85% NE(m). Diet adjustments were made following weekly weighings. GnRH challenges were conducted at four periods: prior to initial energy deprivation, at termination of 40% NE(m) feeding, and twice more at 21-d intervals after 85% NE(m) feeding began. Blood was sampled via a jugular catheter every 15 min for 5 h, and GnRH was injected after the fourth sample. None of the heifers exhibited estrous activity after the initial energy deprivation. Heifers on the bypass fat diet continued to lose weight during the treatment period, while the control heifers gained a slight amount of weight. Baseline and peak concentrations of LH were not significantly affected by time or diet. Time to GnRH-induced LH peak was longer (53 vs 130 min, P < 0.01) after 40% NE(m) and remained greater at all times thereafter. Serum lipid levels increased 82.5% among heifers being fed the bypass fat. Energy restriction had no effect on the magnitude of LH response to GnRH but did delay response time.

Effects of positive-pressure pulsation on several characteristics of milk production.

A premilking phase of positive-pressure pulsation was evaluated by comparing milk production, persistence, peak milk flow, average milk flow, and several indicators of udder health of two groups of high-producing Holstein cows. Forty-four cows, 21 controls and 23 test animals, completed at least 250 days of lactation. After a premilking wash-up regimen of 25 to 30 s, test cows were given an additional 45 s for positive-pressure pulsation. The group with positive-pressure pulsation maintained significantly more persistence during the last two-thirds of lactation, and this resulted in an 8% relative increase of milk production over controls. There were no differences of udder health between the two groups as measured by number of cases of clinical mastitis or California mastitis test.

Failure of hair analysis to reflect excessive sodium chloride ingestion by Holstein heifers.

Mineral analysis of hair samples was to assess its value as an indicator of excessive salt ingestion. Ten pregnant Holstein heifers averaging 499 +/- 32 kg were group-fed 9 kg of alfalfa hay cubes and 2 kg of a 14% dairy concentrate mix per head per day. Salt and dicalcium phosphate were available free choice. Prior to treatment animals received fresh water containing 450 ppm total dissolved salts. During treatment, sodium chloride was added to the fresh water at 3500 ppm. Hair samples together with blood and urine were taken immediately prior to start of the experiment and at 30 and 60 days after initiation. Hair samples from two previously marked areas were composited, washed with hot detergent, rinsed, dried with ethanol and then acetone, and wet-ashed. Blood electrolytes and hair and urinary minerals were measured by atomic absorption spectrophotometry. Analysis yielded no significant treatment effects for sodium, potassium, calcium, magnesium in hair and blood serum, and no correlations between hair, blood serum, and urinary minerals. Only urinary sodium reflected the increased sodium ingestion.

Beta-adrenergic receptor involvement in lipolysis of dairy cattle subcutaneous adipose tissue during dry and lactating state.

The influence of lactation on beta-adrenergic receptor kinetics was studied with adipocytes from eight Holstein cows during two physiological states, dry period 30 days prepartum and 30 days postpartum or early lactation. Physiological state had no effect on binding kinetics of (--)-hydrogen-3 labeled dihydroalprenolol. Affinity rate constants (8.2 versus 7.2 X 10(7) min-1 M-1) and equilibrium dissociation constants (7.1 versus 7.9 nM) for both prepartum and postpartum periods were similar. In contrast, the apparent number of beta-adrenergic receptors varied with lactational state (42,154 versus 72,264 sites/cell) for dry and lactating status as estimated in assays containing 5 nM (--)-hydrogen-3 dihydroalprenolol. Glycerol release and adipocyte concentrations of adenosine-3',5'-cyclic phosphoric acid were assayed with or without 10 microM epinephrine. Epinephrine elicited greater release of glycerol in adipocytes from lactating than dry cows (3.91 versus 2.1 mumol/10(6) cells/120 min). The concentration of adenosine-3',5'-cyclic phosphoric acid rose during the first 5 min of incubation in the presence of epinephrine and then fell to base after 10 min. Maximum concentrations at 5 min were not different in adipocytes from dry and lactating cows (250 versus 280 pmol/10(6) cells).

Relation between mastitis test score, mineral composition of milk, and blood electrolyte profiles in Holstein cows.

The relationship between mastitis, mineral composition of milk, and blood electrolyte profiles was investigated in 54 Holstein cows. Sodium potassium, calcium, magnesium, and chloride in blood and milk were compared under two indices of mastitis, a milk quality test and a milk somatic cell count. Milk from cows with evidence of udder infection had higher sodium and chloride and lower potassium than cows free of mastitis. Although there was a correlation between blood calcium and milk calcium and between milk calcium and milk somatic cell count, as well as differences in mean potassium and calcium in blood between mastitic and nonmastitic cows, there was no direct relationship between the indices of mastitis and electrolyte profiles of blood.

Effect of stress on blood leucocyte and milk somatic cell counts in dairy cows.

Blood and milk samples from Holstein cows were examined for total blood leucocyte count, differential blood leucocyte count, milk quality test, and somatic cell count in milk while the cows were stressed by corticotropin injection, confinement in a heat-humidity chamber, or environmental-heat stress by exposure during the hot summer months of June through November in southern Arizona. All three stressing conditions resulted in a moderate blood leucocytosis. Modest increases in somatic cell counts of milk were associated with corticotropin injection and environmental-heat stress. Positive correlations were recorded between blood leucocytes and somatic cell counts of milk in mastitis-free cows injected with corticotropin and between percent blood neutrophils and somatic cell counts of milk in environmental-heat stressed cows with no evidence of current mastitis.