Effects of body condition and diet in late gestation on the subsequent health and performance of dairy cows

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The objective of the experiment reported here was to compare the effects of a total mixed ration (TMR) with a pasture hay diet on the healthy transition from late gestation to early lactation of dairy cows with different body condition scores [BCS; 4.2 (thin) and 5.8 (fat) units on an 8-point scale]. It was hypothesised that providing the dietary cation-anion differences (DCAD) of the two diets were similar, more clinical and subclinical hypocalcaemia would be recorded in fat cows than in thin cows, and that this would not be affected by type of diet. The design of the experiment included 72 cows in a 2 × 2 factorial (two BCS × two types of diet fed for the 4 weeks before expected calving date) where cows were fed individually indoors before calving, and grazed pasture and fed concentrates after calving. The calculated DCAD of the two diets were similar (22.1 v. 18.4 meq/100 g DM for the hay and TMR diets) as were the precalving urine pH levels (7.73 and 7.74) of the cows. Intakes of hay and TMR during late gestation were on average 8.8 and 9.3 kg DM/cow.day, respectively, but the patterns of intake varied, with hay intake declining over the 3 weeks before calving and that of TMR remaining relatively constant until just before parturition. Cows fed hay had higher (P < 0.05) non-esterified fatty acid (0.90 v. 0.64 mmol/L), β-hydroxybutyrate (0.61 v. 0.43 mmol/L) and calcium (2.50 v. 2.24 mmol/L) concentrations, and lower (P < 0.05) glucose (3.03 v. 3.21 mmol/L) concentrations, than the cows fed the TMR before calving. Immediately after calving, most of these differences had disappeared. Average blood serum calcium concentrations within 12 h of calving were similar (P > 0.05) in cows fed each diet (1.94 and 1.79 mmol/L for hay and TMR, respectively). At calving, concentrations of serum calcium and urea were greater (P < 0.05) in the thin cows than the fat cows. Only fat cows had clinical hypocalcaemia (serum calcium concentrations <1.40 mmol/L) or needed to be treated for milk fever either at calving or in the week following calving, whereas there tended to be more thin cows with subclinical hypocalcaemia (serum calcium concentrations of 1.40–2.00 mmol/L). Subclinical ketosis (serum β-hydroxybutyrate concentrations of 1.00–3.00 mmol/L) was more prevalent in fat cows than in thin cows; otherwise health disorders were evenly spread across treatments. In the first 5 weeks of lactation, there were no significant (P > 0.05) effects of late gestation diet on any aspect of production. In contrast, there were positive (P < 0.05) effects of BCS on milk yield and concentrations of fat and protein in milk. The hypothesis concerning the amount and severity of hypocalcaemia in fat cows was only partially supported by the results, but type of diet had no effects on cow health or subsequent production.