Short communication: The effect of dry period duration and dietary energy density in early lactation on the rennet gelation properties of milk

This study was carried out to examine the effects of decreasing the dry period (DP) duration and altering the energy density of the diet during early lactation on the rheological characteristics of milk. Forty mature Holstein-Friesian cows were used in a completely randomized design with a 2 × 2 factorial arrangement of treatments. Cows were randomly assigned to 1 of 2 dry period treatments and 1 of 2 nutritional treatments. The DP treatments were continuous milking (CM) or an 8-wk standard dry period (SDP), and the nutritional treatments were a standard energy diet (SE) or a high energy diet (HE). Actual DP lengths were 6.3 ± 1.7 d and 62.1 ± 1.9 d for cows on the CM and SDP treatments, respectively. Milk samples were collected at 2, 6, and 10 wk postpartum. The concentrations of fat, protein, and lactose were determined in each sample. The rennet gelation properties were measured at 31°C using dynamic low-amplitude strain oscillatory rheometry. The following parameters were obtained from the resultant elastic shear modulus (G′): gelation time, maximum curd firming rate, and gel strength. Reducing the DP duration from 62 to 6 d resulted in increases in milk protein concentration (31.8 vs. 34.7 g/kg), maximum curd firming rate (2.58 vs. 3.60 Pa/min), and gel strength (69.4 vs. 90.5 Pa). Increasing the dietary energy density decreased percentage milk fat (43.1 vs. 37.7 g/kg) but otherwise had no effect. Gel strength was correlated with maximum curd firming rate (r = 0.99), and both variables were correlated with milk protein concentration (r = 0.71 and r = 0.73, respectively). The results indicate that decreasing the duration of the DP increased milk protein concentration and improved the rennet gelation properties of milk, but that dietary energy density had little effect.