Effect of average daily gain on body size, metabolism, and milk production of Italian Holstein heifers raised on two different planes of nutrition and calving at two different ages

The aim of the present experiment was to verify if average daily gain (ADG) can affect body growth and development, fat deposition, metabolic profile and, by interacting with age at first calving (AFC), also milk production in first lactation. A total of 141 Italian Friesian heifers, in two experimental herds, were allotted to 2 feeding regimens formulated for a moderate (M, 0.7 kg/d) or an accelerated (H, 0.9 kg/d) ADG diet from 5 to 15 mo of age. Half of the heifers on M were programmed to be bred at 15 mo (ME), and half at 18 mo (ML) of age; heifers on H were programmed to be bred at 15 mo of age (HE). Every 28 d, body weight (BW), wither height (WH), hip height (HH), body length (BL), and hearth girth (HG) were measured; in those moments and each week from –14 to 70 d after calving (only in one herd), the body condition score was evaluated. In one herd only, metabolic profile was assessed at 9 and 15 mo of age. Growth curve parameters were estimated for BW, HH, BL, and HG for each heifer. Metabolic profiles from both ages (9 and 15 mo) were processed by Principal Components Analysis. At first calving, milk production and composition were recorded. The BW of M heifers grew slower (0.74 vs. 0.83 kg/d) than H heifers, and this difference was reflected also in a faster HG gain in H than M heifers (0.176 vs 0.165 cm/d). The BCS of heifers fed on H diet grew faster than that of heifers fed on M diet by 9 mo of age. At this age, plasma concentration of urea, Ca, Na, albumin, and -glutamyl transferase activity were greater in H than in M heifers. At 15 mo of age, plasma concentration of bilirubin and alkaline phosphatase activity were lower in H heifers than M heifers. At 9 mo, plasma urea concentration was correlated with HH and HG curve parameters. At 15 mo, plasma glucose concentration was correlated with BW curve parameters. Plasma ceruloplasmin concentration was correlated with HH curve parameters at both ages, and with BW curve parameters at 15 mo. The main results from the PCA on metabolic profile in the growing heifers have shown that PC1 was mainly composed of total protein, albumin, Na, Cp, glucose, Ca, and globulin. This component was correlated with the parameters of the growth functions for BW, HH, and HG. Experimental diet and AFC per se did not affect milk production in first lactation, but there was a difference of about 15 d among groups for what concerns the time at which the peak milk production occurred.