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 this experiment was to study the effect of two average daily gains between 5 and 15mo of age on body measurements, fat deposition, metabolic profile, interaction with age at first calving, and 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, 0.7kg/d) or an accelerated (H, 0.9kg/d) average daily gain diet from 5 to 15mo of age. Half of the heifers on M were programmed to be bred at 15mo (ME), and half at 18mo (ML) of age; heifers on H were programmed to be bred at 15mo of age (HE). Every 28d, body weight, withers height, hip height, body length, heart girth, and body condition score were recorded. Also each week in one herd from −14 to 70d post-calving, the body condition score was evaluated. In one herd, metabolic profile was assessed at 9 and 15mo of age. Growth curve parameters were estimated for the body measurements for each heifer. Metabolic profiles from both ages (9 and 15mo) were processed by Principal Components Analysis (PCA). For the first lactation, milk production and composition were recorded. The M heifers grew slower (0.73 vs. 0.81 kg/d) than H heifers, and this difference was reflected also in a faster heart girth gain in H than M heifers (0.184 vs. 0.169 cm/d). The body condition score of heifers fed on H diet increased faster than that of heifers fed on M diet by 9mo of age. At this age, plasma concentration of urea, Ca, Na, albumin, and γ-glutamyl transferase activity were greater in H than in M heifers. At 15mo of age, plasma concentration of bilirubin and alkaline phosphatase activity were lower in H than M heifers. At 9mo, plasma urea concentration was correlated with hip height and heart girth curve parameters. At 15mo, plasma glucose concentration was correlated with body weight curve parameters. Plasma ceruloplasmin concentration was correlated with hip height curve parameters at both ages, and with body weight curve parameters at 15mo. The results from the PCA have shown that the first principal component was correlated with the parameters of the growth functions for body weight, hip height, and heart girth. Experimental diet and age at first calving per se did not affect milk production in first lactation, but the peak of milk production of HE heifers occurred 15d earlier than that of ML heifers.