title: Variation in plasma concentrations of insulin-like growth factor-I in pasture-fed Holstein cows.
abstract: The objective of this 5-wk study was to determine dietary effects on plasma concentrations of insulin-like growth factor-I (IGF-I), as well as milk production and milk components in pasture-fed dairy cows. Thirty-two Holstein cows 4 to 5 wk postpartum were randomly assigned to 4 dietary subgroups. Feed was provided twice daily ad libitum at 0900 and 1600 h composed of fresh-cut pasture, meadow hay, and pelleted cereal grain to achieve differing levels of DMI and ME density (LL: 16.6 kg of DMI and 174 MJ of ME; HL: 17.3 kg of DMI and 181.1 MJ of ME; LH: 15.4 kg of DMI and 183.1 MJ of ME; HH: 17.9 kg of DMI and 213.3 MJ of ME, with the first letter indicating DMI and the second ME, and with H indicating high and L indicating low, respectively). The first day cows were placed on their diets was designated d 0. Concentrations of IGF-I were measured in frozen-thawed samples of plasma using a verified ELISA. Dietary treatment had affected plasma concentrations of IGF-I by d 7 with cows on high ME diets having greater IGF-I concentrations at d 14 (83.7 vs. 45.6 ng/mL) than cows on the low ME diets. The level of DMI had less effect on plasma concentrations of IGF-I at d 14 (72.2 vs. 57.1 ng/mL). Dietary treatment effects on these concentrations had stabilized by d 21. Day-to-day variation in mean plasma concentrations of IGF-I within each dietary treatment was low during an intensive period of daily sampling for 14 d (from d 22 to 35). Within-cow day-to-day variation was also low compared with that among cows within the same dietary group and was associated with a high repeatability in the day-to-day concentration of IGF-I in individual cows. Intraclass correlation coefficients for IGF-I ranged from 0.56 (+/- 0.14) to 0.88 (+/- 0.06) with a combined (pooled) value for the 4 subgroups of 0.77 (+/- 0.05). The ME and DMI effects (H vs. L) at d 35 were 79.3 vs. 41.4 and 62.0 vs. 55.7 ng/mL, respectively. Although the ME and DMI differences also affected milk yield and compositional parameters, the effects were not as proportionately great as those measured for IGF-I. Altering the ME or DMI components of the pasture-based diets produced changes in plasma IGF-I concentrations that did not become stabilized for 3 wk, but were then highly repeatable for individual cows within each dietary group. Both observations have relevance to interpreting data related to plasma concentrations of IGF-I in lactating Holstein cows.
authors.affiliation: School of Agriculture and Food Systems, the University of Melbourne, Victoria 3031, Australia.
authors.forename: F Y
authors.initials: FY
authors.lastname: Obese
authors.forename: A R
authors.initials: AR
authors.lastname: Rabiee
authors.forename: K L
authors.initials: KL
authors.lastname: Macmillan
authors.forename: A R
authors.initials: AR
authors.lastname: Egan
authors.forename: S
authors.initials: S
authors.lastname: Humphrys
authors.forename: G A
authors.initials: GA
authors.lastname: Anderson
mesh_terms: D000821:Animal Feed; D000818:Animals; D002417:Cattle / Q000097:blood*; D004032:Diet; D002523:Edible Grain; D004797:Enzyme-Linked Immunosorbent Assay; D005223:Fats / Q000032:analysis; D005260:Female; D005511:Food Handling / Q000379:methods; D007334:Insulin-Like Growth Factor I / Q000032:analysis*; D007774:Lactation / Q000502:physiology; D007785:Lactose / Q000032:analysis; D008892:Milk / Q000737:chemistry; D008894:Milk Proteins / Q000032:analysis; D049590:Postpartum Period
chemical_list: D005223:Fats; D008894:Milk Proteins; D007334:Insulin-Like Growth Factor I; D007785:Lactose