title: A method to estimate cow potential and subsequent responses to energy and protein supply according to stage of lactation.
abstract: Milk responses to dietary change are influenced by the relative production level, that is, the distance between observed production and potential production. The closer the animal is to its potential, the smaller the expected response is to extra nutrients. Therefore, the aim of this work was to provide a method to quantify cow potential, to estimate subsequent responses to changes in nutrient supply. The observed efficiencies in net energy for lactation (NEL) and metabolizable protein (MP) are proposed as a basis to estimate the relative production level of the animal. The rationale for using NELand MP efficiency (ratios of milk energy yield/NELabove maintenance supply and milk protein yield/MP above maintenance supply) builds on the uniformity of the observed relationships between size of the milk responses and extra NELsupply and MP supply, when centered on a given efficiency. From there, a pivot nutritional situation where MP and NELefficiency are 0.67 and 1.00, respectively, was defined, from which milk responses could be derived across animals varying in production potential. An implicit assumption of using response equations centered on reference efficiency pivots is that the size of the response to a fixed change in nutrient supply, relative to the pivot, is identical for animals with different production capacities. The proposed approach was evaluated with 2 independent data sets, where different dietary treatments were applied during the whole lactation. In these data sets, MP and NELabove maintenance supply were calculated weekly using the recently updated INRA Systali feed units system. Differences in NELand MP supply above maintenance between the extreme dietary treatments were large, on average 667 g of MP/d and 13 MJ of NEL/d (3.11 Mcal/d) in the first data set, and 513 g of MP/d and 29 MJ of NEL/d (6.93 Mcal/d) for the second data set. Milk energy yield and milk component yields were predicted with root mean square prediction errors between 7.6 and 13.5% and concordance correlation coefficients between 0.784 and 0.934, respectively. Assessed by the Akaike's information criterion, significant differences existed in the accuracy of prediction for milk energy yield and milk component yields between stages of lactation. However, the effects of stage of lactation were not consistent between data sets and, for most of the predicted variables, relatively small. We concluded that the pivot concept can be used to predict milk energy yield and milk component yields responses to dietary change with a good accuracy for diets that are substantially different and across all stages of lactation.
authors.affiliation: UMR Modélisation Systémique Appliquée aux Ruminants, INRA, AgroParisTech, Université Paris-Saclay, 75005 Paris, France; Trouw Nutrition R&D, PO Box 220, 5830 AE Boxmeer, the Netherlands. Electronic address: jean-baptiste.daniel@trouwnutrition.com.
authors.forename: J B
authors.initials: JB
authors.lastname: Daniel
authors.affiliation: UMR Modélisation Systémique Appliquée aux Ruminants, INRA, AgroParisTech, Université Paris-Saclay, 75005 Paris, France.
authors.forename: N C
authors.initials: NC
authors.lastname: Friggens
authors.affiliation: Trouw Nutrition R&D, PO Box 220, 5830 AE Boxmeer, the Netherlands.
authors.forename: H
authors.initials: H
authors.lastname: Van Laar
authors.affiliation: Agri-Food and Biosciences Institute, Hillsborough, Co. Down, BT26 6DR, United Kingdom.
authors.forename: C P
authors.initials: CP
authors.lastname: Ferris
authors.affiliation: UMR Modélisation Systémique Appliquée aux Ruminants, INRA, AgroParisTech, Université Paris-Saclay, 75005 Paris, France.
authors.forename: D
authors.initials: D
authors.lastname: Sauvant
mesh_terms: D000821:Animal Feed; D000818:Animals; D002417:Cattle; D004032:Diet; D005260:Female; D007774:Lactation; D008892:Milk; D008894:Milk Proteins / Q000494:pharmacology
chemical_list: D008894:Milk Proteins
keywords: energy; milk composition; potential; protein; response