Forward Modeling of Fluctuating Dietary 13C Signals to Validate 13C Turnover Models of Milk and Milk Components from a Diet-Switch Experiment

Abstract

Isotopic variation of food stuffs propagates through trophic systems. But, this variation is dampened in each trophic step, due to buffering effects of metabolic and storage pools. Thus, understanding of isotopic variation in trophic systems requires knowledge of isotopic turnover. In animals, turnover is usually quantified in diet-switch experiments in controlled conditions. Such experiments usually involve changes in diet chemical composition, which may affect turnover. Furthermore, it is uncertain if diet-switch based turnover models are applicable under conditions with randomly fluctuating dietary input signals. Here, we investigate if turnover information derived from diet-switch experiments with dairy cows can predict the isotopic composition of metabolic products (milk, milk components and feces) under natural fluctuations of dietary isotope and chemical composition. First, a diet-switch from a C3-grass/maize diet to a pure C3-grass diet was used to quantify carbon turnover in whole milk, lactose, casein, milk fat and feces. Data were analyzed with a compartmental mixed effects model, which allowed for multiple pools and intra-population variability, and included a delay between feed ingestion and first tracer appearance in outputs. The delay for milk components and whole milk was ∼12 h, and that of feces ∼20 h. The half-life (t½) for carbon in the feces was 9 h, while lactose, casein and milk fat had a t½ of 10, 18 and 19 h. The 13C kinetics of whole milk revealed two pools, a fast pool with a t½ of 10 h (likely representing lactose), and a slower pool with a t½ of 21 h (likely including casein and milk fat). The diet-switch based turnover information provided a precise prediction (RMSE ∼0.2 ‰) of the natural 13C fluctuations in outputs during a 30 days-long period when cows ingested a pure C3 grass with naturally fluctuating isotope composition.

Milk production and nutrient partitioning as measured by13C enrichment of milk components during C3and C4plant feeding in purebred Holstein and in Charolais × Holstein F2crossbred cows

“…The plateau of δ 13 C for lactose was achieved almost within the first 2 d of C 4 plant feeding, and half-life for changes in 13 C enrichment in lactose was much shorter than in casein and fat, which support recent findings [8,36]. On the other hand, carbon originating from dietary nutrients is less utilised immediately after feed intake for milk fat and casein synthesis, and endogenous sources such as body fat and muscle tissues provide substrates for milk fat and casein synthesis [6].…”

“…The transfer of nutrients originating from feed into milk can be estimated by feeding C 3 and C 4 plants, because C 4 plants have a higher natural 13 C abundance (δ 13 C) than C 3 plants [3][4][5]. Changes in δ 13 C in milk components in response to switch of diet from C 3 to C 4 plant feeding are used to estimate the dietary carbon fraction for milk production, providing information on nutrient transfer from diet to the mammary gland [6][7][8].…”

13C discrimination between diet, faeces, milk and milk components

“…In consequence, discrimination between diet and product can be greater than 2 ‰ and discrimination among milk constituents can be greater than 3 ‰. Discriminations as determined in this study are considered reliable, given (I) the small isotopic change between the feed of the pre-experimental period (pasture herbage) and the experimental period (pasture herbage and about 10 % of grain maize); (II) the long periods for equilibration and subsequent measurement after shifting to the experimental feed, which was 1 week and about 2-8 times longer than the half-life times in the experiments by Boutton et al [13,19] and lack of any indication that turnover of body material influenced the data over the following 8 weeks of the experiment; (III) the controlled conditions especially with stall-feeding, where feed and orts were determined daily; (IV) the comparably small trend in feed isotopic composition ( ∼ 0.7 ‰ for δ 13 C) during the experimental period, and (V) the large number of animals and milkings, yielding a total of 240 sample pairs per discrimination estimate. Altogether, these conditions underlay 95 % confidence intervals of far better than 0.2 ‰ in the case of diet-product discrimination.…”

“…The assessment of discrimination requires isotopic and energetic flow equilibrium conditions, with chemically and isotopically constant diet components which lead to a steady-state distribution of C isotopes between the different products of metabolism and stores in the animal. Otherwise, mobilization from old body pools that reflect a previous different diet may alter the isotopic composition of products and, hence, cause the apparent discrimination to vary over time until equilibrium discrimination is eventually reached [16,19]. In extremis, a complete equilibrium would require (isotopically and chemically) uniform feed from birth to death, similar to the classical experiments by DeNiro and Epstein [9].…”

“…For specific animal parts or products with fast turnover, near-equilibrium conditions may be obtained earlier. Body proteins in heifers required more than 160 days to attain a new isotopic equilibrium [20], while 1 week was apparently sufficient to reach a new equilibrium in milk carbon and nitrogen isotope composition of dairy cows following a change of diet isotope composition [19,21,22] with equilibration periods varying between 4 days for whole milk [13] and 2 weeks for casein [6]. Digesta of herbivores finally excreted as faeces also have short turnover times of several days [19,23,24].…”