Aspects of rumen adaptation in dairy cattle : morphological, functional, and gene expression changes of the rumen papillae and changes of the rumen microbiota during the transition period

In dairy cattle the nutrient requirements change rapidly around calving. During the dry period nutrients are required for maintenance, recovery from the previous lactation, and fetal growth. After calving, milk production commences and the energy requirements can increase by a factor 3 to ~184 MJ net energy for lactation during the first weeks of lactation, compared with the dry period, whereas feed intake doubles to ~24 kg dry matter (DM)/d compared with the dry period. In addition, high quality lactation rations are fed, usually containing a sizable portion of concentrate, thereby increasing fermentable organic matter (FOM) intake to ~14 kg/d. As a result, daily volatile fatty acid (VFA) production by the rumen microbiota increases from ~60 mol/d during the dry period to ~132 mol/d during early lactation. To maintain rumen pH at levels favorable for microbial fermentation, and prevent a negative impact on production and health, clearance of the produced VFA is essential. This mainly occurs through absorption over the rumen wall. The increase in capacity of the rumen for absorption of VFA is associated with morphological and functional changes of the rumen papillae which cover the rumen wall. However, current knowledge of these changes as they occur around calving is scarce (Chapter 1). Increasing our understanding of the adaptation of the rumen can provide new insights to optimize dairy cattle nutrition and thereby health, welfare, and production. The objective of this thesis was to study the adaptation of the rumen to ration changes during the dry period and early lactation. Changes in rumen papillae morphology, fractional absorption rate of VFA (<em>k</em>aVFA), and changes in the composition of the rumen microbiota were the primary targets for study. In addition, the expression of genes and proteins associated with absorption and metabolism of VFA by the rumen epithelium were studied to better understand the relationship between functional changes and morphological changes of the papillae. Uniquely, all these aspects were studied in parallel in the same dairy cows during the dry period and early lactation using a repeated measurement setup. Two experiments were conducted. In the lactation experiment, the effect of transition from the dry period to the subsequent lactation, and the effect of early lactation concentrate build-up strategy on the adaptation of the rumen were studied. In the dry period experiment, the effect of feeding supplemental concentrate during the late dry period in order to ‘prepare’ the rumen for the lactation was studied. Treatments of both experiments were aimed at creating a difference in FOM intake (kg/d) and thereby VFA production (mol/d), as VFA production was hypothesized to affect rumen papillae development and thereby the capacity for VFA absorption. During the lactation experiment, intake of FOM did not change during the dry period (5.7 kg/d), but increased during the subsequent lactation to 15.0 kg/d at 80 d postpartum (pp). In addition, the rapid increase in concentrate allowance resulted in a temporarily 22% greater FOM intake compared with a gradual increase at 16 d pp (Chapter 2). The total production rate of VFA, measured using an isotope dilution technique (Chapter 3), was affected by these changes in FOM intake and increased 2.3 fold to 123 mol/d after calving, compared with the dry period (53 mol/d). The temporarily greater FOM intake with the rapid increase in concentrate allowance at 16 d pp coincided with a 54% greater propionate production (34 mol/d) compared with a gradual increase in concentrate allowance, whereas acetate (66 mol/d) and butyrate (10 mol/d) production were not affected. Papillae surface area (Chapter 2) decreased by 19% between 50 d antepartum (ap) and 3 d pp to 28.0 mm2, but increased during early lactation to 63.0 mm2. Papillae surface area increased faster with the rapid increase in concentrate allowance and surface area was 38, 34 and 22% larger at 16, 30, and 44 d postpartum respectively, than with a gradual rate of increase of concentrate allowance. Histology (Chapter 2) revealed that rumen papillae and epithelium thickness decreased slightly after calving, but were not affected by the concentrate treatment. Feeding concentrate during the dry period did not affect daily FOM intake (6.0 kg/d) but did increase VFA concentration in the rumen fluid by 21 m<em>M</em> to 121 m<em>M</em>, and increased papillae surface by 29% (Chapter 4). However, the increased papillae surface area in the dry period was not maintained to the subsequent lactation period. After calving, papillae surface area increased by 50% to 58.0 mm2 at 45 d pp. The postpartum development of the rumen papillae was not affected by the treatment during the dry period. These results indicate that rumen papillae respond to changes in FOM and VFA production intake during the dry period and early lactation, and that the magnitude of this response depends on the rate of change in FOM intake. During both experiments, <em>k</em>aVFA was measured using a buffer incubation technique in an empty washed rumen. During the lactation experiment (Chapter 3), in accordance with the developments in papillae surface area, the <em>k</em>aVFA decreased during the dry period from 0.48/h at 50 d ap to 0.34/h at 3 d pp. During the subsequent lactation, it increased rapidly to 0.56/h at 16 d pp and further to 0.72/h at 80 d pp. However, the greater papillae surface area due to the rapid increase in concentrate did not coincide with a greater <em>k</em>aVFA. During the dry period experiment (Chapter 4), <em>k</em>aVFA increased after calving by 50% to 0.48/h at 45 d pp, but the increase in papillae surface area due to supplemental concentrate during the dry period did not affect the <em>k</em>aVFA during the dry period (0.36/h) or the subsequent lactation. These results indicate that papillae surface area is not the limiting factor for <em>k</em>aVFA. Changes in the expression of genes were studied at the mRNA level in papillae tissue from both experiments (Chapter 5). The expression of apoptosis related genes was not affected by sampling day or its interaction with treatment for both experiments, suggesting papillae proliferation during the transition period was mainly the result of an increased mitosis rate. The limited changes in the expression of genes associated with rumen epithelial transport and metabolism of VFA in dairy cows during the transition period do not suggest that these capacities of the epithelium increased per unit of surface area. Thus the major response to the increase in daily VFA production after calving was tissue proliferation. In addition, papillae from the lactation experiment were used to study expression at the protein level using immunoblotting. Results showed that expression of several proteins changed during early lactation indicating modulation of intracellular pH regulation and sodium homeostasis, and VFA metabolism. Only for one gene, a significant but weak correlation between the examined mRNA and protein expression levels was observed, indicating that care must be taken when interpreting results obtained at either level. Ration changes associated with the transition from the dry period to lactation affected the rumen microbiota during the lactation experiment (Chapter 6). The rapid increase in concentrate allowance postpartum temporarily decreased bacterial community richness by as much as 30% compared with a gradual increase in concentrate. This transient depression in bacterial community richness with a rapid, but not a gradual, rate of increase of concentrate allowance pp indicates that the rate of change in ration composition and feed intake has a greater effect than the change in ration composition and feed intake level as such. The relative abundances of most major bacterial taxa were affected by the transition to lactation, but few were affected by the rate of increase of the concentrate allowance. The relative abundances of rumen protozoal taxa changed after calving, and were affected by the concentrate treatment. However, differences between treatments groups disappeared again when concentrate intake became similar. The archaeal community was likewise affected by both the transition to lactation and the treatment. The observed changes in rumen microbiota composition, including changes in bacterial community richness, did not appear to affect the fractional degradation rate of NDF, starch, CP, and OM measured in situ using a nylon bag technique. The results in the present thesis show that morphologically and functionally the rumen papillae can adapt rapidly to the changes in FOM intake and daily VFA production associated with the transition from the dry period into the subsequent lactation. However, the contrast in response of rumen papillae surface area development and the fractional absorption rate of VFA to the concentrate treatments indicates that papillae surface area is not the limiting factor for VFA absorption. This proposition is further supported by the limited histological changes of the rumen epithelium and limited changes in gene expression. Considering that the capacity for absorption and metabolism of VFA per unit of papillae surface area remains similar, an extra-epithelial factor, likely visceral blood flow, limits VFA absorption. The capacity of the rumen to adapt after calving and the limited beneficial effect of supplementing concentrate during the dry period indicate that dry period feeding strategies can best be optimized for the prevention of periparturient diseases.