“Effects of a ration change from a silage and concentrate- to a pasture-based ration on the production, health and rumen physiology of dairy cows”

In pasture-based dairy production systems, dairy cows often receive a silage- and concentrate-based ration (= TMR, total mixed ration) during wintertime and are then gradually introduced to fresh herbage in spring. The present study aimed to investigate how the transition to this new nutritional situation influenced different production and health indicators as well as rumen physiology. A 10-week trial (wk 1-10) was performed in spring 2014, which included 60 dairy cows of the German Holstein breed (166 ± 23 days in milk, 23.5 ± 3.7 kg milk/d; means ± SD) of which 10 were rumen- and duodenum-fistulated. The cows were divided into a pasture- and a confinement group (PG and CG, n = 29 and 31, each group contained 5 fistulated animals). The CG stayed on a TMR-based ration (35% corn silage, 35% grass silage, 30% concentrate; DM basis), while the PG was gradually transitioned from a TMR to a pasture-based ration (wk 1: TMR-only, wk 2: 3 h/d on pasture & TMR indoors, wk 3 & 4: 12 h/d on pasture & TMR indoors, wk 5-10: pasture-only). A continuous grazing system was implemented on a ryegrass dominated pasture and temperature humidity indices (THI) were assessed based on a continuous recording of temperature and humidity; indoors as well as outdoors. Dry matter intake (DMI) indoors was measured using automatic weighing throughs. To estimate the DMI outdoors the n-alkane technique (in wk 7 and wk 9) and exclosure cages (in wk 5-6, wk 7-8 and wk 9-10) were used. Milk production and body weight (BW) were measured twice a day at milkings. Samples for milk components were collected twice a week and body condition score (BCS) was assessed in 14-d intervals. Urine samples were collected once a week and analyzed for creatinine and purine derivatives concentrations to measure alterations in nitrogen excretion and microbial protein production. To assess possible negative effects of this ration change on metabolic and liver health different clinical chemistry measures and complete blood counts were measured weekly. To observe changes in rumen fermentation patterns pH, volatile fatty acids (VFA), NH3-N and lipopolysaccharide (LPS) concentrations were measured in rumen fluid samples collected medially and ventrally on a weekly basis. To evaluate a possible subacute ruminal acidosis (SARA) risk ruminal pH was additionally recorded weekly during 1-4 consecutive days using continuous ruminal pH measuring devices. In wk 1, wk 5 and wk 10 the total rumen content was weighed and the DM and non-DM content determined. Subsequently, rumen papillae were collected at three locations to determine the mean papillae area and for a histopathological evaluation. Additionally, a VFA absorption test (VFA-AT) was performed. DMI from TMR, milk production, body weight (BW) and BCS decreased as soon as the PG had partial access to pasture. Milk production and BW decreased even further in the first week on a full grazing ration, but thereafter BW increased again and milk production stabilized. DMI estimation using the n-alkane method in wk 7 and wk 9 revealed an increase in DMI from pasture between the two time points and indicates an adaptation of grazing behavior and metabolism over several weeks. Increased serum β-hydroxybutyrate and nonesterified fatty acids concentrations at several time points as well as a continuous BCS decrease during the whole course of the trial indicate an energy deficit in the PG. These alterations in production and metabolic parameters in the PG were also reflected in various rumen variables. Mean rumen pH and molar acetate proportions decreased, and molar butyrate proportions increased continuously over the course of the trial, which can most likely be ascribed to an increased intake of rapidly fermentable carbohydrates. During the first weeks on a full grazing ration (wk 5-7) variation of rumen pH decreased and in wk 5 a lower rumen content, papillae surface area and potential for VFA absorption was observed. In wk 8-10 variation of rumen pH and total VFA concentrations increased again, and acetate/propionate ratio decreased. In wk 10 rumen content, papillae area and VFA absorption characteristics similar to initial levels were observed. Although continuous rumen pH assessments and LPS concentrations did not reveal an increased risk for subacute rumen acidosis (SARA) during the adaption period, histopathology of rumen papillae and VFA absorption potential indicate a possible risk for rumen health. An increased risk for SARA was observed in wk 9 and wk 10 in the PG, but rumen LPS concentrations and histopathology were not adversely affected. As expected an increase in N excretion occurred in the PG, as indicated by an increase in serum and milk urea concentrations as well as an increase in the urine total N to creatinine ratio. However, no biological relevant changes were observed for serum albumin, total protein, cholesterol, aspartate transaminase, γ-glutamyltransferase and glutamate dehydrogenase concentrations as well as for white and red blood cell counts. It was therefore concluded that higher metabolic N concentrations did not have any negative transitional impact on the liver. During wk 5 and wk 7-8 mild heat stress was observed in both groups and metabolic variables were consequently altered. Serum glucose concentrations decreased and urine total N/creatinine and purine derivatives/creatinine ratios increased. Concurrently rumen LPS concentrations increased in both groups. Results of the present study suggest that during the transition from a TMR to a pasture-based ration an initial decrease in DMI occurred accompanied by possible negative effects on rumen physiology. After a behavioral and metabolic adaption DMI and ruminal fermentation rate increased again with no adverse effects on rumen morphology and VFA absorption capacity, although rumen pH after adaptation to pasture indicated increased risk of SARA.