Impact of substituting compound feed with increasing levels of fresh grass-clover on nitrogen metabolism and plasma metabolites of sows

Abstract

The purpose of this study was to identify biomarkers for fresh grass-clover intake and determine the digestibility and energy value of fresh grass to estimate voluntary fresh grass-clover intake in sows kept on pasture according to organic production practice. A total of sixteen multiparous dry sows (Danish Landrace x Danish Yorkshire) were housed in metabolism cages for 2 periods of 5 d. To study dietary effects, sows were fed one of four mixed treatments where increasing proportions of a basal commercial organic sow compound feed was partly replaced with 0, 2, 4, or 6 kg fresh grass-clover collected 3 wk after the previous cut. Sows were fed similar amounts of metabolizable energy (ME). Total collection of urine and feces was performed on a daily basis for 5d, and blood samples were collected from the jugular vein on the last day of feeding. Plasma metabolites were analyzed using a non-targeted liquid chromatography-mass spectrometry (LCMS) approach. Analyzed plasma metabolites (area under the curve in arbitrary units from the LCMS) were screened for correlation with grass intake. Data on nutrient digestibility and plasma metabolites were analyzed using a MIXED procedure while accounting for repeated measurements. Apparent total tract digestibilities of dry matter, organic matter, nitrogen, and energy of fresh grass-clover were 72, 64, 71, and 68%, respectively, using the regression method. The N intake increased linearly with increasing fresh grass-clover intake (P < 0.001). There was no evidence of altered N deposition and N utilization in response to increasing grass intake (P > 0.05). There was a linear increase in plasma urea content (2.64 to 4.39 mM) when grass intake increased from 0 to 6 kg/d (P = 0.02). Plasma glucose, lactate, creatinine and NEFA, and triglycerides were not affected by increased grass-clover intake. The daily ME contribution from fresh grass-clover (MJ/d) was found to be highly positively correlated with plasma pipecolic acid (Y = 0.0289 X; r2 = 0.91; P < 0.001) and a metabolite tentatively identified as plasma bisnorbiotin (Y = 0.482 X; r2 = 0.92; P < 0.001). In conclusion, fresh grass-clover intake of sows was highly correlated with plasma pipecolic acid and plasma bisnorbiotin concentration and apparent total tract digestibility of DM, OM, N, and energy estimated for 100% fresh grass-clover intake in dry sows was 64 to 72% using the regression method.