Implications of acute or chronic pasture restriction on indicators of metabolic status in grass-based dairy cows

Abstract Text:

Annual variation in pasture growth rate has a major effect on grass availability for grazing dairy cows, especially at the onset of lactation in early spring. The objective was to determine the effect of imposing acute (two weeks) or chronic (six weeks) periods of varying levels of pasture restriction on indicators of metabolic health and hepatic gene expression in early lactation dairy cows. Holstein Friesian and Jersey crossbred cows (n = 96) were randomly assigned to one of four daily herbage allowances (DHA: 60, 80, 100 and 120% of intake capacity) for either two or six weeks (12 cows per treatment) during early lactation for 2 consecutive years.  During the experimental period no supplemental concentrates were fed. Milk samples were collected three times per week for progesterone analysis to determine effects on estrous cyclicity. Blood was collected once weekly during the study to determine circulating concentrations of glucose, non-esterified fatty acids (NEFA) and β-hydroxybutyrate (β-HBA). In Year 2 of the study, liver biopsies were collected from a subset of cows assigned to the 60% DHA for 2 weeks, 60% DHA for 6 weeks and the 100% DHA for 6 week treatments at experimental weeks 0, 2 and 6. Reverse-transcription quantitative PCR (RT-qPCR) analysis was used to determine the mRNA abundance of 24 target genes related to energy metabolism.  Data were analyzed using PROC MIXED of SAS.

The DHA treatments had no effect on resumption of cyclicity, mean plasma glucose or NEFA concentrations. Mean plasma β-HBA concentrations during the treatment periods were increased in cows on the restricted DHA treatments for two weeks (1.25 ± 0.09, 1.08 ± 0.1, 1.03 ± 0.1, 0.9 ± 0.09 mmol/l; P<0.05; 60%, 80%, 100%, and 120%, respectively) and six weeks (1.43 ± 0.09, 1.32 ± 0.1, 1.06 ± 0.1, 1.04 ± 0.09 mmol/l; P<0.05; 60%, 80%, 100% and 120%, respectively). At week six, 60% DHA increased mean expression of glucose-6-phosphatase, pyruvate carboxylase, carnitine palmitoyltransferase 1A, acyl-CoA synthetase long-chain 1 and decreased mean expression of fatty acid synthase and acetyl-CoA-carboxylase. DHA had no effect on mRNA abundance of IGF-1, but 60% DHA for 6 weeks increased expression of insulin-like growth factor binding protein-2. We conclude that imposing acute periods of restricted DHA had only modest effects on metabolic health in early lactation dairy cows.

Keywords: gene expression, metabolites, restriction