Effect of energy allowance during the dry period on insulin resistance and metabolic adaptation in transition dairy cows on grass silage-based diets.

The research documented in publications I-IV involved studies in dry, latepregnant
Ayrshire dairy cows on grass silage (GS) based diets (I-IV). The
principal aim was to investigate the effect of prepartal plasma non-esterified
fatty acids (NEFA) level (I) and the effect of prepartal dietary energy intake
(II-IV) on the development of insulin resistance (IR) during late pregnancy (IIV)
and changes in insulin resistance in early lactation (II-IV). Detailed,
extensive physiological studies were conducted to understand the mechanisms
underlying the development of maternal insulin resistance and to investigate
the impact of changes in dietary energy level and subsequent changes in
plasma NEFA concentration prepartum. The insulin resistance was assessed
by interpretation of data from intravenous glucose tolerance test (IVGTT) with
minimal model (MM) approach (I-III) and by insulin challenge (IC; I, II) data.
Besides insulin resistance, also the impact of prepartal energy intake on
metabolic adaptations, tissue deposition and mobilisation as well as dry
matter intake (DMI) and lactational performance were investigated in
publications III and IV.
In publication I, the key objective was to evaluate the effects of increment
of plasma NEFA concentration, typically observed during the last weeks of
pregnancy and in early weeks of lactation in dairy cows, on glucose tolerance
and responsiveness or sensitivity to insulin as assessed by IVGTT and IC. The
greater NEFA levels were achieved by abomasal infusion of tallow (TAL) or
camelina oil (CAM). Compared with water infusion (CON), infusion of lipids
increased basal plasma NEFA concentrations by around 50%, to an equal level
than what was found in dairy cows 2 to 1 weeks prepartum on GS-based diets
(II-IV). Elevation of plasma NEFA concentration impaired glucose clearance
and decreased insulin secretion during metabolic challenges. These data
suggest that elevated plasma NEFA concentrations impaired whole-body
insulin responsiveness and sensitivity in dry cows in late pregnancy. As
assessed by MM indices, both the disposition and the insulin sensitivity
indices were greater after CAM than TAL infusion during IVGTT. Compared
with TAL infusion, CAM had an insulin-sensitizing effect which was most
likely caused by alterations in plasma profiles of major long-chain fatty acid
(FA) groups. A 50% increment in the percentage of polyunsaturated FA (C18:2
and C18:3) and a similar decrease in the percentage of monounsaturated FA
(C16:1 and C18:1) was found in plasma FA profiles after CAM infusion when
compared with TAL.
In publication II, the dietary effects on insulin resistance were assessed not
only by the level of energy intake but also by comparing tissue responses to
glucose and insulin in late pregnancy vs. early lactation. Compared with
controlled energy intake (CEI), the effect of prepartal overfeeding and gradual
restriction of energy (HEI) had a minor effect on whole-body insulin resistanceduring the transition period. An attenuated prepartal NEFA response to
endogenous insulin was found in HEI cows suggesting a more refractory
adipose tissue to insulin than in CEI. After parturition, this effect was reversed.
Across the dietary treatments, both basal and stimulated insulin concentration
decreased after parturition as a result of a lower response to a similar secretory
stimulus than before parturition and due to increased clearance of insulin
postpartum. Compared with prepartal IVGTT, glucose disposal was enhanced
postpartum across the dietary treatments. A hyperbolic relationship denoted
as the disposition index (DI) was observed during the IVGTT. Compared with
prepartal glucose and insulin dynamics across the diets, the MM indices point
to increased insulin resistance shortly before than shortly after parturition.
However, low insulin concentration is the major factor regulating the use of
glucose by peripheral tissues in early lactation. The lack of dietary effect on
whole-body insulin resistance in publication II was most likely due to minor
dietary effect on tissue accretion between treatment groups, although the
lower prepartal plasma NEFA concentration in HEI than in CEI cows suggests
enhanced lipid deposition in adipose tissue before parturition, facilitated by
higher plasma insulin (IV). No dietary effect on plasma hormone and
metabolite concentrations or total DMI was found after parturition. High
energy intake during the dry period tended to decrease milk yield after calving
(IV).
In publication III easily applicable diets suitable for loose housing systems
were compared. An ad libitum allowance of GS (HEI) induced a more
pronounced BW and BCS change prepartum when compared with a GS-diet
diluted with wheat straw (CEI). HEI cows demonstrated a compensatory
insulin response to glucose in prepartal IVGTT which preserved glucose
tolerance of peripheral tissues. The HEI diet reduced and delayed NEFA
suppression suggesting decreased insulin sensitivity and responsiveness in
adipose tissue prepartum. The high NDF-content in CEI diet probably
decreased ruminal propionic acid production as reflected by lower prepartal
glucose and insulin CEI cows. Prepartal energy level did not affect metabolic
flexibility of transition dairy cows as assessed by the absence of dietary effect
on mobilisation of body reserves, plasma metabolites and hormones, and DMI
after calving, whereas milk yield was greater from week 5 onward in HEI than
in CEI.
The moderate negative effects of gradual restriction of prepartal energy and
dilution of energy by mixing GS with wheat straw on early lactation production
response demonstrated that these feeding practices were not optimal for
transition dairy cows. A moderate or ad libitum overfeeding affected
peripheral insulin resistance in the level of prepartal lipid metabolism, while
ad libitum overfeeding of GS induced changes in prepartal glucose metabolism
as well. Both the difference in energy intake and the composition of the diet
contributed to the observed effects on glucose and NEFA dynamics
orchestrated via changes in insulin concentration in the transition period.