The liver during transition: central pillar of formulation strategies
Hepatic functionality and health must be one of the aspects that nutritionists must keep in mind as priority, as it is critical for proper transition into lactation. Despite the importance of taking care of liver metabolism, 20 to 40% of dairy cows are unable to cope with the changes caused by lactation and develop metabolic disorders associated with lipid metabolism (McArt et al., 2012; Ribeiro et al., 2013).
To give an idea of the increase in liver activity during parturition, the hepatic release of glucose increases from 1.4, to 2.7, and to 3.5 kg/day between -9, 11, and 33 days relative to parturition, and its weight increase by 23 during the first month of lactation (Greenfield et al., 2000; Reynolds et al., 2003; Selim et al., 2014; Reynolds et al., 2003; Von Soosten et al., 2011). Hepatic removal of fatty acids from blood is 3-fold compared with prepartum values (Reynolds et al., 2003). Because the bovine liver has limited capacity to export very low-density lipoproteins, hepatic’s oxidation capacity can be exceeded and might affect dairy cows during early lactation (Drackley et al., 2006).
Source: own elaboration
This may lead to accumulation of Non-Sterified Fatty Acids (NEFA) within the hepatocytes, which although does not impair gluconeogenesis directly it decreases the conversion of ammonia into urea, which reduces the synthesis of glucose from propionate (Strang et al., 1998; Overton et al., 1999).
Furthermore, High plasma NEFA levels during early postpartum have been associated with periparturient diseases (Ospina et al., 2010; Chapinal et al., 2011; Chapinal et al., 2012b). Ospina et al. (2010) reported that elevated postpartum NEFA was associated with displaced abomasum, clinical ketosis, and metritis.
In parallel, the supply of glucose precursor from rumen (propionate) after parturition, rumen efficiency (fermentation and absorption of nutrients) is reduced due to its small size and reduced length of rumen papillae (as a consequence of a low level of inclusion of carbohydrates in dry period diets). It is even more severe in those farms where prepartum (close-up) diets are not given. Therefore, NEB constitutes a metabolic state with which we must coexist. However, metabolic reasons why it occurs must be understood and controlled.
Insulin resistance in high-producing dairy cows: consequences and Importance
Demand for glucose increases rapidly with the onset of lactation as it is a fast source of energy for tissues (and the main one for synthesis of lactose, then determining milk volume). In order to sustain lactation (feed the newborn), insulin resistance in peripheral tissues increases the release of glucose by the liver postpartum, and its availability to the mammary gland. Insulin suppresses gluconeogenesis in the liver mostly by inhibiting the uptake of glucogenic substrates (Brockman, 1985).
FATTY ACIDS: THEIR FUNCTIONAL ROLE
Palmitic acid (C16:0) supplements seems to come to stay on it, as it is the supplement that is helping farmers to increase (or maintain) milk fat % (see table 1).
Which is the real effect of C16:0 on milk production?
Source: Adapted from Loften et al. (2014)
Despite palmitic acid supplements help to increase milk fat production, potential negative consequences on cows’ metabolism must be taken into account. Early-lactation cows have a genetic predisposition to drive energy to milk production, which is carried out at the expense of body condition loss. Palmitic acid can exacerbate this issue as it has a strong influence on insulin resistance. Hence, adding more palmitic acid to fresh cows may aggravate this natural partitioning of nutrients, then mobilizing body reserves in excess, and increasing the risk of metabolic diseases incidence along with reproductive problems (due to loss of body condition and increase of NEFA and BHB). Also, a series problems have been observed after the use of palmitic acid supplements: changes in butter and cheese quality (due to the increase in their melting point), increase of the saturated/unsaturated ratio in dairy products (less healthy). On the positive side, it must be said that.
In contrast, the stimulatory effects of insulin by C18:1 indicate that this fatty acid is particularly desirable at the onset of lactation to increase the distribution or partitioning of energy to body reserves, thus reducing body condition loss. Also, as already mentioned, fats’ digestibility will be increased. This also has consequences on reproduction: additional energy available for the cows and improvements in the quality of the follicle and better early embryonic development, which implies a significant improvement in fertility (Aardema et al., 2011). More effects can be observed in table 1.
Source: own elaboration from Souza et al. (2017)
In this study, it was found that the proportion 60% of C16:0 and 30% of C18:1 is the best option.
Source: NUTRION Internacional’s own elaboration
Under this context, the author recommends following 2 guidelines
- Formulation strategy 1: rumen-protected glucose precursors
As there is a NEB + a suppression of liver intake of glucogenic substrates, the use of rumen-protected glucose precursors (instead of rumen-glucose as such and/or non-protected glucose precursors) is beneficial.
- Formulation Role of fatty acids and their combination on insulin resistance and metabolic health
The effects of fatty acids may also be due to the effects of lipid intermediates, such as sphingolipid ceramides. Ceramides reduce insulin sensitivity (Rico et al., 2015), improve the mobilization of non-esterified fatty acids (NEFA) in adipose tissue and increase milk production (Rico et al., 2016; Davis et al., 2017). Palmitic acid is the substrate for the de novo synthesis of ceramide (McFadden, 2017), which indicates a link between the increase in the supplementation of this fatty acid and the mechanisms to partition energy towards milk production.
The transition period constitutes a metabolic challenge for dairy cows, as a consequence of NEB and Insulin Resistance, and the consequent cascade of potential imbalances and disorders.
The liver is the major organ metabolizing energy in that period, so that energy balance and nature of the nutrients given is essential. In relation to this, and taking into account the effects of insulin in the liver (uptake of glucose precursors), the use of two formulation strategies are useful.
First, the use of rumen-protected glucose precursors that, by definition, will not be ruminally metabolized nor used by the liver for its own work.
Second, an adequate ratio of fatty acids (particularly C16:0 and C18:1) that has been proven to reduce insulin resistance and improve metabolic health, production results and reproductive performance.