Effects of a supplement containing multiple types of gluconeogenic precursors on production and metabolism in Holstein bull calves during heat stress

Highlights

• We evaluated the effects of a commercial glucose precursors during heat stress in bull calves.

• We assessed the effects of this product on productive performance of bull calves during heat stress.

• We examined changes in the level of blood parameters during heat stress and in combination with a dietary treatment.

• This product changed insulin level but not productive performance and body temperature indices.

Abstract

Glucose appears to be a preferred systemic fuel during heat stress (HS) in a variety of species. Increasing the dietary grain content can enhance the post-absorptive carbohydrate status, but providing excessive fermentable starch can cause rumen disorders and this is especially true during HS. Current study objectives were to evaluate the effects of a glycerol based supplemental product on growth and metabolic variables in Holstein bull calves during controlled HS. Before the start of the experiment, bull calves (n=14; 163.6±30.1 kg body weight) were subjected to thermal neutral conditions [26.5±3.4 °C and a temperature–humidity index (THI) of 70.4±2.8] for 7 d (period 1; P1). During this period, productive parameters as well as blood metabolites were measured and used as covariates for the subsequent HS period. Following P1, a cyclical HS pattern was implemented for 21 d (P2) where daily ambient temperatures ranged from 29.1 to 39.7 °C and the THI was >74 for 24 h/d and >83 for at least 14 h/d. During P2, half of the HS calves (n=7) received a control diet (CON) and the other half received the control diet supplemented with a product (300 g/d) containing gluconeogenic precursors (GLU). Throughout each period respiration rate, rectal temperature and skin temperature at the shoulder and rump were recorded at 0600, 1100 and 1500 h daily. Blood samples were obtained prior to and 4 h post the a.m. feeding during both periods. Although HS markedly reduced DMI (18%) and growth as expected, supplemental GLU did not affect body weight gain. Supplemental GLU decreased the shoulder temperature at 0600 and 1500 h (P<0.01), and decreased respiratory rate at 1500 h (P<0.02). Feeding GLU did not affect blood urea nitrogen (BUN), glucose or nonesterified fatty acids (NEFA) concentrations, but increased circulating insulin prior to the a.m. feeding (P<0.03) and this demonstrates that GLU was effective at enhancing the post-absorptive carbohydrate status. Our results suggest that feeding supplemental GLU improves some body temperature indices but did not enhance growth performance in Holstein bull calves during HS.

Introduction

Heat stress (HS) compromises efficient animal production and although difficult to accurately quantify, the economic impact on the global livestock industries is likely greater than $100 billion annually (Baumgard and Rhoads, 2013). Economic losses due to HS can be attributed to decreased milk production, increased incidence of metabolic disorders and health problems (e.g., rumen acidosis), slowed and inconsistent growth, compromised milk quality, reduced reproductive performance, and mortality (West, 1999).

A variety of amelioration strategies for HS are available, and can be implemented alone or in a coordinated manner. These include: (1) physically modifying the environment (shades, fans, evaporative cooling, etc.), (2) management adaptations (timing of milking, feeding, etc.), (3) genetic selection and (4) dietary modifications. To date, genetic selection for HS tolerance is frequently associated with production drag during thermal neutral conditions (Baumgard and Rhoads, 2013). Investing in adequate HS abatement facilities maybe financially challenging for many producers; this is especially true in developing countries and for small stakeholders. Identifying nutritional strategies that can be easily incorporated into the diet to alleviate the negative effect of HS is beneficial for optimizing production of high quality protein during the stressful summer months.

There are several nutritional strategies to consider during HS and they typically concentrate on increasing the energy density of the diet and minimizing the thermic effect of feeding (see reviews by West, 1999, Kadzere et al., 2002). Recently, some studies have demonstrated that heat-stressed farm animals including growing and lactating ruminants preferentially utilize glucose for processes other than milk and muscle synthesis (see reviews by Baumgard and Rhoads, 2012, Baumgard and Rhoads, 2013). Consequently, heat-stressed animals appear to be in a negative post-absorptive carbohydrate status and thus altering insulin action and increasing glucose availability may be a viable approach during HS (Rhoads et al., 2013). One method of enhancing hepatic glucose output is to feed additional starch, but feeding excessive grain needs to be carefully considered given that heat-stressed ruminants are susceptible to ruminal acidosis (Kadzere et al., 2002). Consequently, safely providing rumen substrates that are gluconeogenic themselves (glycerol; if absorbed intact by the intestine or rumen wall) or are metabolized into glucose precursors (propionate) may increase productivity during the warm summer months (Baumgard and Rhoads, 2012). Dietary ionophores safely increase propionate production and can increase hepatic glucose output (Baumgard et al., 2011). Another dietary option to increase propionate production is via supplementing glucose precursors, such as glycerol and propylene glycol, which have been used for prophylactic and metaphylactic treatment for ketosis (Johnson, 1954, Fisher et al., 1973).

We hypothesized that supplementing a product containing a variety of gluconeogenic molecules could enhance the post-absorptive carbohydrate status during HS and improve animal performance. Therefore, our study objectives were to evaluate the effects of a supplement containing multiple types of glucose precursors (mainly consisting of glycerol and propylene glycol), on productive performance and circulating bioenergetic markers in Holstein bull calves during HS.