Applying the California net energy system to growing goats

Abstract

The aim of this review is to describe the main findings of studies carried out during the last decades applying the California net energy system (CNES) in goats. This review also highlights the strengths and pitfalls while using CNES in studies with goats, as well as provides future perspectives on energy requirements of goats. The nonlinear relationship between heat production and metabolizable energy intake was used to estimate net energy requirements for maintenance (NE_m). Our studies showed that NE_m of intact and castrated male Saanen goats were approximately 15% greater than female Saanen goats. Similarly, NE_m of meat goats (i.e., >50% Boer) was 8.5% greater than NE_m of dairy and indigenous goats. The first partial derivative of allometric equations using empty body weight (EBW) as independent variable and body energy as dependent variable was used to estimate net energy requirements for gain (NE_g). In this matter, female Saanen goats had greater NE_g than males; also, castrated males had greater NE_g than intact males. This means that females have more body fat than males when evaluated at a given EBW or that degree of maturity affects NE_g. Our preliminary results showed that indigenous goats had NE_g 14% and 27.5% greater than meat and dairy goats, respectively. Sex and genotype also affect the efficiency of energy use for growth. The present study suggests that losses in urine and methane in goats are lower than previously reported for bovine and sheep, resulting in greater metabolizable energy:digestible energy ratio (i.e., 0.87 to 0.90). It was demonstrated that the CNES successfully works for goats and that the use of comparative slaughter technique enhances the understanding of energy partition in this species, allowing the development of models applied specifically to goat. However, these models require their evaluation in real-world conditions, permitting continuous adjustments.

Keywords: comparative slaughter; degree of maturity; efficiency of utilization; metabolizable energy; requirements for growth; requirements for maintenance.

Figure 1.

Timeline showing studies applying the California net energy system to goats in Brazil (♂ represents intact male goats, Ol represents castrated male goats, and ♀ represents female goats).

Figure 2.

Age range throughout studies: squares represent studies with Indigenous goats, diamonds represent studies with Boer crossbreds, and circles represent studies with Saanen goats.

Figure 3.

Energy partition across sexes in studies with growing goats. No difference between sexes was observed in energy losses in feces, urine, or gasses (P ≥ 0.35), using MIXED procedure of SAS (SAS Inst. Inc.; version 9.4). Energy losses from gaseous products of digestion were predicted according to Blaxter and Clapperton (1965) as: Gas energy, kcal/d = GEI, kcal/d × (4.28 + 0.059 × DGE,%); GEI = gross energy intake; DGE = digestible gross energy.

Figure 4.

Linear regression of residual (observed ME minus predicted ME = 0.82 × DE) on the predicted values centered on their mean (ME = metabolizable energy; DE = digestible energy). The intercept of the linear regression equation was used to estimate mean biases, whereas linear bias (i.e., systematic bias) was assessed using the slope of the regression equations (St-Pierre, 2003), using MIXED procedure of SAS (SAS Inst. Inc.; version 9.4). Diamonds represent studies with Boer crossbreds, and circles represent studies with Saanen goats.

Figure 5.

Linear regression equation of dietary ME concentration on dietary DE concentration (ME = metabolizable energy; DE = digestible energy). Fitted using MIXED procedure of SAS (SAS Inst. Inc.; version 9.4). Diamonds represent studies with Boer crossbreds, and circles represent studies with Saanen goats.