Applying the California net energy system to growing goats

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.

Can Agriculture Balance Its Energy Consumption and Continue to Produce Food? A Framework for Assessing Energy Neutrality Applied to French Agriculture

In the context of energy transition, agriculture is facing a double challenge, which is to reduce its fossil fuel dependency and provide—in addition to food—bioenergy to society for substitution to fossil fuels. The feasibility of this challenge depends on whether agriculture can achieve energy neutrality, meaning to balance its consumption of external energy by energy recovery from internal sources. Livestock feed is a major determinant in this balance. We use France as a demonstration case of the changing role of feed in the long-term agricultural energy metabolism and the challenge of reaching energy neutrality. Results show that current agriculture is structurally energy deficient to such an extent that its functional energy requirements almost equal its final produce. The energy recovery potential from crop residues and manure could at best cover the primary biomass equivalent of the external energy inputs to agriculture. Only a suppression of feed from cropland combined with very high energy recovery from agricultural residues could allow achieving energy neutrality and making agriculture a net energy source to society.

Recent advances in estimating protein and energy requirements of ruminants

Considerable efforts have been made in gathering scientific data and developing feeding systems for ruminant animals in the past 50 years. Future endeavours should target the assessment, interpretation and integration of the accumulated knowledge to develop nutrition models in a holistic and pragmatic manner. We highlight some of the areas that need improvement. A fixed metabolisable-to-digestible energy ratio is an oversimplification and does not represent the diversity of existing feedstock, but, at the same time, we must ensure the internal consistency and dependency of the energy system in models. For grazing animals, although data exist to compute energy expenditure associated with walking in different terrains, nutrition models must incorporate the main factors that initiate and control grazing. New equations have been developed to predict microbial crude protein (MCP) production, but efforts must be made to account for the diversity of the rumen microbiome. There is large and unexplained variation in the efficiency of MCP synthesis (9.81–16.3 g MCP/100 g of fermentable organic matter). Given the uncertainties in the determination of MCP, current estimates of metabolisable protein required for maintenance are biased. The use of empirical equations to predict MCP, which, in turn, is used to estimate metabolisable protein intake, is risky because it establishes a dependency between these estimates and creates a specificity that is not appropriate for mechanistic systems. Despite the existence of data and knowledge about the partitioning of retained energy into fat and protein, the prediction of retained protein remains unsatisfactory, and is even less accurate when reported data on the efficiency of use of amino acids are employed in the predictive equations. The integrative approach to develop empirical mechanistic nutrition models has introduced interconnected submodels, which can destabilise the predictability of the model if changed independently.