Leucaena leucocephala feeding systems for cattle production in Mexico Sistemas de alimentación con Leucaena leucocephala para la producción bovina en México

The impacts of leucaena (Leucaena leucocephala) feeding systems on cattle production, environmental services and animal welfare in Mexico are discussed. A total of about 12,000 ha of leucaena have been established in the tropical regions of México, where most of the information for the current review was obtained. Incorporating leucaena in a grass pasture increases dry matter intake of grazing cattle and reduces the level of methane produced. This results in improved liveweight gains and milk yields as well as a reduction in the level of greenhouse gas released. Additional benefits are increases in soil carbon and nitrogen levels and less stress on animals as the leucaena plants provide shade and reduce environmental temperatures. While these benefits are substantial, the area developed to leucaena represents less than 0.1% of the area which could potentially be developed. Strategies to increase adoption of these grass-legume systems by farmers need to be developed to make effective use of the systems for increasing beef and milk production while reducing the undesirable environmental outcomes normally associated with ruminant production.


Introduction
In Mexico, animal production systems which utilize leucaena (Leucaena leucocephala) can increase productivity, improve animal welfare and mitigate environmental impacts relative to grass-only systems (Figure 1), but adoption of these systems by commercial farmers is limited. In this review, we discuss the Mexican experience in terms of animal productivity, energy supplementation and environmental services. This analysis provides an understanding of the achievements that have been made and challenges facing the system. Leucaena-grass systems can provide a wide range of ecosystem services. These include: reduction of greenhouse gas emissions through both mitigation of methane (CH4) emissions and increased carbon (C) storage; improved nutrient cycling; increased soil organic matter; and improved atmospheric nitrogen (N) fixation.
Considerable effort has been made in various countries to improve our understanding of mechanisms of enteric CH4 mitigation in cattle fed rations containing foliage of leucaena, notably in Australia (Harrison et al. 2015) and Colombia (Molina-Botero et al. 2016). In Mexico, respiration chamber methodology (Canul-Solis et al. 2017) has been used to measure enteric CH4 emissions in cattle fed leucaena.
In the tropical regions of Mexico, environmental temperatures and relative humidity are high, and at certain times of the day (during summer), above the physiological capacity of livestock to dissipate body heat. This condition leads to low animal productivity due to elevated body temperature and respiratory rate, leading to reduced voluntary feed intake. A strategy to improve animal comfort is the inclusion of woody species in monocrop-grass systems ( Figure 2).

Materials and Methods
Information analyzed in the current review was generated from several regions of the Mexican tropics, particularly the states of Michoacán and Yucatán. In those regions, leucaena has been established by 615 livestock owners (mainly cattle producers) located in 10 different states (Campeche, Chiapas, Guerrero, Jalisco, Michoacán, Quintana Roo, San Luis Potosí, Tamaulipas, Veracruz and Yucatán), with a total livestock production area of 27,307,096 ha (Sagarpa 2014) of which about 14,906,331 ha are appropriate for leucaena establishment. This indicates the enormous potential to expand the planting of this legume, since only about 0.08% (12,000 ha) has been successfully established to date.
Animal performance and energy supplementation trials have been carried out using crossbred (Bos taurus × B. indicus) cattle. Levels of C storage, N fixation and nutrient cycling were quantified in commercial grass-only pastures and leucaena-grass associations under grazing.

Methane mitigation in crossbred cattle fed rations containing leucaena
Trials carried out in open-circuit respiration chambers revealed that, as the level of chopped fresh foliage of leucaena was increased in a basal ration of a low-quality grass (Cenchrus purpureus syn. Pennisetum purpureum), enteric CH4 emissions of cattle decreased linearly (Piñeiro-Vázquez et al. 2018) ( Table 1). This confirmed previous results by Harrison et al. (2015) in Australia. It is possible that condensed tannins contained in leucaena foliage induced changes in the microbial population of the rumen, thus affecting methanogenic Archaea and decreasing CH4 emissions. Energy loss through CH4 production, as percentage of gross energy intake (Ym), fell from 5.2% for the grass-only ration, to 3.6%, when leucaena was fed at 20% of ration dry matter (DM), and it continued to fall as legume levels were increased. DM digestibility also decreased as leucaena percentage in the ration increased, but differences were not significant. These findings suggest that incorporating leucaena at 20% of ration DM would reduce CH4 emissions by around 25%, relative to a grass-only diet. This represents an important outcome from both an environmental point of view (reduction of CH4 production and emission) and animal performance (improvement through reduced energy loss).

Carbon storage and N cycling increased
Carbon storage and N cycling increased by 38 and 47%, respectively, in leucaena-grass systems compared with pure grass pastures. The legume fixed more than 200 kg N/ha/yr and soil organic matter increased by about 200% in the legume association compared with the grass-only system. Environmental temperature was reduced by almost 13% in the legume tree-grass system (Table 2).

Energy supplementation to increase efficiency of nitrogen (N) utilization
Intake of crude protein (CP) by cattle grazing in paddocks with high leucaena plant densities (about 35,000 plants/ha), as currently used in Mexico, could be high, resulting in excessive losses of N in the urine (Bottini-Luzardo et al. 2015). This may increase energy requirements for maintenance of cattle (Jennings et al. 2018) and increase N2O emissions from the urine (Bao et al. 2018). There may also be an imbalance in the protein:energy ratio in the rumen leading to inefficient microbial protein synthesis (Calsamiglia et al. 2010;Barros-Rodríguez et al. 2013). Blood urea nitrogen (BUN) is a good indicator of nutritional balance in ruminants and the protein:energy ratio in total dietary intake is the most important factor related to fluctuations in BUN (Hess et al. 1999). In south-east Mexico, Ruiz-González (2013) found that both BUN and urinary N excretion in cows increased linearly with increasing levels of leucaena in the diet, suggesting inefficient use of N in the rumen. In addition, Arjona (2015) compared cane molasses, sorghum grain, citrus peel and rice polishing as energy supplements for lactating Holstein × Zebu cows fed a diet containing 45% leucaena foliage, relative to a control treatment without energy supplementation. Both BUN and urinary excretion of N were higher for the control group than for the supplemented treatments, suggesting that energy supplementation improved the utilization of N in the rumen. Total feed intake was 25% higher and milk yield was increased by 30% with energy supplementation, with no differences due to the particular energy sources. It was concluded that energy supplementation, regardless of the source used, will improve the efficiency of microbial protein synthesis in the rumen, as well as increase consumption of DM and animal performance, in cattle fed rations incorporating leucaena (Castillo et al. 2000).

Animal welfare
In the central part of Mexico, growing bullocks grazing leucaena associated with Megathyrsus maximus had lower body temperature (measured with an infrared thermometer) and respiratory rates in the morning (36 vs. 38 °C and 42 vs. 65 breaths per minute) and in the afternoon (38 vs. 39.5 °C and 59 vs. 80 breaths per minute) than in a feedlot (Utrilla-García 2013). This suggests that the feedlot system, which is becoming a common practice in Mexico, can have negative impacts on animal welfare and productivity, if shade is not provided, since it could reduce feed intake and liveweight gain and therefore general productivity. In another study, undertaken in southeast Mexico, to evaluate animal welfare of dairy cattle grazing leucaena-grass systems with trees, improved microclimate conditions allowed animals to cope better with heat stress. In addition to increased biodiversity there were advantages for animal welfare in terms of social and affiliative behavior, such as social licking, head leaning and social rubbing (Améndola et al. 2015).

Leucaena-grass system adoption
From 2012 to 2017, leucaena-grass systems have been established over almost 12,000 ha (615 farms), which represents about 0.08% of the total livestock area suitable for leucaena establishment (14,906,331 ha) in 10 states of the tropical region in Mexico. Although this area is significant, the rate of adoption has been slow and limited, as there are more than 350,000 cattle farms in the tropical region (PGN 2018). Possible contributing factors are: a) uncertainty in livestock markets; b) lack of state laws that give long-term support to this initiative; c) lack of longterm extension services for cattle producers; d) high labor demand for leucaena-based systems, which means farmer leaders must have confidence in the system; e) lack of availability of good quality farm inputs and services and the need for an appropriate market-chain for distribution; f) inefficient support from state policies; and g) high costs and limited access to credit.

Conclusions
Leucaena feeding systems could be an important strategy in the tropical regions of Mexico to improve animal productivity and welfare, and to reduce greenhouse gas emissions to the environment. Silvopastoral systems with leucaena have proved highly productive in Mexico. However, these systems have not been adopted widely by farmers, mainly due to lack of readily available technical support, several socioeconomic and political constraints and appropriate strategies to make effective use of the systems for cattle production with minimal impact on the environment.