Intake and digestibility of nutrients
Dry matter intake is the main factor affecting animal production17,18, as this fraction of the food contains nutrients (proteins, carbohydrates, lipids, vitamins and minerals). In the present study, the linear increase in the intake of DM and non-fibrous carbohydrates (NFC), expressed as metabolic weight, can be explained by the increase in the concentration of dry matter silage and starch in the grains with advancing maturity6,9. Starch is the main reserve carbohydrate in cereals and represents around 60–80% of the weight of corn and sorghum grains19,20, which contributes to the increasing the DM content of the crop6,9. Corroborating Ward et al.21, who demonstrated a relationship between increased sorghum silage intake and increased silage DM content.
Additionally, the accumulation of starch in the grains with advancing plant maturity favored the linear increase in the digestibility of DM and organic matter (OM) of the silage, as it is a homopolysaccharide with high ruminal digestibility22. Several studies have shown that, during fermentation inside the silo, the protein matrix surrounding the starch granules is solubilized, primarily due to the enzymatic activity of bacteria23. This increases the digestibility of DM and starch at the ruminal level24,25. Despite the increase in voluntary intake (g kg− 1 of BW0.75 day− 1) and digestibility of DM, O2 consumption and CO2 production by animals were not affected, as these are directly related variables.
Although silage made from plants harvested at 100 days has a numerically higher concentration of NDF compared to other silages, as observed by Hatew et al.11, this result did not affect the voluntary intake of this fraction by the animals in the present study. Previous research has demonstrated that dietary NDF concentration negatively correlates with voluntary feed intake by animals due to the physical capacity of the rumen26,27. As recommended by Mertens28, NDF intake by sheep fed almost exclusively on forage (ranging 350–750 g of NDF in DM) is approximately 35 g kg− 1 BW0,75 d− 1, which is a higher value than the obtained in this study. As plants mature, the concentration of lignin increases, which has a negative correlation with forage digestibility, as reported in previous studies29–31. In this study, there was a reduction of approximately 28 g kg− 1 of DM in NDF digestibility when comparing silage produced with plants in the milk stage with silage from plants in the hard dough stage.
The DM content of the crop to be ensiled is the main factor in deciding the ideal time for harvesting. DM contents between 300–400 g kg− 1 fresh matter are considered adequate to producing good quality silage32. During the fermentation process, forages with low DM levels can favor undesirable fermentations by bacteria of the genus Clostridium, which are responsible for the production of butyric acid and excessive production of ammonia, compromising the nutritional value32,33 and voluntary intake34,35 of the silage, and consequently, animal performance. Another negative aspect is the production of effluent, which not only leads to nutrient loss but also poses an environmental risk36.
The CP levels observed in the silage, which ranged from 63–68 g kg− 1 DM, indicate that there were no significant losses during fermentation, as these values are close to those recorded in previous studies at the time of harvesting the sorghum plants37,38. This effect can be confirmed by the intake of CP by the animals, which was not affected by the different sorghum silages provided, as well as the digestibility of this fraction. Teixeira et al.39 reported intake of CP (4.14 g kg− 1 BW0.75 day− 1) similar to that observed in the present study after feeding ‘BRS 610’ sorghum silage to sheep. It is important to highlight that CP is generally the most expensive nutrient in the diet of ruminants and directly influences voluntary intake, performance, and carcass and meat characteristics of sheep40,41.
Nitrogen (N) balance
Nitrogen balance is a parameter used to estimate the losses of this nutrient relative to what was ingested by the animal. In this study, the positive N balance indicates that the CP concentrations provided via silage were able to meet the requirements of these animals14,42.
The lack of effect on N ingested by animals can be explained by the close CP levels recorded in the different silage, which ranged from 63 to 68 g kg− 1 of DM. However, the linear increase in NR with advancing plant maturity indicates better utilization of N when animals were fed silage produced from more mature plants. This result was likely due to an increase in the availability of fermentable carbohydrates in the rumen. NFC levels increased with grain maturation, which influenced nitrogen balance, as reported by Calsamiglia et al.43. Synchronizing the supply of energy and protein in a diet is essential to maximize microbial growth, increase N retention, and improve the utilization of protein and energy42,44,45.
Energy partitioning and respirometry
The energy use efficiency of ruminants must be estimated, as insufficient energy intake can limit animal performance46.
The linear increase in GE intake by animals fed silage produced from plants at different stages of maturity is a result of the increase in dry matter intake, which is in agreement with the high correlation (r²= 0.99) between these parameters, as reported by Machado et al.37 and So et al.47. As previously mentioned, the accumulation of starch in grains with advancing maturity led to the production of silage with a higher concentration of NFC, which are high-energy compounds18. This also resulted in a linear increase in NE intake, which is effectively available energy for animal production48.
The energy losses found in this study align with the findings of Santos et al.18 and Pires et al.14. These authors concluded that fecal production represented the main source of energy loss, followed by caloric increment, methane production and urine. Therefore, it is necessary to develop nutritional strategies and technologies to improve digestibility and reduce caloric increment, aiming to maximize the energy flow for animal performance37.
In this regard, the use of heterofermentative bacteria during sorghum ensiling can alter the fermentative profile of the silage, leading to an increase in propionate production14,49. Propionate is the main gluconeogenic precursor used by ruminants50, which consequently enhances the efficiency of metabolic energy use and the net energy content of silage14.
The metabolizability of GE (qm) involves energy losses in feces, urine, and methane, while the efficiency of use of ME (km) represents the real NE available for the maintenance and performance of the animal, which is influenced by caloric increment (IC)51. Energy losses in feces were the most impactful on qm, resulting in low values at the four forage harvest ages. km was higher due to energy losses by CI. Machado et al.37, evaluating sorghum hybrids ‘BRS 610’, BR 700, and BRS 655 at three maturation stages, found qm values ranging from 0.42 to 0.52, which are close to those observed in this study. However, the values (0.53 to 0.78) were higher, justified by the lower CI values obtained in their evaluation. Fox et al.52 highlighted km values ranging from 0.576, for diets with ME of 2.0 Mcal kg− 1 (temperate climate grasses in late flowering), 0.651 for diets with ME of 2.6 Mcal kg− 1 (corn forage), and up to 0.686, for diets with ME of 3.2 Mcal kg− 1 (corn grains).
The methane emissions observed in this study agree with the results observed by Machado et al.37 for sorghum silage and McGeough et al.53 for corn silage harvested at different maturity stages. The increase in starch concentration, accompanied by a decline of fibrous constituents, can alter rumen fermentation, favoring the production of propionic acid and resulting in a decline in methane formation12,53,54. Hatew et al.11 suggested that late harvesting of corn could be an effective strategy to reduce methane production in dairy cows without negatively impacting their performance. Ruminants fed diets high in fiber and low in NFC can increase methane emissions13. Consequently, these animals present lower performance and a longer production cycle55,56, compromising the sustainability of production systems18.
In this study, despite the nutritional changes observed in silage produced from plants at different stages of maturation, these were not significant enough to affect methane production by the animals. This lack of effect could possibly be attributed to the fact that the sheep used in the study were in a maintenance situation (low nutritional demand). However, modifications in the nutritional value of forage could potentially yield different results in animals with high nutritional demands, such as growing sheep and lactating cows11,57–59.