Phosphorus (P) is the second most abundant mineral in animals, with approximately 80% stored in bones and teeth, while the remaining portion is distributed across bodily fluids and soft tissues, where it supports critical functions. These include bone formation, energy metabolism, and enzyme activity regulation (1). During the growth phase, P is particularly vital for sheep, playing a pivotal role in skeletal development and maintaining bone density, essential for rapid growth (2). Additionally, P is integral to ATP production and utilization, influencing energy metabolism and enzymatic functions, including protein and nucleic acid synthesis (3, 4). Thus, P is indispensable for optimal growth, energy balance, and biochemical processes in sheep.

Over-application of P in fertilizers and animal feeds has exacerbated environmental concerns, contributing to soil P accumulation and eutrophication of surface waters. While adequate dietary P is essential for physiological functions, over-supplying P beyond the animal’s requirements can lead to environmental challenges. Current estimates suggest that accessible mineral P reserves may be depleted within 50–100 years. Sustainable P management strategies are therefore imperative, focusing on enhancing P utilization efficiency and minimizing P losses from livestock.

The Yunnan Semi-fine Wool Sheep is a dual-purpose breed renowned for its adaptability, rapid growth, and utility in both wool and meat production, making it a cornerstone of southwestern China’s livestock industry (5). Despite advancements in understanding P requirements in sheep, these needs vary across breeds, growth stages, and production purposes. Research on P requirements in wool-meat dual-purpose breeds remains limited. According to the Agricultural Research Council (6), the maintenance and growth P requirements for sheep are 0.014 g/(kg BW·d) and 6.0 g/kg EBW, respectively, using the factorial method. The French INRA system recommends 5.5 g/kg BW for growth (7). Bellof and Pallauf (8) found a higher requirement of 7.5 g/kg EBW for German Merino lambs. Similarly, studies on other breeds, such as Ideal × Ile de France and Santa Inês sheep, reported growth P requirements between 4.14 and 5.39 g/kg BW, which are lower than ARC (6) recommendations (9). Domestic research by Ji et al. (10) reported P maintenance and growth requirements of 0.025 g/(kg EBW·d) and 5.27–5.83 g/kg EBW for Dorper × Small-tailed Han sheep. These findings emphasize variability in P requirements among breeds, highlighting the need for targeted research on wool-meat dual-purpose breeds.

The factorial method is widely used to assess animal nutrient requirements, distinguishing between maintenance and growth needs. P maintenance requirements are typically determined via the minimal endogenous loss method, which evaluates basal P excretion under P-free diets (6). However, factors such as feed intake, diet composition, and breed can influence endogenous P losses, requiring further study (11). The comparative slaughter method, in contrast, calculates net growth requirements by assessing P deposition differences pre- and post-experiment (10). This study investigated the effects of varying dietary P levels on growth performance, slaughter characteristics, and nutrient metabolism in Yunnan Semi-fine Wool Sheep. Using both the minimal endogenous loss and comparative slaughter methods, it determined P requirements for maintenance and growth, providing insights for optimized feeding strategies and P management practices to enhance production efficiency and reduce environmental impacts.

Phosphorus (P) requirements in livestock are influenced by factors such as breed, growth stage, production purposes, and environmental conditions (23). Specifically, for the Yunnan Semi-fine Wool Sheep, a dual-purpose breed used for both wool and meat production, research on its P requirements is lacking, particularly during key growth phases. In the present study, increasing dietary P levels resulted in a trend toward a linear decrease in feed intake, although the differences observed were not statistically significant. This finding contrasts with previous studies on dairy cows, which reported no significant effect of P levels on feed intake, likely due to the diets being within normal P intake ranges (24, 25). The results of our study suggest that insufficient dietary P may fail to meet the nutritional needs of growing sheep, prompting an increase in feed intake to improve P uptake. However, no significant effects on daily weight gain or feed conversion ratio were observed, consistent with earlier studies (25, 26).

There is limited research examining the effects of P on organ indices and slaughter performance in sheep. In our experiment, higher dietary P levels tended to reduce liver and lung indices, although the differences were not statistically significant. Some studies suggest that excess dietary P can influence the regulation of parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23), which could potentially contribute to the increased burden on organs such as the liver and lungs, suggesting a risk of organ damage from elevated P levels (27). Phosphorus absorption primarily occurs in the form of inorganic phosphate salts, with the majority of P being absorbed through the intestine, with a smaller portion absorbed through the stomach (28). When animals consume low-P diets, absorption is facilitated by a calcium-independent active transport mechanism, whereas when P intake meets or exceeds requirements, absorption predominantly occurs passively.

In our experiment, increased dietary P levels led to a reduction in the rumen index, especially at P levels of 0.82 and 0.97%. Studies on cows with right-sided displaced abomasum or abomasal torsion often indicate hyperphosphatemia, potentially associated with dehydration and reduced renal blood flow (29), suggesting a possible link between high P levels and a reduction in rumen index. These results warrant further investigation. The impact of dietary P levels on slaughter performance has been inconsistent in previous studies. Research on poultry has shown that low P levels can reduce carcass weight (30), while studies on larger animals, such as pigs and sheep, have found no significant effects (31, 32). In our study, dietary P levels did not significantly affect slaughtering rate. However, further analysis revealed that the left-side carcass slaughter rate was lowest at the 0.51% P level, with a linear increase observed as dietary P levels increased, except for the lowest group. This suggests that P levels play a role in influencing the slaughter performance of growing Yunnan Semi-fine Wool Sheep.

Phosphorus is critical for maintaining skeletal health (33). As a component of inorganic phosphate, P is essential for bone development and mineralization, particularly during growth and post-pubertal bone strength maintenance (34). These findings are consistent with our study, which observed a linear increase in the net rate of the left rib as P levels increased from 0.51 to 0.97%. However, no significant effects were noted on the calcium (Ca), P, or ash content in the bone. These findings align with previous studies showing that P supplementation in grazing sheep can increase bone and muscle mass without significantly altering protein, fat, P, or ash content in bone (31). Similarly, studies on dairy cows have indicated that low dietary P does not affect the Ca, P, or ash content in the coccygeal vertebrae (35). In poultry, P-deficient diets have been shown to reduce tibial bone density and fracture strength (36), and similar effects have been reported in goslings (37). Some inconsistencies may arise from species-specific differences and the ratio of dietary Ca to P.

While the effect of dietary P on digestive metabolism in sheep is underexplored, our study examined the impact of varying P levels on the digestion and metabolism of P, Ca, energy, crude protein, neutral detergent fiber (NDF), acid detergent fiber (ADF), and ash. As expected, increasing P intake led to a linear increase in fecal P excretion, consistent with findings from Feng et al. (38) and Puggaard et al. (39). However, urinary P excretion did not vary with increasing dietary P, although total P excretion exhibited a linear increase. Both apparent digestibility and retention rates of P increased with higher dietary P, except in the 0.68% group, where lower dry matter intake reduced P intake without a corresponding decrease in total P excretion (40). Some studies have questioned the role of vitamin D in regulating P absorption, suggesting that absorption efficiency may be largely independent of dietary P levels, with variations in fecal P excretion being attributed to changes in salivary inorganic phosphate secretion (41). In our experiment, no significant differences were observed between the 0.82 and 0.97% groups, which may reflect a saturation of the passive transport mechanism at high P intake levels (38).

The fecal excretion of Ca decreased as dietary P levels increased, which is consistent with earlier studies. P deficiency reduces Ca absorption in the intestines, leading to a decrease in plasma 1,25-dihydroxyvitamin D concentration (42). As a result, both the apparent digestibility and deposition rates of Ca increased quadratically with increasing dietary P, with the lowest values observed at the 0.51% P level. The enhanced Ca digestibility and deposition in the 0.40% P group may reflect compensatory mechanisms, leading to increased dry matter intake. Additionally, the higher ratio of dietary Ca to P relative to fecal Ca to P indicates greater intestinal Ca absorption in the hindgut fermentation vessel (43). The NRC also suggests a strong correlation between nitrogen and P content in growing pigs, indicating mutual interrelations in their deposition (44).

P affects nitrogen metabolism and utilization in cattle. Studies in cattle have shown that insufficient P intake impairs rumen fermentation, reducing microbial protein production and nutrient digestibility (45). Experiments in vitro of cattle also demonstrate that inadequate P supply negatively impacts total rumen bacteria, specific microbial species, and fiber degradation (46). Our study supports these findings, with increased dietary P improving nitrogen digestibility and reducing nitrogen excretion. Therefore, enhancing the apparent digestibility and deposition rate of crude protein.

As P is a critical component of energy-producing reactions, playing a role in the formation of high-energy phosphate bonds such as adenosine triphosphate (ATP), it is essential for energy metabolism (47). In our study, increasing dietary P levels was associated with a linear decrease in fecal energy excretion and a linear increase in energy deposition, highlighting the role of P in regulating metabolic processes. The effect of dietary phosphorus (P) levels on ADF digestibility was similarly observed in our experiments. As dietary P levels increased, a linear reduction in fecal ADF content was noted, accompanied by a clear improvement in ADF digestibility. However, no effect of dietary P levels on NDF digestion was detected.

The P maintenance and growth requirements of Yunnan Semi-fine Wool Sheep were determined through metabolic and comparative slaughter experiments. Using the minimum endogenous loss method, we estimated the P maintenance requirement for these sheep during the growth phase to be 0.06 g/EBW. The ARC (53) set the P maintenance requirement for sheep at 42.5 mg/kg BW, later adjusting this to 14 mg/kg BW in the 1980 recommendations, translating to 0.6–0.8 g in our experimental conditions. The NRC (54) recommended a P maintenance requirement of 20 mg/kg BW for growing sheep, which is a 28.6% increase over the ARC (6) recommendation. Studies in China suggested a maintenance requirement of 0.03 g/kg EBW for sheep weighing 25–35 kg, which corresponds to 0.73–0.99 g for sheep in the 30–40 kg range. The INRA (55) recommended 30 mg/kg BW for growing sheep, and the AFRC (56) suggested 40 mg/kg BW, translating to 1.2–1.6 g for lambs in our trial. Our results, ranging from 1.47 to 1.97 g, are consistent with these recommendations and highlight the influence of growth stage, breed, and environment on P requirements.

Estimates of the net P requirement for growth in sheep vary depending on the nutritional system used. Our comparative slaughter experiments determined the net P growth requirement for Yunnan Semi-fine Wool Sheep to be 6.19 to 5.34 g/kg EBW for animals weighing between 30 and 40 kg. The NRC (48) recommends a net P growth requirement of 6.0 g/kg BW for sheep. However, these recommendations are based on the ARC (6) figures, and it is acknowledged that advancements in breeding and changes in environmental conditions have altered nutritional requirements. Recent studies by international researchers have investigated the mineral needs of various animal breeds and growth stages, revealing some inconsistencies in their findings. For example, Bellof et al. (49) reported a net P growth requirement of 7.5 g/kg EBW for German Merino sheep weighing 30–55 kg. Similarly, Gomes et al. (22) found net P growth requirements ranging from 8.8 to 9.0 g/kg EBW for 5–20 kg Saanen lambs, while Araújo et al. (50) reported net P growth requirements of 7.86–8.74 g/kg EBW for Moxoto goats weighing 15–25 kg. In China, the net P growth requirement for meat sheep weighing 25–35 kg was estimated to be between 5.04 and 5.35 g/kg EBW (10). Our findings are in close agreement with those of Chinese meat sheep studies and the NRC recommendations but are lower than the values reported by Bellof, Gomes, and Araujo et al.

The effects of varying dietary phosphorus (P) levels on growth, slaughter performance, and digestive metabolism in Yunnan Semi-fine Wool Sheep during the growing period were systematically investigated. Phosphorus levels influenced the metabolism of phosphorus, calcium, protein, and energy, as well as the apparent digestion rates of acid detergent fiber. Based on the experimental conditions, a dietary P level of 0.40% was identified as optimal. Using the minimum endogenous and comparative slaughter methods, the P requirements for maintenance and growth were determined to be 0.06 g/kg EBW and 6.19 to 5.34 g/kg EBW, respectively. These findings support strategies aimed at optimizing P intake to meet maintenance and growth needs while potentially reducing excess phosphorus use in livestock nutrition.