Abstract
Protein requirements for slow-growing pigs have not been defined. The study was designed to determine the relationship between the level of dietary protein and nitrogen balance in slow-growing Windsnyer pigs. Thirty Windsnyer pigs with an initial bodyweight of 34.7 (SD = 0.85) kg were randomly assigned to six experimental diets containing 193, 174, 154, 135, 117, and 97 g/kg of crude protein (CP). Pigs were penned individually. There were five pigs in each diet. The faecal nitrogen output (NFO) showed a decreasing quadratic relationship with protein level. A linear relationship between protein level and urinary nitrogen excretion was observed (P =0.0001). The total nitrogen excreted showed a significant quadratic relationship to decreasing levels of CP in the diets (P =0.0001). The apparent digestibility of nitrogen had a linear relationship with dietary CP level (P =0.0005). A quadratic relationship between dietary protein level and N utilization was observed (P =0.0001). The optimum protein level for total nitrogen excreted, nitrogen utilization and biological value of feed protein were attained at 135 g/kg protein level. The efficiency of nitrogen utilization supports the low protein diet. The present results suggest reducing the dietary protein level for Windsnyer pigs to decrease amount of excreted N that is deposited into the environment.Protein requirements for slow-growing pigs have not been defined. The study was designed to determine the relationship between the level of dietary protein and nitrogen balance in slow-growing Windsnyer pigs. Thirty Windsnyer pigs with an initial bodyweight of 34.7 (SD = 0.85) kg were randomly assigned to six experimental diets containing 193, 174, 154, 135, 117, and 97 g/kg of crude protein (CP). Pigs were penned individually. There were five pigs in each diet. The faecal nitrogen output (NFO) showed a decreasing quadratic relationship with protein level. A linear relationship between protein level and urinary nitrogen excretion was observed (P =0.0001). The total nitrogen excreted showed a significant quadratic relationship to decreasing levels of CP in the diets (P =0.0001). The apparent digestibility of nitrogen had a linear relationship with dietary CP level (P =0.0005). A quadratic relationship between dietary protein level and N utilization was served (P =0.0001). The optimum protein level for total nitrogen excreted, nitrogen utilization and biological value of feed protein were attained at 135 g/kg protein level. The efficiency of nitrogen utilization supports the low protein diet. The present results suggest reducing the dietary protein level for Windsnyer pigs to decrease amount of excreted N that is deposited into the ironment.
 Keywords: apparent nitrogen digestibility, ideal protein level, nitrogen retention, excreted nitrogen, nitrogen utilization
Highlights
A large number of smallholder pig farmers in southern Africa keep slow-growing pigs (Ramsey et al, 2000; Chimonyo et al, 2006)
The present results with Windsnyer pigs demonstrated that reducing levels of dietary protein resulted in a reduction in total nitrogen excretion
The present results suggest that diets with low levels of protein reduce urinary nitrogen excretion and ammonia volatilization
Summary
A large number of smallholder pig farmers in southern Africa keep slow-growing pigs (Ramsey et al, 2000; Chimonyo et al, 2006). Slow-growing Windsnyer pigs are indigenous pigs that are facing extinction and it is essential to conserve them through sustainable utilization. The commercial pig industry shuns slowgrowing breeds owing to their high propensity to deposit fat (Halimani et al, 2012). These indigenous pigs play a pivotal role in improving and maintaining financial sustainability and in reducing food insecurity (Ncobela et al, 2018). One of the main limitations to Windsnyer pig production by smallholder farmers is the protein supply. Dose-response trials are ideal in estimating protein requirements for these genotypes
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