Environmental regulations as well as economic incentives have resulted in greater use of synthetic amino acids in swine diets. Tryptophan is typically the second limiting amino acid in corn-soybean meal-based diets. However, using corn-based co-products emphasizes the need to evaluate the pig’s response to increasing Trp concentrations. Therefore, the objective of these studies was to evaluate the dose–response to increasing standardized ileal digestible (SID) Trp : Lys on growth performance of growing-finishing gilts housed under large-scale commercial conditions. Dietary treatments consisted of SID Trp : Lys of 14.5%, 16.5%, 18.0%, 19.5%, 21.0%, 22.5% and 24.5%. The study was conducted in four experiments of 21 days of duration each, and used corn-soybean meal-based diets with 30% distillers dried grains with solubles. A total of 1166, 1099, 1132 and 975 gilts (PIC 337×1050, initially 29.9±2.0 kg, 55.5±4.8 kg, 71.2±3.4 kg and 106.2±3.1 kg BW, mean±SD) were used. Within each experiment, pens of gilts were blocked by BW and assigned to one of the seven dietary treatments and six pens per treatment with 20 to 28 gilts/pen. First, generalized linear mixed models were fit to data from each experiment to characterize performance. Next, data were modeled across experiments and fit competing dose–response linear and non-linear models and estimate SID Trp : Lys break points or maximums for performance. Competing models included broken-line linear (BLL), broken-line quadratic and quadratic polynomial (QP). For average daily gain (ADG), increasing the SID Trp : Lys increased growth rate in a quadratic manner (P<0.02) in all experiments except for Exp 2, for which the increase was linear (P<0.001). Increasing SID Trp : Lys increased (P<0.05) feed efficiency (G : F) quadratically in Exp 1, 3 and 4. For, ADG the QP was the best fitting dose–response model and the estimated maximum mean ADG was obtained at a 23.5% (95% confidence interval (CI): [22.7, 24.3%]) SID Trp : Lys. For maximum G : F, the BLL dose–response models had the best fit and estimated the SID Trp : Lys minimum to maximize G : F at 16.9 (95% CI: [16.0, 17.8%]). Thus, the estimated SID Trp : Lys for 30 to 125 kg gilts ranged from a minimum of 16.9% for maximum G : F to 23.5% for maximum ADG.

During growth, pigs convert plant protein into animal protein. The major part of the ingested protein is excreted via manure, with potential nitrogen (N) losses to the environment. To limit N losses and increase sustainability of pork production, the efficiency of protein conversion should be maximized. The aim of this paper is to critically evaluate diet and management strategies linked with N efficiency. Besides nutrition, we discuss three management strategies observed in science and in practice to be linked with improved N efficiency: genetic selection, castration and slaughter weight. Because diet has a marked effect on eventual N losses, it must also be taken into account when evaluating management strategies. A reductionist approach, such as feeding the same diet across all management treatments, may overestimate the effect of a management strategy and eventually lead to incorrect conclusions. The amount of excreted N depends on the amount of ingested N, the amount of absorbed N, the amino acid (AA) balance in the diet and the animal’s N and AA requirements. Daily multiphase feeding adapted to the individual animal’s AA needs is likely to be the most N efficient. For animals housed in groups, phase feeding is necessary. When combined with periods of temporary AA restriction, N efficiency can be further improved. Specific AA consumption must be balanced by applying the ideal protein concept. With better knowledge of the requirements of individual animals and the commercial availability of certain AAs, the total dietary CP level can be lowered within limits. Further research is needed on the minimal CP level that allows maximal performance. For this end a useful parameter may be the ratio of standardized ileal digestible (SID) lysine : apparent total tract digestible CP level. By combining optimal nutrition and management, a whole body N efficiency approaching 60% may be achievable in the near future.

A meta-analysis, using information from 45 experiments on growing-finishing pigs published in 39 manuscripts, was carried out to determine the simultaneous effects of the physical environment (space allowance, group size, flooring conditions, temperature, presence of enrichment), pig traits (initial body weight (BW) for each studied time interval, sex, genetics), feeder characteristics (water provision within the feeder, feeder design (individual/collective), feeder places/pig, presence of feeder protection) and feed characteristics (feed allowance (ad libitum/restricted), net energy content, crude protein (CP) content), as well as their potential interactions, on the feeding behaviour and performance of growing-finishing pigs. The detrimental effect of low temperature on performance was particularly evident for restricted-fed pigs (P < 0.05). At reduced feeder space allowance, a reduction in the percentage of time spent eating was predicted when increasing initial BW, whereas the opposite was predicted for larger feeder space allowances (P < 0.001). The reduction in visit duration to the feeder in higher BW groups became gradually more important with increasing feeder space allowance (P < 0.01), whereas the increase in the ingestion rate and average daily feed intake (ADFI) with increasing initial BW became smaller with increasing feeder space (P < 0.05). The model predicted a reduction in feed conversion ratio (FCR) with increasing group size (P < 0.05) and floor space allowance (P < 0.01) and on solid floors with or without bedding (P < 0.05). In comparison with other feeders, wet/dry feeders were associated with more frequent but shorter feeder visits (P < 0.05), higher ingestion rates (P < 0.001) and higher ADFI (P < 0.10). The use of protection within individual feeders increased the time spent feeding (P < 0.001), reduced the number of visits per day (P < 0.01), the ingestion rate (P < 0.001) and FCR (P < 0.01) in comparison with other feeder types. Sex modulated the effect of the number of feeder places/pig on FCR (P < 0.05), with a gradual reduction of FCR in entire males and females when increasing feeder space allowance. Genetics tended to modulate the effect of diets’ CP content on FCR (P < 0.10). Overall, these results may contribute to the improvement of the welfare and performance of growing-finishing pigs by a better knowledge of the influence of the rearing environment and may help optimize the feeding strategies in current production systems.