Gastrointestinal Nitrogen Metabolism of Equids and Impact on Protein Requirement

Protein Source and Intake Effects on Diet Digestibility and N Excretion in Horses-A Risk of Environmental N Load of Horses

Six Finnhorse mares were used in a digestibility trial, in which six typical horse diets were compared. The diets were: (A) haylage 100%; (B) hay 100%; (C) hay 70% and oats 30%; (D) hay 70% and soybean meal + oats 30%; (E) hay 70%, rapeseed meal + oats 30% and (F) hay 70 %, linseed meal + oats 30%. The trial was conducted according to an unbalanced 6 × 4 Latin square design with four 3-week experimental periods. The experimental period consisted of 17-day preliminary feeding which was followed by a 4-day total faecal and urine collection periods to evaluate N excretion. The digestibilities of DM (p < 0.001) and OM (p < 0.001) in the haylage-only diet were lower compared to the other diets. The supplemental protein feeds improved the diet digestibility of CP (p = 0.002) compared to a hay + oats diet. Furthermore, the DM (p = 0.019), OM (p = 0.006), and CP (p = 0.016) digestibilities of the soya-supplemented diet were better than those of the rapeseed- and linseed-supplemented diets. Faecal excretion was greater for haylage (19.3 kg fresh faeces and 3.6 kg DM/day) and hay (18.7 kg fresh faeces and 3.6 kg DM/day) diets (p < 0.001) compared with the other diets. Urine excretion was also greater for forage-only diets compared with diets including protein supplements. Horses excreted 14.0 L urine/day on haylage-only diet (p = 0.026) and 14.3 L/day on a hay-only diet (p = 0.003). Horses excreted more nitrogen in their urine than in dung. N excretion differed between the diets. Horses on a haylage-only diet excreted 51.6 g N in faeces/day and on hay-only diet 51.4 g N/day. On the other hand, when protein content in forages increased, N excretion via urine increased (haylage vs. dried hay). Horses excreted less N in urine with hay-only diet than with haylage-only or protein-supplemented diets (p < 0.001). When N excreted in faeces and urine was counted together, horses excreted less N with a hay-only diet (p < 0.001) than with a supplemented one (oats and/or protein feeds). The results showed that feed choices affected the amount of nitrogen excreted. Feeding recommendations should consider not only the horse category and work level, but also the protein source. When good quality protein is fed, smaller N intakes can be applied to reduce the N excretion via urine and dung. At the farm level, improved understanding of feed quality, as well as feeding planning and practices, is a way to decrease the risk of N leaching and evaporation.

Effects of Sodium Caseinate and Varying Protein Sources on In Vitro Fermentation of Forages by Mixed Equine Cecal Microorganisms

To assess the impact of protein on fermentation by equine cecal microorganisms, cecal fluid from 4 cecally cannulated horses was used to inoculate fermentation bottles containing buffer, forage, and supplemental protein. In experiment 1, sodium caseinate (SC) provided 0, 0.5%, 1%, 2%, or 4% additional crude protein (CP) to bottles containing alfalfa or native warm-season prairie grass hay. Bottles were equipped with continuous gas pressure monitors and placed into a shaking incubator for 48 hours at 39°C. Cultures with alfalfa had greater (P < .0001) in vitro dry matter disappearance (IVDMD), neutral detergent fiber disappearance (NDFD), acid detergent fiber disappearance (ADFD), cumulative gas production, and total volatile fatty acid (VFA). Sodium caseinate increased gas production (P ≤ .05) and decreased pH (P < .003) in cultures with grass hay. Sodium caseinate at 1%, 2%, or 4% additional CP increased IVDMD, NDFD, and ADFD (P < .01), while 4% additional CP also increased total VFA (P < .01). For experiment 2, SC, fishmeal, soybean meal (SBM), whey, porcine blood plasma, and L-lysine hydrochloride were added to supply 2% additional CP to cultures with grass hay. All protein sources decreased pH and increased IVDMD, NDFD, and ADFD (P ≤ .01), with the largest effects elicited by SC, L-lysine, and whey (P ≤ .05). Total VFA (P ≤ .04) and gas (P ≤ .05) production increased with L-lysine, whey, SC, SBM, and fishmeal. While protein supplementation had minimal effects on cultures containing alfalfa, it altered fermentation of grass hay, more notably with more soluble protein sources.

Dietary nitrogen utilisation and prediction of amino acid requirements in equids

The equine population represents an important sector of animal agriculture and, thus, contributes to environmental contamination. The horse industry lags behind other livestock industries in developing prediction models to estimate N and amino acid (AA) requirements aimed at precision feeding and management to optimise animal health and performance while mitigating nutrient excretion. Effective predictions of N utilisation and excretion are based on knowledge of ingredient protein quality and the determinants of N and AA requirements. Protein quality is evaluated on the basis of N and AA digestibility and AA composition. Amino acid composition of grains, pulses and oil seeds is extensive, but there is large deficit on that of forages. Several studies have reported on pre- and post-caecal N digestibility in horses, demonstrating that a large proportion of N from forages is metabolised post-caecally. Few have reported on AA digestibility. It is proposed that whole-tract (i.e. faecal) N and AA digestibility be used in evaluating feed-ingredient protein quality in equids to begin designing predictive models of N and AA requirements. Nitrogen gain and AA composition in deposited tissues and their corresponding efficiency of utilisation are the key determinants for a prediction model. We estimated that N utilisation for maintenance is 0.74. Maintenance requirements for N and AA were derived from faecal N and AA losses in horses and expressed as a function of dry-matter intake and from integument losses in swine. Relative to our factorial model, the NRC (2007) requirement for lysine and N is overestimated when based on a segmented curve and a breakpoint. When based on N equilibrium, lysine NRC (2007) requirement estimate agrees with our factorial model estimate, while N requirement is underestimated. The pool of AA profile used to express requirements of other essential AA has a large impact on requirement, as shown, in particular, for threonine. Threonine requirement based on faecal endogenous AA profile is higher than is lysine requirement for maintenance and lactation.