You have to know previously the components of the feed and see if the animals present some alergic, not aspected reaction or decrease of the production parameters. First you have to read about the antinutrional factors reported to focus your research

I agree with Willamil: Numerous methods available depending on what you want to analyze (condensed tannins, hydrolysable tannins, heavy metals, lignin,,,.

Volatile oils (terpenes/terpenoids, etc.): steam distillation or solvent extraction

Condensed tannins and hydrolysable tannins: various methods.

Some references to get you started:

1. Juha-Pekka Salminen* and Maarit Karonen. 2011. Chemical ecology of tannins and other phenolics: we need a change in approach. Functional Ecology 2011, 25, 325–338

2. . Irene Mueller-Harvey. Review: Unravelling the conundrum of tannins in animal nutrition and health. J Sci Food Agric 86:2010–2037 (2006).

3. Adams, R. P. 2007. Identification of essential oil components by gas chromatography/mass spectrometry. 4th ed. Allured Publishing, Carol Stream, IL.

4. Adams, R. P., C. A., Taylor, Jr., T. R. Whitney, W. C. Stewart, and J. P. Muir. 2013. Goats and deer do not use terpenoids to select or avoid browsing on Juniperus pinchotii Sudw. trees. Phytologia. 95:238−245.

5. Terrill, T. H., A. M. Rowan, G. B. Douglas, and T. N. Barry. 1992. Determination of extractable and bound condensed tannin concentrations in forage plants, protein concentrate meals, and cereal grains. J. Sci. Food Agric. 58:321–329.

6. Utsumi, S. A., A. F. Cibils, R. E. Estell, and Y. F. Wang. 2006. Influence of plant material handling protocols on terpenoid profiles of one-seed juniper saplings. Rangel. Ecol. Manage. 59:668−673.

Book:

Harinder P. S. Makkar, S. Siddhuraju, Klaus Becker. Plant Secondary Metabolites (Series: Methods in Molecular Biology - 393). Humana Press, 2007

I have this book , you can determining "antinutritional factors " and any phytochemical (such Tannins , saponin . EO) in feed

Harinder P. S. Makkar, S. Siddhuraju, Klaus Becker. Plant Secondary Metabolites (Series: Methods in Molecular Biology - 393). Humana Press, 2007

Lot of methods try this link http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/food_technology/food_analysis_and_quality_control/21.analysis_of__antinutrients_in_foods/et/2626_et_m21.pdf

I hope this can help you. I'm also doing my research on methods of quantification of the antinutritional factors in feeds.

Trypsin inhibitor

Trypsin inhibitor was assayed by determining residual trypsin activity using Kakade et al. (1974) method with slight modification. Briefly, samples (1.0 g) were extracted with 0.01 N sodium hydroxide for 3 hrs in shaker and the supernatant was collected after centrifugation. To the known aliquots, 2 ml of trypsin solution was added and kept in a temperature bath at 37°C for 10 min. 5 ml of Nα-Benzoyl-L-arginine 4-nitroanilide hydrochloride (BAPNA) pre-warmed to 37°C was added. The reaction was terminated exactly 10 min later by adding 30% of acetic acid. A blank and control were also run simultaneously. The absorbance was read at 410 nm in UV-spectrophotometer (Shimadzu, UV-1800). Decrease of 0.019 of ∆A indicates the presence of 1µg of trypsin inhibitor in the sample.

Phytic acid

The estimation of phytic acid was carried out by Davis and Reid (1979) method after extracting the samples with 0.5 N nitric acid for 3 h. To the known volume of extract, 1ml ferric ammonium sulphate was added and placed in a boiling water bath for 20 min followed by 5 ml of amyl alcohol. The test tubes were shaken well, and then centrifuged at 3000 rpm for 10 min. Finally the colour intensity was read at 465 nm in UV-spectrophotometer (Shimadzu, UV-1800) against amyl alcohol blank exactly after 15 min of addition of ammonium thiocyanate.

Tannin

Vanillin-HCl assay (Price et al. 1978) was used to determine the quantity of tannin content after extracted with absolute methanol for 20 min. The content was centrifuged at 3000 g and the supernatant was used for analysis. To the 1 ml of extracted aliquots, 5 ml of vanillin-HCl reagent was added and kept in a water bath for 20 min. The intensity of colour developed was read at 500 nm in UV-spectrophotometer (Shimadzu, UV-1800) against 4% hydrochloric acid as a blank. Catechin as equivalent to tannin was used as a standard with different concentration.

Saponin

Saponin was extracted from the acetone extracted residual matter for 3 h using methanol in Soxhlet extraction unit (AOAC 1997). To 1 ml of methanolic extract, water and organic solvent (chloroform and methanol) was added at the ration of 1:2 and allowed to separate the layers after mixing thoroughly. The upper aqueous layer (1 ml) was kept at 60 110°C in hot air oven till complete evaporation of solvent. To which 0.1 ml and 0.4 ml of vannilin reagent and perchloric acid were added, respectively and kept at 70°C for 10 min. The intensity of colour developed was read at 540 nm in UV-spectrophotometer (Shimadzu, UV-1800) after adding 2.5 ml acetic acid. Diosgenin was used as a standard at different concentration to calculate the saponin content.

Glucosinolate

Glucosinolate was estimated by McGhee et al. (1965) method. Briefly, 10 g of samples were extracted with hot distilled water for 5 min and the contents were filtered using Buchner funnel. The residue was repeatedly washed with hot water and made up to known volume. To 25 ml of extract, 10 ml of silver nitrate, 25 ml of ethanol were added and kept in a boiling water bath for 45 min. After cooling to room temperature, it was titrated against 0.01 N potassium thiocyanate in the presence of 6 N nitric acid and ferric ammonium sulphate till the pale salmon colour was obtained.

Guar gum

The estimation of gum content in guar meal was carried out by Das et al. (1977) method with slight modifications. Briefly, guar meal was soaked with distilled water at the proportionate of 1:10 for overnight. The soaked material was filtered using Whatmann filter paper and the residue was rinsed for 3 to 5 times with distilled water. All the filtrates were pooled together in a container. The gum content was precipitated by the addition of 50 to 100 ml of isopropanol and the process was repeated for 5 to 10 times. Meanwhile the precipitated gum was collected completely in a pre-weighed petridish and the exact quantity of gum content was measured after drying the excess of moisture.