Introduction concentrate feeds by difference. As part of this

Livestock Production Science 68 2001 41–52 www.elsevier.com locate livprodsci The effect of dietary protein content and hay intake level on the true and apparent digestibility of hay F.J. Mulligan , P.J. Caffrey, M. Rath, M.J. Kenny, F.P. O’Mara University College Dublin , Department of Animal Science and Production, Lyons Research Farm, Newcastle, Co. Dublin, Ireland Received 14 September 1999; received in revised form 25 January 2000; accepted 8 June 2000 Abstract The digestibility of concentrate feeds has been determined by difference using hay as a basal forage in many institutions. The objective of this experiment was to examine the effect of hay intake level and dietary crude protein CP content on hay digestibility. Groups of wether lambs 1 year old were fed three levels 600, 700 and 800 g day of hay each at two CP contents 103 and 182 g CP kg DM, achieved by using a urea solution. An extra group of lambs were fed hay unsupplemented with urea 42 g CP kg DM at a level of 800 g day. The true organic matter digestibility True OMD, calculated using two methods: True OMD 1 and 2 and apparent OMD of all treatments fed the 182 g CP kg DM diets were significantly higher P , 0.05 than treatments fed the 103 g CP kg DM diet OMD: 632.8 g kg vs. 604.2 g kg; True OMD 1: 720.6 vs. 694.1 g kg; True OMD 2: 711.8 vs. 681.0 g kg. Apparent and true digestibility also increased as hay allowance increased and a significant linear effect was observed for GED. For the three groups fed 800 g day of hay, significant differences in digestibility due to CP content only occurred between the 182 and the 42 g CP kg DM diets with the exception of CP digestibility. Although the effect of CP content was significant in some cases, the magnitude of the observed digestibility response is of little consequence for concentrate feed evaluation.  2001 Elsevier Science B.V. All rights reserved. Keywords : Hay digestibility; Intake level; Protein content; Sheep

1. Introduction concentrate feeds by difference. As part of this

procedure, it is necessary to assume that the di- Ruminants require some source of forage to ensure gestibility of the hay is unchanged when it is fed normal rumen function. This being the case, diets with the concentrate being evaluated. If the diges- used to evaluate the digestibility of concentrate feeds tibility of the hay were to change markedly, then the often include hay as a small proportion of the overall digestibility of the concentrate when calculated by diet. The digestibility of this hay must be pre-de- difference would be distorted. Two factors which termined to allow calculation of the digestibility of may affect the digestibility of hay are dietary protein content and hay intake level. Although several reports exist in the literature on Corresponding author. Tel.: 1 353-1-6012-167; fax: 1 353-1- 6288-421. the effect of dietary protein content on the diges- 0301-6226 01 – see front matter  2001 Elsevier Science B.V. All rights reserved. P I I : S 0 3 0 1 - 6 2 2 6 0 0 0 0 2 0 9 - 8 42 F .J. Mulligan et al. Livestock Production Science 68 2001 41 –52 tibility of feeds, considerable ambiguity still exists 2. Materials and methods with regard to the level of dietary protein required for optimum digestibility. Several authors Satter and 2.1. Animals and treatments Roffler, 1977; National Research Council NRC, 1984; Boggs et al., 1987; Willms et al., 1991 have Twenty-eight Suffolk or Texel cross wether lambs suggested that a crude protein CP content of 100– 1 year old of mean weight 41.6 kg C.V. 8.54, 120 g kg DM is optimum for digestibility. However were used to determine the true and apparent di- Cronin 1996 reported large increases in the di- gestibility of hay. The experiment had a 3 3 2 gestibility of hay as the CP content of a mixed factorial design groups A to F with three feeding hay soyabean meal ration was increased up to 202 levels of hay 600, 700 and 800 g d at two dietary g kg DM. Krishna Mohan et al. 1987 also reported CP contents 110 and 200 g CP kg DM. As it was increases in the apparent digestibility coefficients of desired to effect a change in the CP content of the isocaloric, mixed, rations with a roughage sun- diet without markedly changing the DM intake or nhemp hay and paddy straw to concentrate maize introducing a concentrate into the diet, urea solution and de-oiled groundnut-cake ratio of 1:1 as CP was used as the supplementary protein source. content was increased up to 160 g kg DM. Another treatment group G which was not part of Most of the evidence on intake effects and di- the factorial, was fed 800 g d of hay 42 g CP kg gestibility is concerned with mixed diets, or the DM unsupplemented with urea. The inclusion of 10 concentrate proportion of the diet. Several authors g of a mineral vitamin mix 14 Ca, 7 P was Tyrrell and Moe, 1975; Colucci et al., 1982; Robin- common to all diets. Details of the experimental son et al., 1987; Edionwe and Owen, 1989; Zinn et groups are given in Table 1. al., 1994; Woods et al., 1999 report depressions in digestibility for various diets with increasing level of 2.2. Experimental procedure intake. However other researchers Blaxter et al., 1956; Ulyatt et al., 1983; Mbwile and Uden, 1997 A period of 18 days was allowed for acclimatisa- have reported no significant effect of increasing tion to both the diets fed and the experimental intake on digestibility. In all cases, where no differ- environment. This included a period of 7 days on full ence in digestibility was reported as intake increases, dietary allowance, then movement to metabolism mostly forage diets were fed. crates for 4 days before total faecal collections began The digestibility of large numbers of concentrate for a period of 8 days. During this period, account ingredients has been determined at this institute in was taken of any feed refusals. Each days faecal recent years Coyle et al., 1996; Cronin et al., 1997; collection was dried at 55 8C for 3 days and a Mulligan et al., 1998; O’Mara et al., 1999. Hay was constant proportion of the daily faecal DM output used as the basal forage and the level of hay fed was used to make a composite sample for the 8 days. would have differed when hay was fed as the basal The protein source urea solution was sprayed on forage typically 15 of dietary DM than when its the previously weighed hay allowances which were digestibility was measured separately typically 90 of dietary DM. The CP content of the diet would Table 1 also have varied depending on the CP content of the Details of treatments and urea supplementation daily allowances concentrate ingredient being evaluated. It was gener- Group Hay g d Urea g d CP g kg DM ally between 100 and 200 g CP kg DM, in com- A 600 11.9 103 parison to a level of 100 g CP kg DM when the B 600 29.2 182 digestibility of the hay was being determined separ- C 700 14.8 103 ately. In that context, this study was important to D 700 34.1 182 validate these measurements of concentrate diges- E 800 17.0 103 tibility. The objectives of the experiment were to F 800 39.0 182 a G 800 42 determine the effect of hay intake level and dietary a protein content on the true and apparent digestibility Group G received 30 ml of water sprayed on hay twice daily of hay. instead of urea solution. F .J. Mulligan et al. Livestock Production Science 68 2001 41 –52 43 fed daily in equal halves using manual pump-action meal and 100 g d of a mineral–vitamin mix over a spray bottles, at the time of feeding. This took period of 10 days. Degradability constants a, b and c approximately 1 h, animals being fed at 8–9:00 a.m. for hay DM were determined according to the 2ct and 4–5:00 p.m. Two urea solutions were used, equation P 5 a 1 b1 2 e Orskov and which consisted of either 264 g 110 g CP kg DM McDonald, 1979 where ‘a’ is the rapidly soluble treatments or 700 g 200 g CP kg DM treatments DM, ‘b’ is the potentially degradable DM not of granular urea 46 N added to 1 l of distilled including a and ‘c’ is the fractional rate of degra- water and heated to approximately 80 8C. The daily dation per hour of the ‘b’ fraction with time ‘t’. Dry allowance of minerals and vitamins was sprinkled matter solubility was also determined using the over the dampened hay. method of Weisbjerg et al. 1990 using Whatman No. 1 filter paper. 2.3. Chemical analysis 2.5. True digestibility calculations A sample of the hay was collected over the 8-day faecal collection period and sprayed with both urea Two separate estimates of true OMD true organic solutions at the end of the experiment. This sample matter digestibility were made. The first method was used for determination of the actual CP content called True OMD 1 Mason, 1968 estimates true of the diets. Crude protein was determined as digestibility as 1 2 faecal NDF total OM intake. Kjeldahl nitrogen N 3 6.25 using a Buchi 435 This method attempts to quantify undigested diet digestion unit and a Buchi 323 distillation unit faecal NDF exclusive of other faecal products by Buchi, Postfach, Flawil Schweiz, Switzerland ac- assuming that the converse of NDF i.e., cell con- cording to the Association of Official Analytical tents are 100 digestible and that animal and Chemists AOAC 1980. Neutral detergent fibre microbial cells are soluble in neutral detergent NDF and acid detergent fibre ADF were de- reagent. The second method of estimating true termined using a Fibertec extraction unit Tecator, digestibility, True OMD 2, estimates it as 1 2 [total Hoganas, Sweden according to the methods of Van faecal OM 2 microbial and endogenous faecal OM Soest et al. 1991 and Van Soest 1973, respective- total OM intake]. This method for estimating true ly. Acid detergent lignin ADL was determined after digestibility assumes that the difference in total re-suspension of ADF residue in 10 acid detergent faecal nitrogen and faecal NDF nitrogen represents reagent. The re-suspension was boiled and simmered microbial and endogenous faecal nitrogen Mason for 5 min, then filtered and washed. The crucibles and Frederiksen, 1979. Faecal NDF N was de- were then transferred to a Fibertec cold extraction termined on the residue of faecal samples subjected unit where 25 ml of sulfuric acid was added. After 3 to neutral detergent extraction using the filter bag h the crucibles were filtered and washed until the technique and the Ankom Fiber Analyser Ankom, acid had disappeared and ADL was then determined Fairport, NY, USA. Microbial and endogenous as the residue. The gross energy GE content of faecal organic matter may then be determined by both feed and faecal samples was determined using a assuming that microbial and endogenous faecal Parr 1201 oxygen bomb calorimeter Parr, Moline, nitrogen is 7 of microbial and endogenous faecal IL, USA. Ash was determined after ignition of feed organic matter Van Soest, 1994. or faeces at 550 8C for 4 h and used to calculate organic matter OM. Dry matter DM was de- 2.6. Statistical analysis termined in an oven at 104 8C for a minimum of 16 h. The effect of hay intake level and CP content on 2.4. In-sacco analysis the digestibility D of DM, OM, CP, NDF, ADF, GE, True OMD 1 and True OMD 2, as well as total Dry matter degradability was determined using faecal N, faecal NDF N Feed N and microbial and three Holstein–Friesian steers mean weight: 573 kg; endogenous faecal N output, were compared by C.V. 7.5. Animals were acclimatised to a basal diet analysis of variance ANOVA using the PROC consisting of 7 kg d of hay, 1 kg d of soyabean GLM statement of the SAS Institute 1985. The 44 F .J. Mulligan et al. Livestock Production Science 68 2001 41 –52 model included feeding level and CP content and their interaction as sources of variation. This was carried out on the data of groups A to F i.e., all the supplemented groups. This data was also subjected to regression analysis using the PROC GLM state- ment of the Statistical Analysis Systems SAS Institute to test the linear and quadratic effects of hay allowance on all digestibility coefficients. Data from the seventh group G who were fed 800 g d of hay unsupplemented with any protein 42 g CP kg DM Fig. 1. In-sacco analysis of untreated hay. were then combined with the other two groups who received 800 g d of hay, group E 110 g CP kg DM and group F 200 g CP kg DM. These data were is potentially degradable in the rumen Fig. 1. Using analysed as a completely randomised design using the method of Weisbjerg et al. 1990 291.1 g kg the PROC GLM statement of the SAS Institute DM of the hay was soluble in water. 1985. The model included CP content i.e., treat- Where digestibility coefficients for groups A to F ment as the source of variation. The mean di- were compared in the factorial analysis, no signifi- gestibility coefficient for OMD, True OMD 1 and cant interaction of protein content and hay intake True OMD 2 for all animals were also compared level was found. With regard to the effect of protein using the PROC GLM statement of SAS in order to content, the digestibility of the hay in the 182 g compare the two methods of measuring true di- CP kg DM diets was found to be significantly higher gestibility and the estimates of apparent OMD. For P , 0.05 than the digestibility of hay in the 103 g the in-sacco analysis, parameters were estimated CP kg DM diets for all digestibility coefficients using the PROC NLIN statement of the SAS Institute except NDFD and GED Table 3. 1985. Table 3 a Effect of dietary CP content on digestibility coefficients g kg

3. Results