Livestock Research for Rural Development 13 (6) 2001

Citation of this paper

Digestibility and nitrogen balance of diets containing cottonseed meal, alfalfa, or pigeon pea as the protein source

 W A Phillips and S C Rao

USDA-ARS Grazinglands Research Laboratory
7207 West Cheyenne Street, El Reno, OK, US 73036



The objective of this study was to determine the DM and protein digestibility and N balance in lambs of diets that contained either alfalfa (Medicago sativa), cottonseed meal (Gossypium), or raw cracked pigeon pea (Cajanus cajan) as the protein source. Eighteen lambs were blocked by genotype and within blocks assigned to one of three diets. All diets were formulated to provide similar amounts of crude protein (CP), which was equal to the daily CP requirement for a lamb with an average body weight (BW) of 38 kg.  Diets provided adequate energy to support an average daily gain (ADG) of 100 g. Lambs were confined to metabolism crates for the 28-days experiment and limit-fed each diet once daily. A 5-days total collection of feces and urine was conducted at the end of the experimental period.

Dry matter digestibility was similar among the diets containing pigeon pea, cottonseed meal or alfalfa, but protein digestibility was different.  The diet containing pigeon peas had a lower protein digestibility than the diet containing alfalfa. However, N retention (g/d) was not different among the three diets, although fecal N was higher in lambs fed the diet containing pigeon peas.

From these data, we conclude that pigeon peas can be used as a protein source in the diets of lambs and that one unit of raw cracked pigeon peas can replace 0.6 units of maize and 0.4 units of cottonseed meal in diets fed to lambs without lowering diet digestibility and N retention.

Key words: Pigeon pea, lambs, protein supplement, digestibility 

[NB: Mention of a trade name in this article does not imply an endorsement by the USDA, but is provided for clarity] 


In the southern portion of the United States, both warm- and cool-season grasses are used as pasture for grazing livestock (Phillips and Coleman 1995; Phillips et al 1996). Young beef cattle that graze warm-season grass pastures need supplemental protein during the last half of the grazing season to continue to make adequate weight gains (Phillips and Horn 1998), because forage CP levels have declined. Producers can utilize off-farm sources of protein such as by-products from the oil seed industry (cottonseed meal) or dehydrated forage legumes (alfalfa pellets), but the cost of these products can be volatile and requires the investment of additional capital. At the USDA-ARS Grazinglands Research Laboratory, forage legumes, such as lespedeza (Kummerowia stipulacea) (Rao and Phillips 1999), and grain legumes such as pigeon pea (Cajanus cajan) are being explored as potential new crops.  These crops can be used to utilize land resources during the normally fallow summer period between annual crops of winter wheat and to provide the producer with another crop to market. 

No data are available on the feed value of raw pigeon peas when fed to ruminants. Therefore, the objective of this experiment was to determine the dry matter (DM) and protein digestibility and N balance of a diet containing pigeon peas in comparison to diets containing more traditional protein sources.

Materials and methods 

Eighteen fall born wether lambs were blocked by breed into three groups. Lambs were either Dorset x Dorset, St. Croix x St. Croix or reciprocal crosses (Dorset x St Croix or St. Croix x Dorset). Within each breed group, lambs were randomly assigned to one of three dietary treatments (Table 1).  All diets were limit-fed at 2.4% of BW and were formulated to provide enough net energy for an ADG of 100 g and enough CP to meet the daily maintenance requirement (NRC 1985).  Cottonseed hulls were added as a source of fiber to the diets containing cottonseed meal and pigeon peas, but were not added to the diet containing alfalfa pellets.  The different protein sources were added to provide about 50% of the total protein intake. 

Table 1. Composition of diets containing different sources of protein and fed to lambs in digestion and N balance experiment.






Cottonseed hulls




Maize, ground




Cottonseed meal




Alfalfa pellets












Salt, plain white




Crude protein, % DM b




a Major source of dietary protein was either Alfalfa Pellets (ALF), Cottonseed meal (CSM), or raw cracked Pigeon pea (PP). Amounts are % of DM.
Diet crude protein concentration was calculated from the laboratory analysis of individual ingredients

.Lambs were randomly assigned to one of 18 metabolism crates designed for collection of feces and urine in separate containers. Lambs were given 10 days to adjust to the stalls and offered a diet of 50% maize and 50% alfalfa pellets at 2.5% of BW. Beginning on day 11 of the experiment, lambs were offered the assigned diet at 2.4 % of BW. No dietary transition period was used. 

Each morning at 08:00 any left-over feed present in the feeders was removed, weighed dried at 60EC for 72 h, weighed again to determine the DM content and stored for later chemical analysis. The diet assigned to each lamb was then prepared by weighing each ingredient into a container, mixing the ingredients by hand and transferring the mixture to the feeder. Lambs had ad libitum access to water provided adjacent to the feeder. After a 10-day diet adaptation period, a 5-day fecal and urine collection period was initiated. Feces were collected daily, weighed, dried at 60EC for 72 h, weighed again to determine DM content and stored. At the end of the collection period, all the feces collected from each lamb were composited and a sample was taken for chemical analysis. 

Urine was collected daily during the 5-day collection period in 9.4 liter plastic containers containing 10 ml of 50% sulfuric acid. Each day the weight of the urine collected was determined, diluted with water to a constant weight and a 100-ml sample taken. Daily samples of urine were composited by lamb in a 1000 ml capacity plastic container with screw on cap. Urine containers were kept refrigerated and 10 ml of 50% sulfuric acid were added to each container on day 1 of the collection period to maintain a low pH and prevent any N losses.  

With the exception of the raw pigeon peas, all dietary ingredients were purchased locally from commercial sources and required no additional processing. Pigeon peas (var. ‘Georgia- 2') were produced at the USDA-ARS Grazinglands Research Laboratory during the summer of 1999.  Peas were planted in small plots as a second crop grown during the normally fallow summer period between consecutive crops of hard red winter wheat (Triticum aestivum L., var. ‘Pioneer 2163') on Brewer silty clay loam soils. Following the harvesting of the winter wheat crop as grain, in June, peas were planted in a prepared seed bed in rows 60 cm apart at a spacing of 15 cm within the row. At the end of the 110-day growing season, pigeon pea grain was harvested with a small plot combine, air-dried and stored in sacks as intact peas without pods. Prior to the initiation of this experiment, peas were processed through a small, gasoline-powered hammer mill (John Deere Shredder Model CS-8) operated at an idle throttle speed. The purpose of this processing was to break the outer shell of the pea without greatly reducing particle size.  

Samples of dietary ingredients were collected daily for 5 consecutive days beginning 2 days before the initiation of the collection period. These samples were composited across the 5-day period, dried at 60EC for 72 h to determine DM content and saved for later chemical analysis.  Dried samples of left-over feed , feed ingredient and feces were grounded through a Wiley Mill equipped with a 1-mm screen. Nitrogen concentration of left-over feed , dietary ingredients and feces was determined by using complete combustion nitrogen analyzer (Leco-1000). Urine samples were freeze-dried and digested in a Technicon BD-40 block digester with sulfuric acid.  Nitrogen concentration was then determined colorimetrically (Technicon Methodology No. 329-74W). 

Lambs were weighed at the initiation (May 22, 2000) and at the termination (June 20, 2000) of the experiment. Both weights were taken following a 16-hour fast without feed and water.  Collection of feces and urine ended on June 17, 2000 and all lambs were fed the maize-alfalfa pellet diet previously described until removal from the crates on June 20.  

Data were analyzed as a randomized complete block design using the General Linear Model (GLM) procedure of SAS (1998). In the statistical model, breed was used as the block and diet was used as the treatment. Lamb was used as the experimental unit. If a significant (P < 0.10) ‘F’ value was observed in the statistical analysis, differences among means were determined using the LSD option of the GLM procedure.

Results and Discussion

Average BW was similar (P= 0.29) among the three treatment groups (Table 2).  Because the amount of DM fed was based on BW, DM intake was also similar (P = 0.14) among treatment groups. Crude protein intake ranged from 107 to 122 g/d, which was equal to the daily CP requirement for maintenance for these lambs (NRC 1985). On the other hand, energy intake was calculated to be sufficient for an ADG of approximately 100 g. 

Table 2. Average body weight, daily DM intake, and digestibility of DM and N by lambs fed diets containing alfalfa pellets (ALF), cottonseed meal (CSM), or pigeon peas (PP) as the source of protein








Average body weight, kg





DM intake, g/ d





DM digestibility, %





N digestibility, %





ab Means in the same row with different letters are different, P< 0.05; SEM standard error of means

Diet DM digestibility was similar (P > 0.10) among the three diets. Assuming a constant DM digestibility coefficient for the other ingredients in the diets, we were able to estimate the DM digestibility for cottonseed meal (74%), alfalfa pellets (56%), and raw cracked pigeon peas (75%).  The estimated digestibility can be used to compare the relative feed value of these three sources of protein and energy.  

The term “total digestible nutrients” (TDN) is used as an index of the dietary value of feedstuffs and is determined by summing the digestible CP, digestible carbohydrates, and 2.25 X digestible crude fat. Dry matter digestible is similar to TDN but is usually lower in feedstuffs with a high concentration of crude fat. Published data on pigeon pea DM digestibility was not available, but NRC (1971) assigned pigeon pea seed a TDN value of 67%, which is lower than the DM digestibility value we observed. Our calculated DM digestibility values for the two other protein source agree with published values of 75 % TDN for cottonseed meal and 56% TDN for alfalfa (NRC 1985).  

The CP concentration of the pigeon peas used in this experiment was 19.4%, which is similar to 20.6% reported by the NRC (1971) and 21.2% for cream colored pigeon pea and 22.5% for brown smooth and brown wrinkled pigeon pea reported by Ene-Obong (1995). Raw pigeon peas have been reported to contain trypsin inhibitors and tannins, which would can lower DM and protein digestion, which may explain why using pigeon pea as a protein source for diets fed to lambs in this trial did decrease (P<0.05) N digestibility (Table 2). It appears that while DM digestibility of pigeon pea is high, the N found in pigeon pea is less digestible than that found in alfalfa pellets but similar to that found in cottonseed meal. 

To calculate the relative value of pigeon pea in terms of other known ingredients, we assumed that the cottonseed hulls provided very little digestible DM and deleted them from the equation.  Because the DM and CP digestibility of diets containing cottonseed meal and pigeon pea were similar, we set these two equations as equal to one another (74.4 maize + 13.4 CSM = 53.3 maize + 34.6 pigeon pea), collected common terms and solved the equation for 1 unit of pigeon pea in terms of maize and cottonseed meal. One unit of pigeon pea was equal to 0.61 unit of maize and 0.39 unit of cottonseed meal.  

Due to slight variation in the amount of DM consumed and the N concentration of the different diets, N intake was significantly (P<0.05) different among diets, but the range was only from 16.2 to 19.6 g/d (Table 3). Replacing alfalfa pellets or cottonseed meal with pigeon pea increased (P<0.05) the amount of N excreted in the feces (6.66 vs 4.7 g/d) but did not (P = 0.18) increase the amount of N excreted in the urine.  

Table 3. Nitrogen balance in lambs fed diets containing either alfalfa pellets (ALF), cottonseed meal (CSM), or pigeon pea (PP) as the source of protein







N intake, g/d





Fecal N, g/d





Urinary N, g/d





N retention

  g /d





  Percent of N intake, %





  Percent of N absorbed, %





ab  Means in the same row with different letters are different, P <0.05

The amount of N retained (g/d) was not different (P = 0.47) among the three diets, but the lambs fed the diet containing pigeon pea tended (P = 0.13) to retain a smaller proportion of the N consumed (Table 3). This was primarily due to a greater N intake and lower N digestion by lambs fed pigeon pea diet as compared to the other dietary groups. Swanson et al (2000) fed a similar amount of N to lambs and reported N retention values of 2 to 4 g/d, which agree with the present observation. There were no differences (P = 0.23) among the three diets for the amount of N retained when expressed as a percent of N absorbed, suggesting that utilization of pigeon pea protein, once it was absorbed, was not different from cottonseed meal or alfalfa. 

Ene-Obong (1995) reported that the in vitro protein digestibility was 76.3% for cream, smooth, brown and wrinkled brown pigeon peas.  He also noted that the tannin concentration in these varieties was 7.5 to 14.4 mg/g, which was slightly lower than the tannin concentration of cowpeas grown in the same area. In a review on secondary plant compounds, Reed et al (2000) concluded that the presence of condensed tannins in the diet can lower apparent and true digestibility of protein.  But in small quantities, tannins can reduce unnecessary proteolysis and deamination of amino acids in the rumen resulting in increased post-ruminal non-ammonia N flow. They also noted that in some cases, the increase in fecal N losses due to tannins was offset by a decrease in urinary N, resulting in no change in N retention. Although we did not quantify the presence of tannins in the pigeon peas used in this experiment, we assumed that if tannins were present, any negative impact on N balance was buffered by a higher N intake. 

In summary, raw cracked pigeon peas can be used as a protein source for ruminants. Total diet DM digestibility was not affected by replacing cottonseed meal and maize with pigeon peas. We conclude that one unit if pigeon pea can replace 0.61 units of maize and 0.39 units of cottonseed meal without decreasing DM digestibility.  


The authors are gratefully to H Cantrell, J Garrison and R Bosueman for technical assistance and to Langston University, Garza Institute for Goat Research, Langston, OK, USA for N analysis of urine samples. 


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Received 19 September 2001

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