Cowpea ( Vigna unguiculata L. Walp) and groundnut ( Arachys hypogea L.) haulms as supplements to sorghum ( Sorghum bicolor L. Moench) stover: intake, digestibility and optimum feeding levels

Abstract

Two feeding trials were conducted to study the combined effects of (i) varying degrees of selective consumption and (ii) supplementation with cowpea (Trail 1) or groundnut haulms (Trial 2), on intake of organic matter (IOM) from sorghum stover, and total intake of digestible organic matter (IDOM). In both trials, 36 rams were allocated to 36 different treatments: six levels of feeding sorghum stover (25, 40, 60, 90, 120 and 160 g organic matter (OM) kg −0.75 per day) combined with six levels of supplementation (0, 5, 12.5, 20, 40, and 60 g OM kg −0.75 per day). Each diet was offered for a 21 day period; intake and digestibility were recorded during the last 9 days. Non-linear regression models were used to describe the combined effects of varying amounts of stover ( x) and supplements ( s) offered. All animals ate the full amount of supplement offered, but not the sorghum stover. For animals without supplement, maximum intake ( m) of stover (i.e. IOM at high levels of x) was estimated at 50.7 g kg −0.75 per day in Trial 1 and 45.7 g kg −0.75 per day in Trial 2. In both trials, m decreased linearly with s at the rate 0.4 g g −1. Also, the shape of the curve relating intake of stover to x was affected by level of supplementation. Digestibility of whole stover (0.47 in Trial 1; 0.40 in Trial 2) was much lower than that of cowpea haulms (0.70) and groundnut haulms (0.62), but in both trials animals selected material of higher digestibility when excess stover was offered. Thus, the negative effect of supplementation on intake of stover was partly compensated by higher digestibility of consumed stover. For rations without cowpea in Trial 1, the maintenance level of IDOM was reached by offering 61 g sorghum OM (kg −0.75 per day) of which 47 g was consumed. With the same amount of offered sorghum 9, 18, 28, 38 and 48 g cowpea OM were needed to attain intake levels equivalent to 1.2, 1.4, 1.6, 1.8 and 2.0 times maintenance, respectively. In the trial with groundnut, maintenance was not reached with sorghum alone and larger amounts of supplement were required for the levels of intake mentioned above. Iso-production curves describing which amounts of stover and supplement gave the same IDOM, were not linear with constant slope corresponding to the comparative digestibility of whole stover and supplements, but strongly curved. Such curves can be used to derive optimum combinations of stover and higher quality feeds, depending on feed prices, desired production level and taking into account the effects of selective consumption. With prices of cowpea and groundnut haulms 4 times higher than that of stover, the results of Trial 1 indicate that least cost rations for feeding at 1.2 M (maintenance) would consist of 72 g sorghum OM (kg −0.75 per day) combined with 7 g cowpea haulms. For feeding at 1.6 M this would be 70+26 g , and at 2 M, 54+50 g . Similarly, with the feeds used in Trial 2, optimum combinations of sorghum stover and groundnut haulms for the same levels of feeding would be 58+24, 51+47, and 11+78 g OM kg −0.75 per day .