Studies on the nutritive value of silage with particular reference to the effects of energy and nitrogen supplementation in growing heifers

Abstract

SummaryFour digestibility and balance experiments were carried out in which 33 growing heifers were fed basal diets of low dry matter silage offeredad libitum, supplemented with various levels of barley and/or groundnut meal or urea.Voluntary intake of the silages was low although apparent digestibility of D.M. was high (66–75%). There was a significant linear negative relationship between the intake of D.M. as supplement and silage D.M. intake.Apparent digestibility of crude fibre in the silages was high (74–83%). It was depressed significantly by addition of barley but not by addition of groundnut. There was a highly significant linear relationship between the digestibility of D.M. and G.B. in the mixed silage concentrate diets used.Metabolizable energy (M.E.) values for the silages varied from 2·04 to 2·52 Mcal/kg (8·5–10·5 MJ/kg) D.M. Supplementation with barley at approximately 1% of live weight, increased M.E. values of the mixed diets to 2·57–2·85 Mcal/kg (10·8–11·9 MJ/kg) D.M. Additional N had little effect on these values.Nitrogen retention was significantly increased in all experiments by barley supplementation. It was further significantly increased by groundnut in three experiments and by urea in one experiment.Multiple regression analysis showed that N retention was significantly related to both M.E. intake and intake of absorbed N, and to M.E. intake and intake of supplementary N.Apparent availabilities of Ca and Mg in silage based diets were low and the availability of Na varied with Na intake. Supplementation with either energy or nitrogen increased apparent availability and retention of Mg but did not exert consistent effects on availability or retention of Ca or Na.There was a highly significant linear relationship between ingested and digested Na but not for Ca or Mg. There was a highly significant linear relationship between absorbed and retained Ca and Mg, but not for Na.Animals fed second cut silages had rather low blood glucose levels and higher than normal levels of blood ketones.