Ruminal silage degradability and productivity of forage and grain-type sorghum cultivars

Abstract

Forages for feeding dairy cows should conciliate the potential for high dry matter yield per unit area, maximizing animal stocking rate, with high nutritive value, which enables decreasing the amount of concentrate feedstuffs per milk unit produced. Eighteen sorghum cultivars were cultivated 0.70 m apart and with 12 plants per linear meter. Plants harvested at the milk-to-dough stage were ensiled in the laboratory. Eleven agronomic and chemical traits were evaluated. Silage samples were incubated in situ and the effective ruminal dry matter degradation (DEF) was calculated assuming a fractional passage rate of -0.04 h-1. Dry matter yield was 13.0 ± 2.3 (mean ± S.D.) t ha-1. The neutral detergent fiber content was 50.3 ± 3.8 and the DEF was 48.8 ± 2.8% (dry matter basis). The average sorghum DEF was 84% of the DEF of a composite sample of 60 corn hybrids simultaneously cultivated, ensiled and ruminally incubated. The 12 forage-type cultivars were taller, more productive, had higher fiber content and lower DEF than the 6 grain and dual purpose-type cultivars. The acid detergent fiber content had the greatest correlation with DEF (r= -0.64). The linear model correlating DEF with productivity was: DEF=54.694 - 0.4449 x t of dry matter ha-1 (r²=0.14). Although there are cultivars that show high productivity and digestibility, it seems to be difficult to conciliate the maximum nutritive value with the maximum productivity of the sorghum crop.