Silagem de milho ou cana-de-açúcar e o efeito da textura do grão de milho no desempenho de vacas holandesas

Abstract

Brazilian corn hybrids have primarily hard type endosperm of high vitreousness, while in other countries, as in the United States, dent type hybrids ofsoftendosperm prevails. Dentcornhas greater ruminal starch digestibility and smallerrate of decay in digestibility with advancing maturity than hard hybrids. In the fírst experiment it was tested the hypothesis that the use of soft cultivars may enlarge the ensilage period of corn. A hard texture hybrid (P3041) was ensiled at the half milk line stage ofgrowth (MD) and a dent hybrid (AG4051) at the black layerstage (MM). A third treatment tested the viability of sugarcane (CA) as a forage for high producing dairy cows. Nine lactating Holstein cows were allocated to a sequences ofthe three treatments in three 3x3 Latin squares with 21-day periods. The experimental diets contained 200 g offorage NDF per kg of dry matter. There was no detectable difTerence between corn hybrids in production (34.2 vs 34.6 kg/d) and composition ofmilk, dry matter intake (23.0 vs 23.2 kg/d) and total tract apparent digestibility of nutrients CP>0.37 for the contrast MM vs MD). Sugarcane decreased intake (21.5 kg/d) and milk production (31.9 kg/d) (P<0.05 for the contrast CA vs MM+MD). The lower sugarcane NDF digestibility compared with corn silage NDF (23.1 vs 42.1%, PO.001) was offset by the higher nonfíber carbohydrate digestibility (79.8 vs 74.8%, P=0.02), equalizing the organic matter digestibility (63.9 vs 65.5%, P=0.52). Chewing activity and rumen pH were not different between treatments CP>0.13). In the second experiment, grain texture and ruminal degradability of Brazilian corn hybrids were compared to those of North American hybrids. Fourteen North American hybrids were cultivated in the experimental fíelds of the University of Wisconsin-Madison and harvested at the half milk line stage (ML), black layer (LN) and 21 days after black layer (mature, MD). Five Brazilian hybrids, cultivated in the south ofMinas Gerais State, were harvested at the MD stage. The vitreous endosperm as a percentage of the total endosperm (Vitreousness) and the density (g/cm3) of the grains were evaluated in ali samples. In North American hybrids the vitreousness increased linearly with advancing plant age (PO.001 for the linear effect of maturity stage). The correlation between grain vitreousness and density was 0.87. The vitreousness of the five Brazilian hybrids was 73.1% and density was 1.292 g/cm3, greater than the vitreousness and density ofthe 14 North American hybrids atthe MD stage, 48.2% and 1.201 g/cm3 (PO.001 for the country effect). Subsequently, three North American and three Brazilian hybrids at the MD stage ofmaturity were sub-sampled to represent extremes of vitreousness for each country. These hybridswere degraded insitu in three lactating cows fitted with rumen cannulae. The fractional rate ofstarch degradation ofthe Brazilian hybrids was 2.7 times smaller than the rate ofthe North American hybrids (PO.001). Ruminal starch availability ofNorth American hybrids was 77.4% and for Brazilian hybrids it was 48.5% (PO.001). Samples by stage ofmaturity ofthe most and the least vitreous North American hybrids were also degraded in situ. Ruminal degradation decreased with advancing maturity (PO.001 for the linear effect of stage of maturity). Correlation between vitreousness and ruminal starch degradation using ali in situ degradability data was 0.75. Sugarcane may be an alternative to feed groups ofHolsteins producing up to 32 kg ofmilk per day. The use ofdent corn seemsto be a way to enlarge the ensilaging period ofcorn without reducing animal performance. Brazilian corn grains had harder texture and smaller ruminal starch degradation than North American grains. Vitreousness seems to be an useful parameter for the selection ofcorn hybrids with high ruminal starch degradability.