1070
Lactational response of Holstein cows to brown midrib or leafy-floury corn silage
The objective of this experiment was to measure the response of lactating Holstein cows to total mixed rations containing either brown midrib-3 (BMR; Mycogen F2F387) or an experimental leafy-floury hybrid (LF; Healthy Herd Genetics HHG39HF13) bred for improved NDF and starch digestibility compared with similar leafy corn hybrids. Sixteen cows (6 primiparous; 10 multiparous) averaging 130±16 days in milk were assigned to one of two dietary sequences in a crossover design with 28-d periods (21 d adaptation and 7 d collection). Each diet contained 49% BMR or LF corn silage, 9.8% haycrop silage, and 41.2% concentrate mix (DM basis). The BMR silage contained 30.2% DM, 7.3% CP, 41.5% NDF, 60.2% 24-h NDF digestibility, 31.0% starch, and 77.5% 7-h starch digestibility (DM basis). The LF silage contained 31.9% DM, 7.8% CP, 42.6% NDF, 47.5% 24-h NDF digestibility, 30.3% starch, and 73.0% 7-h starch digestibility which was unexpectedly low. The BMR diet contained 17.2% CP, 32.2% NDF, and 24.6% starch, and the LF diet was similar with 17.8% CP, 33.5% NDF, and 23.0% starch. Data were analyzed as a crossover design using the MIXED procedure of SAS with model effects of diet, sequence, and period, with cow within sequence as a random effect. Compared with the LF diet, the BMR diet resulted in greater DMI (29.7, 27.2 kg/d, SE=0.9; P < 0.001), greater SCM yield (47.0, 41.7 kg/d, SE=1.4; P < 0.001), and greater SCM/DMI (1.59, 1.54, SE=0.04; P < 0.01). Chewing during eating and ruminating was greater for the LF versus the BMR diet (89, 84 min/kg NDF intake, SE=3; P < 0.001). Compared with the LF diet, the BMR diet resulted in greater total tract OM and NDF digestibility (81.4, 77.7%, SE=0.6; 63.8, 55.4%, SE=0.8; P < 0.001), although starch digestibility was similar for the LF and BMR diets (99.0, 98.9%, SE=0.1; P < 0.001). Compared with the BMR diet, the LF diet resulted in higher fecal pH (6.92, 6.71, SE=0.03; P < 0.001), lower fecal starch (0.82, 1.15%, SE=0.09; P < 0.008), and lower fecal P (0.53, 0.63, SE=0.02; P< 0.001). The LF hybrid assessed in this experiment constrained DMI and milk production compared with the BMR hybrid. To achieve dietary fermentable carbohydrate content similar to BMR, a LF hybrid will require higher NDF and starch digestibility than the hybrid evaluated in this experiment.
Keywords: Leafy-floury corn silage, brown midrib corn silage, dairy cattle