Digestibility and milk performance of dairy goats fed two dietary starch concentration, with whole or ground maize

The purpose of this experiment was to evaluate the effects on intake, digestibility and milk performance of goats fed diets with whole or ground maize at two dietary starch concentration obtained substituting beet pulp for maize. For this aim eight second parity Saanen goats (57.4 kg BW, on average) in mid lactation (83 DIM, on average), confined in individual metabolic cages, were utilized for a digestible trial in a duplicated 4 x 4 Latin square design balanced for carry-over effect with a 2 x 2 factorial arrangement of treatments. The goats were fed four diets, with a forage:concentrate ratio of 40:60, containing: 33.5% (on dry matter basis) whole maize grain (HSMG) or 33.5% maize meal (HSMM) in the diets with high starch content; 22.5% dried beet pulp and 11.0% whole maize grain (LSMG) or 11.0% maize meal (LSMM) in the two diets with low starch content. The other ingredients were (on DM): barley meal (13.3%), soybean meal (9.1%), maize gluten meal (3.0%) and mineral-vitamin complex (3.0%). The diets were isonitrogenous and with NDF and starch content (% on DM) of 32 and 33% or 40 and 17%, for high or low starch diets, respectively. The diets were offered ad libitum, allowing for about 5% orts. Individual data for DM intake, digestibility and milk production were statistically analysed using GLM procedure of SAS. Tukey’s test comparisons of treatment means was conducted if P<0.05 for interaction of main effects. The animals made a strong feed selection leaving hay as the sole refusal. As a consequence, the forage:concentrate ratio (DM basis) of the ingested diet resulted to be 32:68 instead of the planned 40:60, on average for the four treatments. The DMI obtained during the experimental trials was not significantly different among treatments, despite a higher value registered by low starch diets (2333 and 2229 g/d, for low and high starch diets, respectively, P<0.05). Concerning DM digestibility, the interaction between the effects of starch concentration and type of maize was statistically significant (P<0.05); particularly, the DM of HSMG diet resulted better digested (71.6%) in comparison with HSMM diet (69.0%) and with LSMG diet (69.1%). Substitution of beet pulp for maize significantly improved NDF and ADF digestibility (56.4 and 55.5% and 42.2 and 38.9%, for low and high starch diets, respectively, P<0.001) due to the high content of pectins, confirming the results obtained by Voelker & Allen (2003) in a trial with dairy cows. Starch digestibility was influenced (P<0.001) by the physical form of maize: whole grain (92.6%) was less utilized than maize meal (97.7%), despite it did not impair total DM digestibility. Anyway, the high utilization of starch in diets with whole maize (94.2 and 90.9% for high starch and low starch, respectively) indicates a low passage of indigested grain trough the gut. This seems due to an extensive damage of maize kernels during mastication (Beuchemin at al., 1994) which determines a relevant grain utilization already at rumen level. The milk production was not affected by the level of starch (4323 and 4328 g/d, for low and high starch diets, respectively): this was probably due to the higher DMI by goats fed the low starch diets. On the other hand, looking at the effect of the type of maize in the diets with high starch content, it is evident that maize meal depressed feed intake and milk yield (4075 and 4570 g/d, for HSMM and HSMG, respectively, P<0.05). Milk fat was not affected neither by starch level nor by maize type and its percentage for all treatments resulted lower than protein percentage (2.52 and 2.66%, for fat and protein content on average, respectively). This might be due to the low ether extract content of the diets (2.24%). The proportion of lactose was significantly influenced by the level of starch (4.49 and 4.39%, for high and low starch diets, respectively, P<0.001): this could be ascribed to the presumably higher rumen propionate production from amylolytic microbes. In conclusion, substitution of beet pulp for maize in diets rich in concentrates significantly improved DMI and NDF and ADF digestibility due to their high content of soluble fibre. Moreover the utilization of whole grain instead of maize meal does not depress diet digestibility while increases milk yield. The data obtained suggest that the grinding of maize given to lactating goats is likely to be of no value.