Effect of grinding of maize and level of starch on digestibility and lactation performance of Saanen goats

 

 

L. Rapetti^#; L. Bava

Istituto di Zootecnia Generale, Facoltà di Agraria, Université degli Studi di Milano, via Celoria 2, 20133 Milano, Italy

 

 

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ABSTRACT

The purpose of this experiment was to evaluate the effects on intake, digestibility and lactational performance of goats when fed diets containing whole or ground maize grain at two different dietary starch levels through substitution of beet pulp for maize. Eight second parity Saanen goats in mid lactation were used in a digestibility trial. The experimental design was a duplicated 4 x 4 Latin square 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 or 33.5% ground maize grain in the diets with high starch content; 22.5% dried beet pulp and 11.0% whole maize grain or 11.0% ground maize grain in the two diets with low starch content. The high starch diets significantly decreased DMI in comparison with the low starch diets (2333 and 2229 g/d, for low and high starch diets, respectively). 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) due to the lower content of starch. Starch digestibility was influenced by the grinding of maize: whole grain (92.6%) was less utilized than ground grain (97.7%), despite not impairing total DM digestibility. The ground maize grain depressed feed intake and milk yield in the high starch content diets. The data obtained suggest that the grinding of maize is not likely to be worthwhile for lactating goats.

Keywords: Whole maize grain, beet pulp, goat, milk performance

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Introduction

Goats are classified as "intermediate feeders" due to their ability to change their feeding behaviour according to the availability of forage (Van Soest, 1994). In particular, Abijaoudè et al. (2000) demonstrated that goats tend to prefer diets rich in concentrate and this suggests some forms of adaptation to avoid ruminal acidosis.

Starch is an important source of energy for lactating ruminants and its digestion is affected by a variety of factors such as type of grain, processing method, conservation method and type of endosperm (Oba & Allen, 2003). This topic has been thoroughly researched for cattle, but very little research has been conducted on dairy goats. An alternative for maize grain is beet pulp because of the high content of non-fibrous carbohydrates (NFC), especially pectins and sugars, which is characterized by a high degradation rate (Rapetti, unpublished data). Broderick et al. (2003) demonstrated that dietary sources rich in pectins altered rumen fermentation through increased acetate and butyrate concentrations and acetate-to-propionate ratio, but also caused depressed dry matter intakes. This is in agreement with the results reported by Voelker & Allen (2003b) who substituted beet pulp for high-moisture maize. Furthermore, in contrast to starch, pectin fermentation does not inhibit cellulose and hemicellulose digestion, primarily because pectinolytic bacteria are also inhibited at a low rumen pH (Voelker & Allen, 2003a).

The purpose of this experiment was to evaluate the effects on intake, digestibility and lactational performance of goats when fed diets containing whole or ground maize at two different dietary starch levels through substitution of beet pulp for maize.

 

Materials and Methods

Eight second parity Saanen goats (57.4 ± 5.3 kg BW, on average) in mid lactation (83 ± 13 days in milk (DIM), on average) were used in a digestibility trial. The experimental design was a duplicated 4 x 4 Latin square balanced for carry-over effect with a 2 x 2 factorial arrangement of treatments. The four diets (Table 1) contained either ground maize grain (ground through a 3 mm screen) or whole maize grain at two different levels of starch (high or low). In the diets with low starch content, dried pelleted beet pulp substituted the same amount of maize grain.

The diets were offered ad libitum, allowing for about 5% orts, twice daily at 8:30 and 18:30. The goats were milked at 8:00 and 19:00. Each experimental period lasted 21 d (13 d of adaptation and 8 d of collection). The goats were confined in individual metabolism cages. Diets, orts and faeces of each goat were weighed and sampled daily, pooled within period and animal, and frozen for analysis. Dry samples were analysed for dry matter (DM), ash, organic matter (OM), nitrogen (N), ether extract, and starch according to the Italian Scientific Association for Animal Production recommendations (ASPA, Commissione Valutazione degli Alimenti, 1980). Neutral detergent fibre, ADF and ADL were determined according to Van Soest et al. (1991). Milk samples were weighed, and stored at 4 °C before analyses. Total N, non-protein nitrogen (NPN), casein and fat concentrations (Gerber method) of milk were determined according to the Italian Scientific Association for Animal Production recommendations (ASPA, 1995). Milk samples were also analyzed for lactose (Milkoscan 605, Foss, Hillerod, Denmark), somatic cell count (SCC) (Fossomatic 360, Foss, Hillerod, Denmark) expressed as linear score (LS = log2 (SCC/12,500) and urea (CL10, Eurochem, Italy). Data were statistically analysed using the GLM procedure of SAS (2000).

 

Results and Discussion

The animals selected feeds, leaving hay as the sole orts. As a consequence, the forage:concentrate ratio (DM basis) of the ingested diets resulted 32:68 instead of the planned 40:60, on average for the four treatments. The dry matter intake (DMI) registered during the experimental trial (Table 2) was increased (P < 0.05) by low starch diets. The DM digestibility of the high starch diet with whole maize grain was significantly higher than the high starch and ground maize grain diet and the low starch and whole maize grain diet. Ether extract digestibility was influenced by starch level, due to the higher fat content and utilization in maize. Substitution of beet pulp for maize grain (P < 0.05) improved NDF and ADF digestibility due to the low content of starch, confirming the results obtained by Voelker & Allen (2003a) in a trial with dairy cows. Starch digestibility was influenced by the physical form of maize: whole grain was less digested than ground grain, even though it did not impair total DM digestibility. 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 through the gut. This seems to be due to extensive damage of maize kernels during mastication (Beauchemin et al., 1994) thus resulting in considerable grain utilization at rumen level. The comparison between treatments showed lower starch digestibility for the diet with beet pulp and whole maize grain.

The milk production (Table 3), expressed both as measured milk yield and 3.5% fat corrected milk (FCM), was not affected by the level of starch. This was probably due to the higher DMI by goats fed 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 ground maize grain depressed feed intake and milk yield. Milk fat was not affected by starch level or by maize type and its percentage for all treatments resulted in lower than protein percentages. This might be due to the low ether extract content of the diets. The level of starch significantly influenced the proportion of lactose. This could be ascribed to the presumably higher rumen propionate production from amylolytic microbes on these diets.

 

Conclusion

In diets rich in concentrates the substitution of beet pulp for maize significantly improved DMI and NDF and ADF digestibility due to its high content of soluble fibre. Moreover, the utilization of whole grain instead of ground maize grain does not depress diet digestibility while it does increase milk yield. In conclusion, the data obtained suggest that the grinding of maize is not likely to be worthwhile for lactating goats.

 

Acknowledgements

This research was supported by funds from Regione Lombardia, Piano per la Ricerca e lo Sviluppo 2002 d.g.r. n. 7/9182 31/5/2002.

 

References

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# Corresponding author. E-mail: luca.rapetti@unimi.it