Abstract
The aim of this experiment was to study the effects of substituting dietary barley grain with orange pulp or soybean hulls on energy, nitrogen and carbon balance, methane emission and milk performance in dairy goats. Twelve Murciano-Granadina dairy goats in midlactation were selected and divided into three groups based on similar body weight (42.1 ± 1.2 kg) and milk yield (2.16 ± 0.060 kg/goat/day). The experiment was conducted in an incomplete crossover design where one group of four goats was fed a mixed ration of barley grain (BRL), another group of four goats replaced barley grain with orange pulp (OP) and the last group of four goats with soybean hulls (SH). After adaptation to diets, the goats were allocated to individual metabolism cages and intake, faeces, urine and milk were recorded and analysed. Then, gas exchange measurements were recorded by a mobile open-circuit indirect calorimetry system using a head box. Dry matter intake was similar for all three groups (2.03 kg/d, on average). No influence of the diet was observed for energy balance and the efficiency of use of metabolizable energy for milk production was 0.61. The OP and SH diets showed greater (P < 0.05) fat mobilization (-42.8 kJ/kg of BW0.75, on average) than BRL (19.2 kJ/kg of BW0.75). Pentadecanoic acid (15:0) and heptadecanoic acid (17:0) were potential biomarkers of rumen function because the higher contents found in the milk of OP and SH goats than BRL suggest a negative impact of these diets on rumen bacterial metabolism; probably linked to the lower nitrogen supply of diet OP to synthesize microbial protein and greater content of fat in diet SH. Replacement of cereal grain with fibrous by-products did not increased enteric methane emissions (54.7 L/goat per day, on average). Therefore, lactating goats could utilize dry orange pulp and soybean hulls diets with no detrimental effect on milk performance.
Highlights
To achieve maximum milk production potential by means acceptable to consumers, feeding systems for dairy ruminants need to ensure high energy intake, among other factors
The higher neutral detergent fibre (NDF), ADF content and greater fat added in soybean hulls (SH) diet than barley grain (BRL) diet appeared to be the main factor responsible for the lower DM, OM and CP apparent digestibility found in SH diet
Diet orange pulp (OP) has lower CP content and lower CP digestibility than the others, with a decrease in ether extract (EE) digestibility of 38 points compared to the BRL and SH diets
Summary
To achieve maximum milk production potential by means acceptable to consumers, feeding systems for dairy ruminants need to ensure high energy intake, among other factors. This might be accomplished by raising the dietary concentration of rapidly degraded non-fibrous carbohydrates (NFC), such as starch from cereal grain. From a nutritional point of view, by-products are included in the ration to supply energy and protein, but are often characterized by high fibre content. This is the case of orange pulp or soybean hulls, which are typically used as grain substitutes. Soybean hull has a similar CP content to that of barley grain (11%) and is high in NDF (58%, high in cellulose) but low in lignin (2%), NFC (24%) and sugars (1.5%), with no starch content (barley grain has 51% starch)
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