Effect of energy and rumen undegradable protein supplementation on milk production and nitrogen losses in dairy cows

Abstract

The rising milk production potential of the NZ dairy herd is reducing the ability for farmers to meet cow energy demands. Providing a higher proportion of the CP in the diet as RUP has been identified as a possible way to increase the protein supply to the animal. Alternately, modulating the nutrient dynamics in the rumen has the potential to capture more of the excess CP available from pasture. The objectives of this research were to measure the milk solid, urinary and faecal nitrogen responses of dairy cows fed iso-energetic diets supplemented with wheat and canola meal, compared to pasture-only diets and to measure these same variables when the timing of supplementation, relative to pasture allocation was altered (1 h or 9 h post-pasture). It was hypothesised that the supplemented treatments would have increased milk production and decreased urinary and faecal N losses, compared to the control. It was predicted that the supplemented treatments would not differ significantly from each other in milk production, but that both supplemented groups would have lower urinary and faecal N losses. It was also hypothesised that the 9 h treatments would have significantly higher milk production and lower urinary and faecal N losses than the 1 h treatments. A total of 30 Friesian x Jersey cows (liveweight: 455 kg) were all matched for parity, body condition score (BCS) and days in milk (DIM), and allocated to five treatment groups (six cows/ treatment). The cows underwent a 7 d adjustment period before undergoing a 24 d treatment period. Four of the treatment groups received either 5.1 kg DM rolled wheat grain or wheat and canola meal, as well as 11.5 kg DM pasture. The treatment groups included: C: pasture (p); E9: p + wheat (w) 9 hours (h) before p; E1: p + w 1 h before p; P9: p + w and canola meal (c) 9 h before p; P1: p + w and c 1 h before p. All of the supplemented treatment groups were designed to be iso-energetic; with the high-RUP supplemented group being iso-nitrogenous with the pasture-only treatment group. Measurements included: milk yield, milk composition, faecal, urine, blood, pasture and concentrate samples. The data were analysed in a two-step analysis that first included the control group in an ANOVA model in GenStat and then the four supplemented groups were analysed using a 2 x 2 unbalanced factorial ANOVA model.

Providing a RFCHO source above a pasture-only diet significantly increased MS production in one of the wheat treatments (2.2 kg/c/d, compared to 2.0 kg/c/d in the control), but not in the other (1.9 kg/c/d) (P<0.001). Nitrogen losses in the wheat-supplemented treatments were lower than in the C group. When the N percentage in the urine and faeces of the E groups was compared with the C group, it was shown that urinary N was reduced by 12.1% (P=0.038) and faecal N by 7.4% (P<0.001).

Feeding a high-RUP diet to cows (compared to an iso-energetic low-RUP diet) increased the level of MS production by 0.2 kg/d and 0.1 kg/d in the P9 and P1 groups, respectively (P<0.001). The P groups lost an average 0.33 of a body condition score (BCS) during the experiment, compared to a 0.07 BCS loss in the E groups (P=NS). The P groups urinary and faecal N losses (as a percentage of N intakes) were higher than the E groups (2.6% and 1.8% increases in urinary and faecal losses, respectively).

Timing supplementation to occur 9 h pre-pasture intake was seen to decrease MS production, compared to offering the supplement 1 h pre-pasture allocation (2.04 MS/d, compared to 2.15 MS/d) (P=NS). The 9 h treatment groups increased in LWT over the experimental period by 18.2 kg; compared to 6.3 kg in the 1 h groups (P=NS). There was a non-significant 4.1 % and 3.2 % reduction in urinary and faecal N losses in the 9 h group, compared to the 1 h group.

The implications of the trial are that feeding a concentrate supplement will normally result in an increase in milk production, although this is dependent on cow intake. Provision of an energy supplement was shown to reduce urinary and faecal N losses. The effect of timing produced some anomalies, with the 9 h group having lower N losses, but having a reduced level of MS production.