Protein supplementation of cows grazing limited amounts of paspalum (Paspalum dilatatum Poir.)-dominant irrigated pasture in mid-lactation

Abstract

Two experiments were undertaken in summer to investigate production responses by dairy cows offered paspalum (Paspalum dilatatum Poir.)-dominant irrigated pasture to the inclusion of protein supplements with different rumen degradable protein characteristics in cereal grain-based concentrates. In experiment 1, cows in mid lactation were offered limited amounts of herbage, at an allowance of 28 kg DM/cow. day together with 8.0 kg DM/cow. day of a pelleted supplement containing: (i) mainly barley and wheat; (ii) barley, wheat and canola meal; or (iii) barley, wheat and cottonseed meal for 35 days. The 3 supplements were formulated to have similar in vitro DM digestibilities. In experiment 2 aspects of rumen function were measured in 9 rumen fistulated lactating cows, individually fed indoors on paspalum-dominant herbage harvested daily with a finger bar mower. Cows were offered the same amount of supplements for 28 days. Pasture intakes were similar to those of the grazing cows in experiment 1.

In experiment 1, there were no effects (P>0.05) of dietary treatment on herbage intake (10.2 kg DM/cow. day) or selection of nutrients from herbage. The concentration of crude protein in the total diet consumed was lower (P<0.05) for the barley + wheat treatment (137 g/kg DM), compared with the barley + wheat + canola meal and barley + wheat + cottonseed meal treatments, which were 179 and 183 g/kg DM, respectively. In vitro DM digestibility of the feed consumed by cows was similar across the treatments, and all diets contained greater than 400 g neutral detergent fibre/kg DM, indicating there were no limitations due to dietary fibre. There were no significant effects (P>0.05) of dietary treatments on milk production (21.8 kg/cow. day), milk fat concentration (34.1 g/kg) or milk protein concentration (29.8 g/kg). Urea-N was higher (P<0.01) in the milk of cows fed the barley + wheat + canola meal and the barley + wheat + cottonseed meal treatments compared with the barley + wheat treatment. In experiment 2, rumen fluid pH was higher in cows consuming barley + wheat pellets than in cows consuming barley + wheat + canola meal on most sampling occasions. Rumen ammonia-N concentration was lowest in the barley + wheat treatment and highest in the barley + wheat + canola meal treatment.

When cows grazing paspalum-dominant irrigated pasture were supplemented with 8 kg DM of a cereal-based supplement, metabolisable protein was calculated to be present in sufficient quantities to support at least 22 kg milk/cow. day and, hence, was not limiting milk production. It is concluded that protein supplements offered to cows in mid-lactation, grazing irrigated pastures in summer, are unlikely to give a milk production response beyond that achieved with cereal grain.